Zoning Application Manual For 950 / 1050 Zone Systems

Pub # 034-4819-02

Residential Zoning Design

For Single Centralized HVAC Systems

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Preface: The term 950 & 1050 will be used throughout this manual to describe the intelligent zoning system of Trane and American Standard. This zoning system is called AccuLink® for American Standard and ComfortLink II® for Trane. Zoning is advanced system design. The intent of this application guide will discuss the challenges and proper techniques of manipulating air flow with zoning. This publication does not cover all aspects of zoning and assumes the reader has a fundamental knowledge of HVAC system design and experience with the following ACCA Manuals: Manual J for heat load calculations Manual D for duct design Manual S for HVAC system selection Manual T for terminal (register) selection Manual RS for an overview of homeowner comfort & zoning strategies Manual Zr for a detailed guide in zone design

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Table of Contents:

Chapter 1 Getting Into the Zone Page 4 Chapter 2 When Zoning is Recommended Page 5 Chapter 3 Heat Load Calculations & Duct Design Page 9 Chapter 4 Home Evaluation Page 12 Chapter 5 Equipment Selection Page 13 Chapter 6 How to Configure Zones within a Home Page 14 Chapter 7 Zoning Components Page 20 Chapter 8 Excess Air Page 22 Chapter 9 Excess Air Management Strategies Page 24 Chapter 10 Design Conditions Specific to Zoning Page 39 Chapter 11 Pitfalls and Misconceptions of Zoning Page 41 Chapter 12 Summary Page 43

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Chapter 1

Getting in the Zone

Zoning fulfills one of human’s most basic desires. The desire for comfort. A conventional HVAC system is typically controlled by one thermostat in one location. This thermostat cannot sense changes in load patterns throughout the home. Variables such as shifts in solar gains, differences in cooling verses heating loads, changes in internal heat loads for social gatherings and a desire for different temperatures throughout the house are all good reasons to apply zoning. Zoning exists to provide comfort conditions that exceed the ability of a conventional system.

Zoning is the intentional manipulation of cooling or heating to specific areas or “zones” of a home. A home can be zoned via two methods:

• A single centralized HVAC system that incorporates dampers in the duct work to control air flow. Each zone will have a sensor that manipulates a damper to control comfort.

• Applying multiple systems to control individual sections of the home. The homeowner and system designer must come to an agreement on the type of equipment and zoning strategies to be used.

The impact of solar loads on a home change from hour to hour and season to season. This shift in solar loads will generate temperature variations within the home. Zoning applies an intentional manipulation of system capacity to maintain even temperatures throughout the individual zones. Zoning is often advertised as turning off air to rooms when not in use—comparing zoning to a light switch. Unfortunately, this extreme strategy can generate wide temperature swings with increased zone pressurization & infiltration. A system running in this manner tends to be inefficient and may contribute to premature component and system failures. This zoning strategy is not recommended.

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Chapter 2 When Zoning is Recommended

Home architecture has made tremendous strides over the past decades. Building designs are more elaborate and materials are more efficient.

The homes from a few decades ago were generally smaller with simpler floor plans, where a single conventional system would provide comfort. Many homes that are built today are more complex and homeowner expectations for comfort have increased. A single conventional system may no longer suffice and zoning may be required to deliver comfort. There are several scenarios when zoning is highly beneficial.

Multi Level Structures

In multilevel structures, warm buoyant air will rise towards the upper levels while cooler air will fall to the lower floors. The centrally located thermostat in this home cannot sense this shift in temperatures. This type of home is one of the most common applications for zoning. A simple two zone system will assist in balancing the conditions between the upper and lower floors. Additional zones may be added for increased comfort on individual levels.

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Time of Day & Seasonal Load Patterns

Temperature variations are not limited to multi story structures. Single story homes are also susceptible to temperature swings between seasonal and time of day load patterns. This shifting solar load pattern will peak in the east rooms in the morning and the west rooms in the afternoon. A home with one centralized thermostat cannot sense this load shift. It can only maintain the temperature at its specific location. The central hallway will remain comfortable while the rest of home will experience temperature swings.

A home is also susceptible to seasonal load patterns. A home with a north / south exposure will require the majority of the BTU’s to be delivered to the southern rooms in the cooling season.

The winter season combines an increase in solar gains along with the majority of the heating air flow impacting the southern side of the home. Temperature variations are likely to occur; and in extreme cases the home may require heating on the north side and cooling on the south side simultaneously.

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Special Indoor Temperature Requirements

Some homeowners have special indoor temperature requirements for entertaining large groups; have family members who like their rooms at different temperatures, or simply have areas of the home that are infrequently used. There may be a need to set these zones at different temperatures at different times.

An exercise room may have a cooler

setting than the rest of the home.

An “in law” suite may wish to maintain a different temperature than the rest of the home.

Unique Architecture

Rooms with large glass loads and rooms that do not share the same design as the rest of the house will typically require zoning. These rooms behave differently from the rest of the home and may exhibit large temperature swings.

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Zoning Realizations & Areas of Caution

Critical questions must be answered when evaluating a home for zoning:

1) Is it possible that the equipment will turn on with only the smallest zone calling? a. If so, can the HVAC system reduce the capacity output to meet this small demand? b. If not, how will the zone system manage this excess capacity?

2) Is the HVAC system & duct design proper for this smallest zone? a. For instance, a house has four zones with a 4-ton AC and a 100K single stage furnace. The smallest

zone has an 8 inch duct and requires 4,000 cooling & 5,000 heating BTU’s. - Will this 4-ton system applied to this zone function properly? - Will this system applied to this zone suffer from premature failures? - Will this zone maintain comfort, or might it suffer from extreme temperature swings? - Would you install this standalone system to handle a zone this size? - If you came across this design installed by another contractor, what would you tell the

homeowner? - Does the application of zoning make all the above conditions automatically disappear, or are

there real concerns that must be evaluated before applying zoning?

3) Is zoning being applied to maintain even temperatures throughout the home, or is the homeowner going to keep different zones at different temperatures?

- Is it possible that some zones may require cooling while other zones require heating simultaneously? How is a HVAC system designed to handle this?

- Is there a return air path dedicated to each zone? Return air traveling across zones to a central return grille will influence the temperature in other zones. Maintaining different zone temperatures is difficult (if not impossible) when all zones share a central return.

Zoning is advanced system design, and a zone system will only work as well as the design it is applied to. The questions and points noted above are realities that must be evaluated and answered. The goal of this application guide is to walk the designer through the requirements & restrictions of a proper zone design, and show some real examples of what happens when zoning is improperly applied.

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Chapter 3 Heat Load Calculation & Duct Design

The first step in designing a zone system is a room by room heat load calculation.

Previous to the release of ACCA Manual J8, HVAC systems were designed for “average conditions”. A calculation of 8,000 BTU’s informs the designer that the average heat gain per hour is 8,000 BTU’s. The designer could not tell how the heat gain in the zone varied throughout the day.

