Heat Load Calculation

ME 425 – Heating Load Calculation

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Heating Load Calculation

ME 425

Air Conditioning System Design

Keith E. Elder, P.E.

Heating Load Calculation

The heating load calculation begins with the determination of heat loss through a variety of building envelope components and situations.

� Walls Roofs

� Windows Doors

� Exposed Floors Slab-on-Grade Floors

� Basement Walls Basement Floors

� Infiltration Ventilation

� Duct Loss Building Pickup


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The Heat Loss Equation

q = Uo x A ( Ti - To )

Where:

Uo =the overall heat transfer coefficient, Btu/hr/ft2/°F

A = the surface area, ft2

Ti = the indoor design temperature, °F

To = the outdoor design temperature, °F

Heat Loss Equation Assumptions

Steady State Conditions

� No Thermal Storage

� Heat Transfer is Instantaneous

One-Dimensional Parallel "Heat Flow"

� Thermally Homogeneous Construction

� Construction Anomalies can be Accounted for by Transmission “Weighting” Corrections

� Heat flow path “ignores” temperature variations due to construction anomalies


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Solving the Heat Loss Equation

Area Takeoffs (from plans with est. zoning)

Design Temperatures (‘05F28 CD & Std. 55)

Assembly U-factors, Uo:

Uo = 1/ RTotal

RTotal = R1 + R2 + Rn + ...

� R1, R2 and Rn represent the thermal resistance of each of the elements in the path of the "heat flow.”

� Properties of typical construction materials can be found in Table 3 of ‘05F25.

Construction Material Properties


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Air Film PropertiesFrom ’05F25.2

Air Gap PropertiesFrom ’05F25.4


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Wall with no Framing Elements

What is the U-factor of a 3-1/2 inch wall composed of 4 inch brick, R-11 insulation and 5/8 inch gypboard?

Ri

Outside Air Film

4" Brick

R-11 Batt Insulation

5/8" Gypboard

Inside Air Film

Total R =

Uo = 1/R

= 1/12.81

= 0.078 Btu

0.17

0.40

11.00

0.56

0.68

12.81

Hr-SF-deg F

Correcting for Cavity Depth

The R-value of batt insulation is rated by manufacturers for predetermined depths. Corrections must be made to calculations for installation in non-standard cavities.

TABLE 20-A

R-Value of Fiberglass Batts Compressed within Various Depth Cavities

Insulation R-Value at Standard Thickness

R-Value 38 30 22 21 19 15 13 11 8 5 3

Standard Thickness 12" 9-½ " 6-¾ " 5-½ " 6-¼ " 3-½ " 3-5/8" 3-½ " 2-½ " 1-½ " ¾ "

Nominal

Lumber

Sizes,

Inches

Actual

Depth of

Cavity,

Inches

Insulation R-Values when Installed in a Confined Cavity

2 x 12 11-1/4 37 -- -- -- -- -- -- -- -- -- --

2 x 10 9-1/4 32 30 -- -- -- -- -- -- -- -- --

2 x 8 7-1/4 27 26 -- -- -- -- -- -- -- -- --

2 x 6 5-1/2 -- 21 20 21 18 -- -- -- -- -- --

2 x 4 3-1/2 -- -- 14 -- 13 15 13 11 -- -- --

2 x 3 2-1/2 -- -- -- -- -- -- 9.8 -- -- -- --

2 x 2 1-1/2" -- -- -- -- -- -- 6.3 6.0 5.7 5.0 --

2 x 1 3/4" -- -- -- -- -- -- -- -- -- 3.2 3.0


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Correcting for Wood Framing

What is the U-factor of the previous example if the wall is framed with Douglas Fir studs spaced 16" on-center.

Through

Insulation 16" OC Stud

Outside Air Film

4" Brick


3-1/2" Fir Stud

5/8" Gypboard

Inside Air Film 0.68 0.68

Total R =

Uo =

Uavg = 0.20 x 0.188 + 0.80 x 0.078

= 0.100 Btu/(hr-S.F.-F)

Through

0.56 0.56

3.50

11.0

0.40 0.400.17 0.17

12.81 5.31

0.078 0.188

What About Metal Studs?

Transmission “weighting”

is not appropriate if the

conductivity of the

penetration exceeds that

of the layer being

penetrated by more than

an order of magnitude.

Isotherms


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Correcting for Metal Studs

ASHRAE Standard 90.1 Correction Factors

Size of Insulation Correction

Members Framing R-Value Factor, Fc

2 x 4 16 in. O.C. R-11 0.50

2 x 4 24 in. O.C. R-11 0.60

2 x 6 16 in. O.C. R-19 0.40

2 x 6 24 in. O.C R-19 0.45

Using Metal Stud Factors

What is the heat loss of the wall in the first example if the wall is framed with 3-1/2" metal studs spaced 16" on-center?

Ri

Outside Air Film

4" Brick

R-11 + Mtl. Stud (11.0 x 0.50) =

5/8" Gypboard

Inside Air Film 0.68

Total R =

Uo = 1/7.31 =

0.56

5.50

0.40 0.17

7.31

0.137 Btu

Hr-SF-deg F


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ASHRAE Window Performance

Because of conductive spacers, frames & new coatings available, window U-factors cannot be calculated accurately.

ASHRAE Window Performance

Reasonable estimates can be found in Table 4, ‘05F31.8.


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ASHRAE Window U-factors

ASHRAE Window U-factors


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Door U-Factors

Door U-factors can be found in Table 4, ‘05F31.11.

Floors on Grade

qslab

= F2 x Perimeter x ∆∆∆∆T


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Floors on Grade - F2 Factors

Infiltration

The uncontrolled inward air leakage through cracks and interstices in any building element and around windows and doors of a building, caused by the pressure effects of wind and/or the effect of differences in the outdoor/indoor air density.

qinfiltration = 0.018Q∆T

Q = air flow due to infiltration, ft3/hr


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Infiltration Estimates

From the 1972 Fundamentals Manual:

Number of Air

Changes Taking

Kind of Room or Building Place per Hour

Rooms with no windows or exterior doors 1/2

Rooms with windows/exterior doors on one side 1

Rooms with windows/exterior doors on two sides 1-1/2

Rooms with windows/exterior doors on three sides 2

Entrance Halls 2

Note: Values may be reduced by 1/3 if windows & doorsweatherstripped

In-Class Heating Load Exercise

What is the heat loss for a Seattle office space 12 feet deep by 50 feet long, with 10 foot ceilings and floor-to-floor spacing of 12 feet? The office zone is located on the third floor of a five story building. The wall is constructed of 6 inch concrete, with R-11batt insulation installed between metal studs 16 inch on-center, and 5/8 inch gypsum board. The window is insulated 1/4 inch glass with 1/2 inch air space and composes 40 percent of the gross wall area. The window framing is metal and has no thermal break.


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Building & Space Sketch

50’

12’

10’

12’

Section Sketch

Outside Air Film

6 Inch Concrete


between mtl studs 16” O.C.

5/8 Inch Sheet Rock

Inside Air Film

Total R =

Uwall =

Business

Heat Load Calculation