Manual J8 provides two separate load calculations to assist with zoning design. 1) The average block load condition is used to size the HVAC system. The HVAC system must be sized as

close to the heat load calculation as possible. System oversizing can generate comfort issues such as poor humidity control, drafty conditions and premature system failures. Applying zoning to an oversized system exacerbates this problem. ACCA recommends that the capacity of an air conditioning system in non-zoned applications should not be more than 15% larger than the heat load calculation of a home. This limit should be minimized when zoning is applied.

2) The hourly peak load condition is used to design the duct system in zoning applications. Since the load conditions change throughout the day, the duct system must be sized large enough handle and absorb peak load conditions. The duct dampers will reduce air flow during reduced load conditions.

The line at 8,000 BTU’s shows the average sensible heat gain of the zone. This is the 12 hour average hourly gain

throughout the day and is used to size the AC system. It is also used to determine duct sizing for non-zoned systems.

The line starting at 4,000 and peaks at 9,500 is the actual

heat gain of this zone. The 9,500 BTU’s is 1,500 (or 18%) more than the conventional heat load calculation. This

additional 18% load is the excursion for the zone.

The line at 10,400 BTU’s represents a 30% buffer zone. The temperature swings in this zone should be relatively

minor (within 3 degrees) as long as the excursion does not exceed the 30% buffer zone.

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This chart shows an excursion as the actual heat gain exceeds the 30% buffer zone.

This is representative of a zone with a lot of glass and

western exposure. This area will have extreme temperature swings throughout the day and will require

zoning to assist with temperature control.

A conventional duct design would be sized to deliver 8,000 BTU’s to this zone. A zoned system would be

sized to deliver 14,400 BTU’s to this zone. This “oversized” duct is now large enough to deliver full air flow in the afternoon, and the dampers will reduce air

flow during lower load conditions.

Ideally, rooms with extreme excursions should be zoned through individual HVAC systems.

Using the example above, we’ll compare the conventional duct design verses zoning design and see why the duct configuration is so critical. The equipment selection for this example is a 4-ton AC & furnace with PSC blower. The total external static (with coil, filter and all accessories) is 0.7 and the blower is delivering 1500 CFM. The blower is starting to ride the blower curve and slipping a little from the designed 1600 CFM. This example will be for peak summer cooling operation.

Duct Design for a Conventional Non-Zoned Application. The designer is still using the 7th edition of Manual J which does not generate peak conditions. All that is known is this zone requires 8,000 sensible BTU’s. The designer chooses a10 inch duct which will deliver 350 CFM at 650 FPM velocity.

But because of the extreme peak load conditions, this zone experiences large temperature swings through the day. The homeowner is not pleased and is demanding that the issue be resolved. The designer decides to apply zoning to this existing duct system.

As zone dampers begin to close, the duct static and velocity rates through the open dampers will increase; delivering more air into the zones that require it. The blower will need to deliver 670 CFM with a velocity of 1250 FPM to handle the peak load condition in this zone.

Unfortunately, the PSC blower cannot deliver this air flow against the increased duct restrictions. This zone will still experience temperature swings, or the homeowner may lose cooling altogether if the indoor coil freezes. The compressor may also be at risk for premature failures due to liquid refrigerant flooding. Applying zoning to this conventional system is not recommended.

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Design for a Zoned Application. The designer rips out the entire duct system and designs the system with zoning in mind. The designer is now choosing a 12 inch duct that can deliver the 670 CFM with a velocity of 835 FPM to absorb the heat load during the peak conditions.

The modulating zone damper will be wide open during the peak afternoon conditions and modulate down to reduce air flow during the morning (and other lower load conditions).

One advantage of a conventional duct design is it’s simple and static. The CFM and velocity rates remain constant through all zones at all times. This is not the case once zoning is applied. The CFM & velocity through the ducts is constantly changing as dampers move. The designer must evaluate the branch duct locations and register selection to ensure each room will be comfortable in a variety of conditions.

- Will a zone exhibit high velocity rates and noise as other dampers close? - Will a zone maintain proper air “throw” to blanket the room when all dampers are open and velocity rates

are low? .

Oversized HVAC systems and / or undersized duct systems can generate air velocity noise, sweating ducts, premature system failures and capacity / humidity issues.

It is difficult if not impossible to resolve application issues with equipment modifications.

An inadequate design will deliver poor performance.

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Chapter 4 Home Evaluation

The first step in any system design is to know the configuration of the home and the requirements of the homeowner. The home for this application has a simple floor plan and the homeowner is requesting balanced temperatures. The designer decides on a simple two zone system, one zone for the living areas and one zone for the bedrooms. The homeowner agrees to the strategy.

The living area is the largest zone and has the largest heat gains and losses.

The chart on the left shows the sensible BTU’s and CFM required for each zone in heating & cooling operation.

Discussions with the homeowner are critical. The homeowner & designer must have an understanding & agreement of the system design. The homeowner must realize that a specific system design may not satisfy all desires.

There is often a balance between system cost & comfort desired.

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Chapter 5 Equipment Selection

The heat load calculation for the house in this example requires 31,306 sensible cooling and 55,877 heating BTU’s. The equipment selected should match the load as closely as possible. The homeowner is asking for a base 13 SEER AC system and 80% furnace with a PSC blower.

• The 3.5 ton AC delivers 29,500 sensible BTU’s—6% short on capacity. • The 4-ton AC delivers 34,700 sensible BTU’s—9% over on capacity. • The 5-ton AC delivers 43,400 sensible BTU’s—28% over on capacity.

The 5-ton is too large and will dramatically increase the amount of excess air. This system should not be used. The 3.5 ton is slightly low on capacity, but would be an acceptable option if the homeowner does not mind a slight temperature slide during extreme summer conditions. For this example, the equipment selected is the 4-ton system which is oversized by 9%.

Never neglect the latent loads when selecting equipment. The equipment must be able to absorb the sensible & latent loads of the home. Any excess latent capacity can be allocated to the sensible capacity of the equipment. For example, if the system has an extra 1,000 BTU’s of latent capacity; then 500 BTU’s can be added to the sensible capacity of the system.

The furnace is an 80,000 BTU with a 4-ton drive and design temperature rise of 30 to 60 degrees. This furnace has an output of 64,000 BTU’s—13 percent over on capacity. The air flow delivery from this furnace is:

--1641cooling CFM with a high speed tap at 0.6 inches of external static --1275 heating CFM with a medium high speed tap to deliver a 46 degree temperature rise

This system configuration is a good match for the home when using the high speed tap for cooling and the medium high speed tap for heating. The use of PSC is for example purposes only. The 950 / 1050 zone system requires our variable speed indoor blower motor,

• Never choose equipment sizes at random or based on square footage of a home • Never assume that an existing system is properly sized

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Chapter 6 Configuring Zones for a Comfortable Home

The designer must confirm two items before proceeding with a zone design: • Will the zones maintain comfort for the homeowner? • How much excess air must be managed in the worst case scenario?

This chapter will focus on the comfort side of the zoning equation. Chapters 8 & 9 will focus on the equipment’s ability to manage excess air.

The Worst Cooling Scenario The worst case in cooling is when the bedroom is calling and the living zone damper is closed. The bedroom zone only requires 36% of the system capacity (592 CFM), so the system must be able to manage 64% (1049 CFM) of excess air.

The Worst Heating Scenario The worst case in heating is when the bedroom is calling and the living zone damper is closed. The bedroom zone only requires 43% of the system capacity (554 CFM), so the system must be able to manage 57% (721 CFM) of excess air.

Excess air exists any time a damper closes, and the system designer must employ strategies for managing this excess air. This application guide will evaluate bypass, relief, air flow reduction with variable speed blowers, over blow and capacity control with multi stage systems. All of these options are available to the designer when creating an HVAC system. Each job site must be evaluated to determine which method, or group of methods are required to accomplish the successful management of excess air.

The first step in designing a zoning system is to walk the house and anticipate zones based on the room layouts and duct design. This same process should be applied to a new construction project with the involvement of the builder and homeowner. This visual and “common sense” approach of laying out zoning is a valuable first step. There are however 6 conditional steps that must also be evaluated. Missing any one of these conditional steps may result in uncomfortable zones and homeowner complaints.

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All six of the conditions below must be evaluated. Creating a zone system based on only a few of these conditions may lead to comfort issues.

Condition 1: Evaluate the openness of the rooms (see the example below). When evaluating a heat load calculation, the kitchen may have a very different load pattern than the breakfast or family room. But due to the openness of these rooms they should be on the same zone. Generally speaking, two adjoining rooms can be combined into one zone if they are more than 25% open to each other. In this example, the family room and kitchen are completely open to each other. The air between these two rooms will mix and the temperatures be fairly balanced. The kitchen and dining room are separated by a wall. The air between these two rooms will not mix as easily and there is a potential for temperature imbalances--even if the door is left open. Rooms in multi story applications are not considered to be adjoining rooms. A loft overlooking a living room may be open to each other, but they should be treated as independent zones.

Condition 2: Understand the homeowner desires for temperature control. A homeowner may desire to maintain specific rooms at different temperatures; such as a guest room or game room that are used on a minimal basis. These rooms must be treated as independent zones and cannot be applied to areas that are used on a regular basis.

Condition 3: Perform a heat load calculation and evaluate the duct design. The air conditioner, heat strips or gas furnace should be sized as close to the heat load calculation as possible. System oversizing dramatically increases the amount of excess air that must be managed, so the negative impacts of system oversizing will be greater with zoning applications than with conventional systems. Undersized ducting generates effects that are similar to system oversizing and increases the challenges of managing excess air. Applying a zoning system to a poorly designed HVAC system may not resolve comfort complaints and may contribute to premature system failures.

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Condition 4: Zones must exhibit similar excursion patterns. Placing an east and west facing room into one zone may cause uncomfortable conditions, especially if these rooms have a large glass load. A thermostat centrally located for these two areas cannot sense this shift in load pattern and a consumer complaint could follow. The ideal practice is to group rooms that have similar solar patterns. The chart below is taken from ACCA Manual RS and shows the general grouping pattern for rooms. The importance of this grouping increases with the amount of glass load.

Condition 5: Evaluate seasonal load patterns. A heat load calculation will show the sensible cooling and heating loads for each room. The cooling to heating ratio (C/H ratio) is obtained by dividing the sensible cooling BTU’s by the heating BTU’s. It is possible to design a perfect system for cooling that generates disastrous results in heating. The goal here is to combine rooms with similar cooling and heating requirements. Ideally, the C/H ratio between the rooms in a zone should be within 15% of each other. The chart below shows a summary of this process. ACCA Manual RS describes this in greater detail.

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Bedroom 1 requires 6176 sensible cooling BTU’s and 13325 heating BTU’s. The C/H ratio of 46% tells us that this room will require about twice as many BTU’s for heating as it does for cooling.

The low limit of 39% represents rooms that may have a more extreme C/H ratio, and the high limit of 53% represents rooms that may have a less extreme C/H ratio. Any room that falls within this 39% to 53% range should experience similar seasonal load patterns and remain fairly balanced between seasons. Condition 6: Evaluate the capabilities of the equipment. Determine the size of the smallest zone. This will be used to calculate the amount of excess air that must be managed: The Total Air Flow – the Air Flow of the Smallest Zone = Excess Air. For example:

The smallest zone on a 5-ton system is sized at 25%. If the total air flow design is 2,000 CFM, then this zone can only handle 500 CFM. The zone system must be able to manage 1,500 CFM of Excess Air. If the HVAC system is unable to manage this excess air, then either the system or the zoning plan must be modified.

It is unlikely to find a home that will satisfy all of the conditions above; so some sacrifices must be made. Follow the steps below to generate a sense of balance in the zoning plan.

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1) Physically evaluate the home and design the zones based on the layout and duct design. 2) Consult with the homeowner and obtain their requirements for the zone system. 3) Evaluate the zones and determine if they fall into the acceptable standards of directional exposure,

excursions and seasonal ratio loads. Make a note of any conflicts. 4) Establish the smallest zone and the amount of excess air that must be managed. Determine if the system

can manage this excess air. Multi stage or modulating equipment may be required. 5) Consult with the homeowner once again in regards to the challenges and issues found. Reduce the number

of zones and evaluate multi-capacity systems that can deliver homeowner comfort without stressing to the equipment.

There is often a relationship between the number of zones and the type of equipment required. The greater the number of zones, the more complex the HVAC system must be to manage excess air. A simple list showing all this info is helpful when discussing a zoning design with the homeowner. It allows the designer to see how the individual rooms will work within their zones.

The designer and homeowner can sit down to view the design and ensure the six comfort conditions above are met.

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Condition 1: The breakfast, kitchen, entrance and family rooms are all open to each other. These rooms should be in the same zone. The other rooms will need further analysis.

Condition 2: Through discussion, the homeowner stated that he wants a balanced temperature throughout the house and has listed the desired temperatures for 77 cooling (50% RH) and 71 heating. This is not to say that the homeowner can never deviate from these temperatures. These are simply the indoor conditions that the designer will reference for the heat load calculations and equipment selection process. The HVAC system should provide the greatest comfort at these conditions and there may be some comfort issues (capacity, humidity or balanced temperatures) when the homeowner moves way from these set points.

Conditions 3, 4 & 5: This checklist allows the homeowner to see the heat load calculation for each room. In evaluating the bedrooms, we see that the SW facing bedroom #2 has a higher excursion than bedrooms 1 & 2. Due to solar exposures, these rooms may experience some temperature swings through the day. The seasonal cooling to heating ratio is very close between these rooms. Outside of the daily excursion, these rooms will work well in one zone. Ensure the homeowner understand this and is acceptable of some temperature swing within these rooms. The excursion pattern for the living room area is fairly mild and the daily temperature swings in this zone should be minor. The biggest concern here is the season cooling to heating ratios which range from 32% in the entry to 184% in the kitchen. Luckily, the openness of these rooms should minimize any temperature deviations.

Condition 6: The equipment selected must be able to manage the excess air when only one zone is calling. We’ll evaluate excess air management in Chapter 8. Using a simple checklist such as this will allow for an informative conversation between the homeowner and designer, and ensure both have an understanding of any challenges involved.

Walking through the home (or blueprint) with a homeowner is critical when discussing comfort. The homeowner must understand that multiple rooms within a zone may have some temperature deviation—especially if these rooms have different patterns in solar gains.

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Chapter 7

Zoning Components There are multiple components that must be added to a central HVAC system when employing a zone system. These additional components must work together to minimize stress on the HVAC system and provide comfort for the homeowner.

System Controller: The heart of the zoning system is the system controller. The controller evaluates the conditions of each zone and delivers the appropriate output signals. The 950 / 1050 zone system can:

• Stage equipment operation based on the individual load values for each zone (most zone systems will stage equipment on time or the percentage of zones calling—they cannot determine the true load of any zone)

• Modulate variable speed blower motors and reduce air flow up to 30% when managing excess air

• Position each modulating zone damper to deliver the required amount of air into each zone.

• Calculate and employ multiple strategies to manage excess air (see chapter 9)

Zone – Thermostat Control

Zone Panel – Damper Control

Zone Dampers: The zone damper adjusts the amount of air to each zone. The dampers with the 950 / 1050 zone system can modulate to any position between fully open and fully closed.

This modulating range allows the air flow to match the load in the zone, providing superior comfort than a standard damper system that is either open or closed.

The fully modulating dampers are also used to employ intelligent relief strategies when managing excess air.

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Zone Sensors: A typical zone thermostat has a set of dry contacts that can only communicate an on or off signal. The zone controller cannot determine if the zone is ½ a degree or 5 degrees away from set point, it only knows on or off. The zone sensor with the 950 / 1050 communicates the zone conditions to the system controller and allows the controller to make decisions regarding system staging and damper positions. These zone sensors provide true communication of zone conditions (Load Value) to deliver better comfort to the homeowner.

Discharge Temperature Sensor—DTS: The discharge temperature sensor (DTS) is a safety device that is placed in the supply plenum. Reduced airflow generates increased temperature splits across the indoor unit. The DTS informs the zone control if the system is operating in safe temperature ranges.

The DTS must not allow the system to trip on heating limits or allow cooling coils to freeze. Remember, a zone system is designed to provide comfort. Premature system failures due to poor air flow will not make a homeowner comfortable or happy.

Static Pressure Sensor: The static pressure sensors are mounted in the return and supply duct to monitor external duct static. The system controller uses static pressure readings during auto zone sizing to determine how much air each zone duct can handle.

The static pressure sensors are used for auto zone sizing only. They are not required for system operation.

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Chapter 8 Excess Air

System capacity, efficiency and reliability are all directly related to air flow. Reducing air flow as dampers close may assist with comfort, but it may stress the HVAC system.

The Blower CFM must be set at 400 CFM per ton with the 950 / 1050 zone system.

While designing and applying a conventional HVAC system to a home is not always easy, but it is a relatively straightforward process.

• Perform an ACCA Manual J heat load calculation • Chose the optimal system to match the sensible & latent load requirements (Manual S) • Design & install the duct system based on Manual D • Chose the optimal registers for proper air distribution throughout the rooms (Manual T)

The HVAC system will be quiet and efficient with even air distribution patterns when these ACCA standards are followed. However, a perfect design based on these standards cannot resolve temperature swings due to daily and seasonal solar shifts.

Remember, zoning is the intentional restriction of heating or cooling to a particular zone. Since the home in our example only has one system; this restriction must be applied to airflow. Any air that is restricted from entering a zone is considered Excess Air, and this air must be managed.

A reduction in air flow will cause: • An increase in external static pressure; possibly beyond the performance curve of the blower • An increase in air velocity through the open ducts (which increases turbulence & noise) • A reduction in total CFM (especially with PSC motors). This creates a reduction in system capacity, and

possibly efficiency Excessive air flow reduction may cause refrigerant flooding in air conditioning mode and extreme temperature rises in heating mode—premature system failures may occur.

Since the temperature split across the indoor unit will increase as air flow decreases, a discharge temperature sensor that monitors supply air temperature is required. It will shut down the heating / cooling operation if the temperature exceeds the trip points noted in the chart below. The system will come back on line when the time & temperature requirements have been met.

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DTS LIMIT TABLE Safety Trip

Point

Turn On

Temp

Minimum Off Time

Blower On

Normal Cooling 38⁰ F 5 minutes Yes Adjusted Cooling 34⁰ F 5 minutes Yes Normal HP Heating 116⁰ F 5 minutes Yes Adjusted HP Heating 128⁰ F 5 minutes Yes Normal HP + Strip Heating 170⁰ F 130⁰ F 3 minutes Yes Adjusted HP + Strip Heating 170⁰ F 130⁰ F 3 minutes Yes

Normal Gas Furnace 135 or ∆T

>60⁰F ∆T< 15⁰F 3 minutes Yes

Adjusted Gas Furnace 135 or ∆T

>60⁰F ∆T< 15⁰F 3 minutes Yes

Normal Oil Furnace 170 or ∆T

>85⁰F ∆T< 35⁰F 3 minutes Yes

Adjusted Oil Furnace 170 or ∆T

>85⁰F ∆T< 35⁰F 3 minutes Yes

Normal Strip Heat only 170⁰ F 130⁰ F 3 minutes Yes Adjusted Strip Heat Only 170⁰ F 130⁰ F 3 minutes Yes

Misconceptions and improper management of Excess Air may generate air flow issues, homeowner complaints and premature system failures. The designer must have a complete understanding of Excess Air:

• What Excess Air is • How to manage Excess Air • How much Excess Air must be managed

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Chapter 9 Excess Air Management Strategies

There are six general strategies for managing excess air: 1) Over blow 2) Reduction in blower air flow (variable speed motors in compressor only operation) 3) Staging of equipment (multi stage & modulating systems only) 4) Relief 5) Bypass 6) Dump

It is rare that a single excess air management strategy will properly control a zone system and multiple strategies are often recommended—especially for systems with 3 or more zones. Properly applying multiple excess air strategies will:

• Ensure proper air flow—the blower motor will remain in its blower curve. • Ensure temperature rise—heating systems will not trip on thermal limits and cooling coils will not freeze or

flood compressors with liquid refrigerant. • Maintain homeowner comfort. Remember, air flow patterns and velocity will change with the opening and

closing of dampers. Poor management of excess air may generate noise or poor air delivery into a zone.

1) Over blow & oversizing duct systems. The theory of over blow recognizes that duct static and velocity rates will increase as dampers close. This will deliver more air to the calling zones than what the duct design had originally intended. System blower performance and external static must be evaluated before counting on over blow to assist with excess air. For example, an 8 inch duct will move 210 CFM at 600 FPM. As dampers close, the velocity may increase to 900 FPM and deliver 310 CFM. This additional 100 CFM into the calling zone is over blow. A blower that is riding the upper limits of the blower curve with all dampers open (high static condition) is already maxed out and cannot deliver additional pressure to the duct system. System air flow will decrease with the closing of the dampers. Over blow cannot be achieved (or is minimal) on undersized duct systems.

2) Variable speed air flow reduction. The zone system has the ability to communicate directly with the variable speed blower motor. If the calling zones cannot handle 100% of the air flow, the system controller will begin managing excess air by reducing blower air flow as much as 30%. This strategy only applies to compressor operation in the cooling and heating (without heat strip) operation. This blower reduction does not apply when using fossil fuel systems or electric heat operation.

3) Equipment staging. When a homeowner asks for zoning, what they are really asking for is the ability to modulate heating and cooling capacities at any given time. Multi stage and modulating systems are ideally suited in zoning applications. A variable speed outdoor unit can modulate capacity to meet the requirement of an individual zone, ramping down to 30% capacity and air flow in most conditions. A homeowner can now maintain comfort without the frustration of high velocity duct noise or stress to the equipment when smaller zones are calling. Multi capacity systems are highly recommended on all zone applications and are essential on systems with 3 or more zones. The sum of the zone sizes determines the operational stage of the equipment. The minimum sum of zone sizes to engage 2nd stage on a two stage scroll is 68%. The minimum sum of zone sizes to engage a 2-stage furnace is 66%. The minimum sum of zone sizes to begin the modulating range of a modulating furnace is 41%.

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4) Relief. If excess air remains after system staging & air flow reduction; then additional air management strategies are required. The relief air strategy allows a portion of the supply air to enter a zone that is not calling. This relief air strategy can either be intelligent or fixed.

A fixed relief strategy is usually a mechanical stop in the damper. This fixed mechanical stop will only allow the damper to close to a predetermined minimum position and always allows air to relive into the zone. The downside to this fixed strategy is there is no control over the relief process. Air will always flow into non-calling zones; regardless if excess air exists or not.

The zone system uses an intelligent relief strategy. When managing excess air, the 950 / 1050 will: • Calculate the load value and damper position of all calling zones.

- The zone control knows exactly how much air the zones can handle. • Set system staging on multi capacity systems and reduce the air flow (with variable speed

products in compressor only operation). • If excess remains, it will relieve this air based on the following progression:

1) Increase damper positions in calling zones up to 100% open. 2) If excess air remains after all dampers in the calling zones are at 100%, the zone control will open dampers in non-calling zones. The minimum damper relief position is 25%. 3) If excess air remains after all dampers in non-calling zones are at 100%, the zone control will open dampers in opposing mode zones.

Unlike damper stops, intelligent relief will continually evaluate the amount of excess air required and minimize the amount of relief air into non-calling zones.

5) Bypass. The most common technique to manage excess air has been bypass. Bypass is not allowed with the 950 / 1050 zone system. This technique takes air from the supply plenum and redirects it into the return duct. Bypass allows the blower to maintain air flow; but can generate extreme load conditions for the HVAC system. In regards to system performance, bypass is similar to air flow reduction.

6) Dump. A dump strategy is similar to bypass, but the location changes. Rather than diverting the supply air directly into the return, the dump strategy will direct this excess air into a low priority zone. A dump zone may be a basement, a hallway or any other area that is rarely used. The advantage of a dump zone is it allows the supply air to mix with the room air before returning to the system. The HVAC system will see a more constant return temperature and fairly stable load conditions. Dump zones however can see extreme temperature and humidity conditions which are undesirable for homeowner comfort.

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Excess Air Strategies Strategy 1: Over Blow

Over blow is a balance between blower performance and duct design. Over blow is a given when zoning is applied; unfortunately, over blow is constantly changing and difficult to calculate.

The duct design for this home required a 12 inch duct to feed the bedrooms and a 16 inch duct for the living area

Let’s say the two supply runs have identical equivalent lengths and the total external static on the system with filters & accessories is 0.6

The 12” duct delivers 592 CFM at 750 feet per minute The 16” duct delivers 1049 CFM at 750 feet per minute

The furnace has the ability to move 1641 CFM at 0.6 inches of external static pressure. The blower is perfectly suited for the ducting when both dampers are open.

When the bedroom zone is satisfied, the system now tries to move the 1641 CFM through the 16 inch duct in the living room zone.

• External static will increase from 0.6 towards 0.7. • Velocity rates will increase from 750 feet per minute to 1100 feet per minute. • Total system air flow will be reduced from 1641 to 1573—a reduction of only 68 CFM. • The air flow into this zone has increase from the 1049 design to 1573—an increase of 514 CFM. This 514

CFM is over blow. The reverse situation is that the bedroom is the only zone calling. The system now tries to move 1641 CFM through the 12 inch duct that feeds the bedroom zone.

• External static will increase to well over an inch. This is off the blower performance charts and we cannot accurately determine blower air flow (but will extrapolate and “guess” 1200 CFM).

• The velocity rates will increase from 750 feet per minute to 1500 feet per minute. • Total system air flow will be reduced from 1641 to 1200—a reduction of 441 CFM. • The air flow into this zone has increased from the 592 to 1200 CFM—an increase of 686 CFM. This 686

CFM is over blow.

The example above is based on an ideal duct system and represents how effective over blow can be. Unfortunately, this zone system cannot rely solely on over blow to manage excess air.

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Many duct systems have high static readings; some well beyond the blower performance tables (even with all dampers open). Remember, a duct system for zoning must be designed on “peak” load conditions. This generally creates an “oversized” duct system that maximizes the opportunity for over blow to manage excess air.

Caution: Air velocity rates will increase as dampers close, and this can generate an air noise complaint. Evaluate the following criteria when designing a zoning duct system:

1) What are the minimum and maximum velocity rates in the main truck lines 2) What are the minimum and maximum velocity rates in the branch ducts 3) Is there a potential that the maximum velocity rates will generate drafts or noise? 4) Is there a potential that the minimum velocity rates will create stagnant air pockets within a room?

There may be benefits to increase the number of supply terminals to ensure air coverage throughout a room while maintain lower velocity rates. This will maintain comfort without generating noise.

Excess Air Strategy #2

Variable Speed Blower

The 950 / 1050 zoning system has the ability to manipulate variable blower air flow through pulse width modulation. This signal is delivered through the BK circuit on 24 volt systems and the data wire on communicating systems. The zone control can reduce blower air flow up to 30% during compressor only operation. Blower reduction is not allowed with fossil fuel or resistance heat is in use.

When evaluating over blow, we saw that a PSC blower will reduce air flow in high static conditions (riding the blower curve). Variable speed motors retain better air flow patterns than PSC motors and are advantageous when designing for over blow. Also, since variable speed blowers retain better air flow performance, the zone control can confidently employ a 30% air flow reduction without sacrificing system reliability. Once air flow is reduced by the maximum of 30%, the zone control will evaluate other methods to manage excess air.

Air flow reduction and bypass have similar impacts on system performance, as both will change the load conditions across the coil and the temperature split from return to supply will increase. Because of this, air flow reduction & bypass may not be incorporated simultaneously. An air flow reduction of 30% plus a bypass of 30% equates to a 60% reduction in load (return air) across the indoor unit.

There are two main advantages of variable speed air flow reduction over PSC bypass: 1. Since variable speed motors perform fairly evenly across a wide range of static pressure readings, the zone

control has accurate control over the amount of air flow reduction. 2. The blower motor power consumption will drop along with air flow. An air flow reduction of 30% will

often reduce the blower motor power consumption by 50%. The chart below shows the wattage Vs CFM of a variable speed motor.

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Excess Air Strategy #3 Multi Capacity Equipment

When a homeowner asks for zoning, what they are really asking for is the ability to modulate heating and cooling capacities at any given time. HVAC systems with capacity control (multi staging or modulating) are a perfect fit with zoning.

Applying a variable speed condenser with modulating furnace to the home referenced in this application guide eliminates excess air alltogether. Zoning design becomes very easy when there is no excess air to manage.

In the cooling mode, the worst case scenaro is when the bedroom is the only zone calling—it can only handle 36% of the system capacity.

• The variable speed outdoor unit will run the compressor at a speed to meet the 36% zone capacity request • The variable speed blower motor will ramp down and deliver 36% air flow. • There is no excess air to manage

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The modulating furnace will run on first stage heating and deliver 43% of the system capacity. This furnace can modulate capacity to handle the needs to either zone individually, or both zones together.

• With system staging, the air velocity rates remain fairly constant regardless of which zone is calling. This minimizes the risk of low or high velocity conditions.

• Staging system capacity allows the equipmet to operate in design conditions. The system will run without cycling on temperature limits or flooded compressors. System longevity is maximizied.

Applying multi capacity systems with zoning provides the ultimate in comfort and system efficiency.

Excess Air Strategy Option #4 Relief

Relief is another strategy that is used to manage excess air.

The relief strategy allows air to be distributed to zones that are not actively calling.

Relief is designed to protect the equipment; it can however generate comfort complaints.

There are two methods when providing air flow relief: • Fixed damper stops • Intelligent relief

Fixed damper stops are typically mechanical locks on the damper actuator. Since duct static and velocity rates are continually changing as dampers move, the amount of relief air with a fixed damper stop will never be constant. To ensure proper equipment operation, fixed damper stops must always be set for worst case conditions. In doing this, the homeowner will typically receive more relief air into non-calling zones than is required.

The bedroom zone in cooling mode is the smallest zone and only requires 36% of the system air flow. The designer must evaluate all possible methods to manage the 64% of excess air; whether through multi stage systems, air flow reduction and over blow. The remaining amount of excess air after all other strategies have been exhausted is the amount of air that must be relieved into the living room zone. These calculations are cumbersome and must be performed for each zone in heating & cooling modes of operation.

The zoning system uses an intelligent relief strategy that is far superior to fixed stops. This intelligent relief strategy continually calculates the amount of excess air to minimize the amount of relief air into non-calling zones.

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This zone system knows: • The size of each zone (how much air flow each zone can handle) • The position of each damper (the percentage of air flow that each zone is receiving) • The blower speed (the system will always begin managing excess air with system staging and air flow

reduction before engaging in relief strategies).

If excess air remains, the zone panel will begin a relief strategy based on the following hierarchy: 1. Open dampers in calling zones. For example, only two zones on a 4-zone system are calling. One

damper is fully open and the other damper is only 40% open due to a lower load value in that zone. The zone control will start to open the 40% damper as the first strategy to reduce excess air. Since this zone is already calling for capacity, opening this damper will have minimal impact to homeowner comfort.

2. Open dampers in the same mode zones. If the calling zones cannot manage the full amount of excess air, then the zone control must look at non-calling zones. It will open dampers anywhere between 25% and 100% to manage excess air. A zone sensor in the “off” mode is considered a same mode zone.

3. Open dampers in opposing mode zones. If excess air remains, the zone control will open dampers in opposing modes as well. It will follow the same calculation as above and position the dampers anywhere between 25% and 100% to relieve the excess air. This is an extreme condition to relieve excess air and is indicative of a poorly designed duct / zone system.

The steps above happen instantaneously. The zone control always knows how much excess air must be managed and will set adjust the dampers in the appropriate zones as needed.

Following a few good zoning practices will minimize the amount of excess air that must be managed:

• The duct system must be designed for peak load conditions. This increases the amount of over blow that each zone can handle.

• Multi stage HVAC systems create a closer relationship between the zone requirements and system capacity. The amount of excess air flow is minimized.

• Variable speed motors provides an automatic 30% reduction in excess air when in compressor only operation.

• Maintain the grouping of rooms into the largest zones possible--the fewer the amount of zones the better. Remember, always evaluate the smallest zone when determine the amount of excess air the system must manage.

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Excess Air Strategy #5 Bypass

Bypass is one of the most common and most complicated methods to manage excess air.

In air conditioning mode, bypass introduces cool supply air into the return air stream. This generates a low load condition that is similar to air flow reduction. In the heating mode, bypass introduces warm supply air into the return air stream. This generates a high load condition that is similar to air flow reduction.

Bypass and air flow reduction produce similar results in regards to system performance, unfortunately bypass is much more difficult to control. Bypass is not allowed with our zone system. This application guide will however discuss the configuration and challenges of bypass.

ACCA Manual D informs us to design a duct system based on the longest run (the critical path) and to use hand dampers to control air flow through shorter runs. The bypass duct is typically the shortest run and provides the easiest air flow path. To compound the problem, the bypass duct has a greater pressure drop than any other duct in the system.

All ducts move air based on some static pressure differential between the ducting and the home. - A supply duct may have a static pressure of + 0.3 inches that delivers air through the registers into a room. - A return duct may have a static pressure of - 0.3 inches that obtains air through a grille in the room. - The bypass duct has a pressure differential of both the supply and return ducts. The bypass will be moving

air at a static pressure difference of 0.6.

The combination of the shortest run plus the greatest pressure differential generates high air flow & velocity rates. Generally speaking, the bypass duct will move more air than a supply duct of the same size.

It is critical that the amount of bypass air is evaluated with only the smallest zone calling (worse case condition).

The easiest way to determine if the amount of bypass is excessive is to monitor the supply air temperature. Insert a thermometer next to the discharge temperature sensor (DTS). Turn all zone sensors to the off position; then set the smallest zone to call for cooling. Allow the system to run for at least 15 minutes and monitor the supply air temperature. The supply air temperature should remain at least 5 degrees above the DTS trip point. Reduce the amount of bypass air until this 5 degree safety barrier is reached. Additional air management strategies must be incorporated if the supply air is still too cold after throttling the bypass damper to the closed position.

Run this same test in heating once cooling operation is confirmed.

The exact amount of bypass air can be easily calculated with three temperature readings. 1) The temperature of the bypass air (the same as the supply air minus any heat gains/losses in the ductwork) 2) The temperature of the return air from the home 3) The mixed air temperature

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Make two calculations: 1) The temperature difference between the return air and bypass air 2) The temperature difference between the return air and mixed air

An example in cooling operation: The return air is 78 degrees The bypass air is 52 degrees The mixed air reading is 70 degrees In referencing the chart below: The delta T between the bypass and return is 26 degrees The delta T between the mixed air and return is 8 degrees The amount of bypass air is 31%

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Bypass in the Cooling Mode

Introducing supply air into the return generates low load conditions. This will reduce system capacity, efficiency and possibly reliability. Equipment variables (sensible capacity & sensible heat ratio), environmental variables (indoor dry bulb, indoor wet bulb, return duct heat gains, outdoor temperature & the amount of bypass) will all impact system performance and the supply air temperatures. All calculations for this example are performed at 75DB / 63WB indoor and 95 outdoor. ACCA Manual ZR covers calculations such as this in detail.

The best case scenario for the house noted in this document is when the living zone is calling and the bedroom damper is closed. The system is delivering 64% of air into the living area and 36% is diverted through the bypass duct.

The chart below shows the temperature readings (and temp splits) through four progressions of bypass air.

The temperature split across the coil goes from 19.9 degrees to 25.3 degrees and the supply air drops from 55.1 down to 49.7 degrees. This is within the minimum supply temperature range and a bypass of 36% air flow is perfectly acceptable in this cooling situation.

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The flip side of this situation is when the bedroom zones are calling and the damper for the living area closes. 64% of excess air is diverted through the bypass and back into the return ducting.

This chart shows the temperatures with 64% of bypass air. The temperature split goes from 19.9 to 32.4 degrees, and the final supply air temperature goes from 55.1 down to 42.6 degrees. This amount of bypass is causing a very low load condition and the air entering the furnace has dropped to 56 degrees (from the 75 degrees inside the home). The expansion valve will have a difficult time managing refrigerant flow and the compressor is at risk of refrigerant flooding. The zone control will shut the system down on the discharge temperature sensor to prevent damage to the equipment. This amount of bypass is not acceptable.

Unfortunately there are no good rules of thumb about how much air can be bypassed. ACCA Manual Zr notes that the maximum allowable bypass may be between 0 and 90% all depending on the HVAC & duct system configuration.

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System Performance

System performance degrades when air flow is reduced or bypassed. The chart below shows the system performance when the living room dampers closes. At 64% bypass: --Total system capacity is reduced by 22% --Sensibly system capacity is reduced by 36%

--Latent system capacity is increased by 20% --EER drops by 20% Based on these figures, you must ask: Will zoning offer a cost benefit to the customer?

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Bypass in the Heating Mode

We saw that system capacity drops with air flow in the cooling mode. But BTU delivery rate of a furnace stays relatively constant, regardless of what happens to indoor air flow.

It is common for an air conditioner to function okay with a reduction in air flow, but the furnace will trip on temperature limits (this is why our zone system allows air flow reduction in compressor only operation).

The *UD1B080A948 furnace delivers 64,000 output BTU’s with a 46 degree temp rise at 1275 CFM. The temperature rise on this furnace is rated between 30 and 60 degrees. An indoor temperature of 72 degrees will generate a supply temp of 118 degrees. Perfect!

The best case scenario for this home is when the living room is calling for heating and the bedroom damper closes (43% bypass). The fist pass of recirculating air will create a temperature rise of 68 degrees with a supply temperature of 140. This temperature rise will increase to over 80 degrees by the fourth pass with a supply temperature of 154 degrees. The furnace will continually cycle on the discharge air temperature sensor. While bypass was perfectly acceptable in the cooling mode, it is not sufficient to manage excess air in the heating mode. Additional strategies must be incorporated.

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Challenges with Bypass

Bypass is generally controlled by duct static pressure (either electronically or by a counter weight). The bypass duct will open as duct static increases. Some of the challenges with bypass are:

• The amount of bypass will vary from one installation to another. There is no set amount or generic calculation to determine the maximum allowable amount of bypass air.

• The amount of bypass will vary between heating & cooling modes. The bypass damper cannot know the mode the system is in, so bypass must always be set for worst case conditions (heating mode in this example). Limiting the amount of bypass for heating also minimizes the amount of bypass that can be applied while in air conditioning mode.

For this home, the furnace reaches the 130 degree temperature rise with only 18% of bypass (225 CFM). Remember, the AC system can handle 36% bypass in cooling without issue. But since the furnace can only manage 18% of bypass air; the maximum bypass limit must be 18%. The worst case scenario will always determine the maximum bypass air flow design.

The system designer has multiple strategies to manage excess air. Each strategy has its own advantages & disadvantages. The designer must choose which strategy or strategies will work best for each system application.

Poor air management strategies will generate homeowner complaints and premature system failures.

18% bypass

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Chapter 10 Design Considerations Specific to Zoning

Each zone must have a dedicated return to the indoor unit. Air that transfers across zones reduces the ability to maintain temperature control. In the example pictured below, there should be a minimum of one return in the bedroom area and one return in the living area. Applying multiple returns per zone may assist with the return air flow path and improve comfort.

The sensor must be placed in the air stream for each zone. Some homes may have zoned areas that are relatively open and air may mix between zones. Sensors should be placed as close to the dedicated zone air paths as possible. Sensors placed outside of the zone air pattern may sense temperatures of other zones—generating homeowner complaints.

Register selection becomes increasingly difficult as velocity rates change. The air velocity into a zone will be reduced as the damper for that zone starts to close. The supply register that encompassed a room with air at a velocity of 600 feet per minute may now have dead air spots when velocity is reduced to 300 feet per minute. The homeowner may feel hot or cool spots as well as stagnant areas within the room.

On the flip side, the velocity into a zone will be high when this is the only zone calling. The register that worked well with 600 feet per minute may now generate drafts and noise at 900 feet per minute.

The amount and velocity of air that leaves a register will continually change with zoning. The system designer must choose a register and placement pattern that will deliver blanketing air flow without drafts in a wide range of velocity rates.

Velocity rates and throw will vary depending on register manufacturer and terminal selection. Reference the manufactures performance data to determine if the register selection and location will maintain comfort with varying velocity rates.

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Lower static pressure and velocity rates will deliver

short air throw patterns. At 300 feet per minute velocity, this register will deliver:

125 CFM with 5 feet of throw

Static pressure and velocity rates will increase as other zone dampers close. At 900 feet per minute velocity,

this same register will deliver: 380 CFM with 15 feet of throw

Register selection & placement becomes more critical with zoning. Evaluate each room and determine register throw based on different velocity rates. The register must deliver enough throw during low velocity conditions, and not generate drafts and noise during higher velocity conditions. Using multiple smaller registers will often deliver better air flow performance than one large register. Ask the supplier for air flow performance tables.

Register performance table captured from the Hart & Cooley web site

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Chapter 11 Pitfalls and Misconceptions of Zoning

Zoning is most often sold with the promise of one or more of the following benefits:

1) You can save money by turning off zones that are not in use 2) You can keep any room at any temperature 3) You can keep the temperatures in your home perfectly balanced

Misconception #1: You can save money by turning off zones when not in use. An efficient home is one that is well insulated and evenly pressurized. Zoning may disrupt this pressurization. Let’s say that the homeowner turns the living area off and turns the cooling temperature down in the bedroom. The supply and return duct runs were sized to move the CFM noted in the table below. In chapter 9, we calculated that with over blow the system could deliver an additional 592 CFM into the bedroom zone.

• The bedroom now has a supply duct moving 1200 CFM and a return duct designed to move 592 CFM. This room is likely to go positive in pressure.

• The supply damper to the living room zone is closed, but it still has a return duct that is designed to move 1049 CFM. This room is likely to go negative in pressure.

• The bedroom zone with positive pressure will have increased infiltration as the air blows through exhaust fans as well as cracks around windows and seams. This room “blows”!

• The living room zone with negative pressure will have increased infiltration as air is being sucked in from the attic & outside. This room “sucks”!

The efficiency of the air conditioning system is also diminished with the reduction in air flow. We noted that either bypass or air flow reduction will generate a low load on across the evaporative coil. This system lost 20% of its EER when the living room damper closed.

This home running in this condition is inefficient due to pressure imbalances throughout the home. The system running in this condition is inefficient due to the low load conditions. The homeowner will not see the energy savings that were promised.

Applying multi stage system and having floor plans that are relatively “open” will minimize these inefficiencies.

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Misconception #2: You can keep any room at any temperature.

The outer shell of most homes is well insulated and air leaks are minimized. There is insulation through the attics, crawl spaces and exterior walls. The goal is simple: Minimize unintentional infiltration & thermal heat transfer. The care that is taken to seal the outer envelope is rarely taken between interior rooms.

Let’s say the home pictured above is designed with three zones—the dining room has been separated from the living room area. As per good zone design instructions, it has a dedicated return and supply duct. The homeowner wants to keep the living space at 76, but the dining room that is rarely used is set at 84. Due to the lack of insulation and open infiltration (even with the door closed), these rooms may never see the desired temperature deviation. Granted, they will likely have some temperature difference, but it is impossible to guarantee the offset. The homeowner may complain that the dining room is still cooling and never goes above 81 degrees. Remember, this room has a return and will remain as a negative pressure zone anytime the HVAC system is running. It will “suck in” cooler air from the living room zone. Zoning may allow for some desired temperature deviation between zones, but quoting exact temperature ranges is impossible.

Or even worse, let’s say it’s the 4th of July weekend and the dining room has reached 84 degrees and the rest of the house is satisfied at 76 degrees. The homeowner now enters the dining room (which has an 8 inch duct from this 4-ton system) and cranks the thermostat down to 74 degrees for a dinner party.

How would a system handle the 1250 CFM of excess air? How would a system designer attempt to manage this excess air? Temperature deviations through a home are difficult if not impossible to manage. It may create an inefficient home & inefficient HVAC system, and may lead to premature system failures.

Misconception #3: Temperatures throughout the home will be perfectly balanced.

We noted early on that the thermostat controls the HVAC system, and the problem with single zone systems is the thermostat does not always reside in the same location as the homeowners. A thermostat located in the hallway may keep the hallway comfortable, but the living room and bedrooms may be hot or cold.

Using remote temperature averaging can help. The thermostat still cannot know the temperature of the individual zones, but it will control the HVAC system based on an average temperature between all zone sensors. The HVAC system has no ability to modulate air flow or capacity to individual zones that are warmer or cooler than desired. Zoning is the only way to modulate capacity between zones to assist with balanced temperatures.

We noted in chapter 6 that dividing this home into two zones would help to maintain even temperatures throughout the day, but not all rooms within each zone have the same load patterns. Bedroom 2 with its south & west exposure has a higher excursion than bedroom 1 & 3. The heat gain patters through these rooms will differ throughout the day; therefore the temperatures within these individual rooms will also vary. The zone for the bedrooms has one sensor and it cannot see the temperature fluctuations between the individual rooms. Temperature averaging may assist with keeping all rooms closer to the desired set point, but this cannot guarantee balanced comfort. The application of zoning is an improvement over a single zone system to maintain even temperatures. But as an improvement, it cannot guarantee a perfectly balanced house.

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Chapter 12 Summary

Zoning is an exceptional strategy to improve comfort conditions. Zoning can be achieved either through the application of multiple systems, or by manipulating air flow through the use of dampers on a single system. This application manual focuses on air flow damper control.

Zoning is advanced HVAC system design. Randomly applying a zoning system to a conventional HVAC design may not improve comfort and can generate premature system failures.

Always involve the homeowner when configuring zones. Write down the desires of the homeowner alongside the zoning layout. Evaluate all possible conditions in chapter 6 to determine if the zone design can provide the comfort the homeowner desires.

Pay attention to the equipment selection and ensure it is sized as close to the heat load calculation as possible. Oversized equipment (this includes multi stage equipment) will increase the amount of excess air that must be managed. Excess air can lead to homeowner discomfort and stress to the HVAC system.

Design the ductwork on Peak Load conditions. This allows the system to deliver full air flow and capacity when required. Dampers will modulate down to reduce air flow and capacity during times of lower load conditions. This “oversized” ducting will also increase the amount of over blow for each zone—in effect reducing the amount of excess air that must be managed.

Evaluate the velocity rates with all dampers open, then the velocity rates with each zone calling by itself. Supply registers must be strategically sized and placed to ensure each room will remain quiet and comfortable throughout a wide range of velocity rates.

Calculate the amount of excess air that must be managed during worst case conditions. Determine what method(s) will be employed to manage the excess air and make modifications as needed:

• Multi stage equipment may be required • The amount of zones may need to be reduced (combining some zones together)

Ensure the homeowner understands the importance of managing excess air and what strategies the designer must incorporate. Work with the homeowner in combining zones if necessary.

Never design a zone system that stresses the HVAC system. A homeowner will not be comfortable or pleased with premature system failures. A properly designed zone system will never allow the HVAC system to trip on thermal limits or allow liquid refrigerant to flood the compressor.

Zoning is advanced system design. Applying zoning on a conventional system to solve comfort issues may generate disastrous results. Please review this publication alongside ACCA Manual Zr before installing zoning systems

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References: ACCA Manual J 8th Edition; Version 2 ACCA Manual D 3rd Edition; Version 1 ACCA Manual Zr 1st Edition; Version 1 ACCA Manual RS ACCA Manual S ACCA Manual T Hart & Cooley Terminal Velocity Charts