Venting Fundamentals and Air Supply

Topics Covered: • The Natural Draft Gas Appliance

• The Purpose of Venting

• The Combustion Process

• Venting Action of the Natural Draft Appliance

• Venting Problems

The Natural Draft Gas Appliance

• Gas appliances that rely on the buoyancy of warm flue gases for venting, are called natural draft appliances.

• Until the arrival of mid and high efficient appliances, with induced draft firing and power venting most residential appliances were natural draft appliances.

The Natural Draft Gas Appliance

• The purpose of venting a gas appliance is to remove the products of combustion.

• For natural draft venting, the sole energy source is the heat above room temperature in the combustion products that are delivered to the draft hood.

• The difference in density between the hot flue gases and the room air causes the draft making the venting system function.

The Natural Draft Gas Appliance

• If this energy source is insufficient, problems will occur in the venting process.

• Spillage of the flue gases at the appliance draft hood is one common problem, carbon monoxide is commonly associated with this problem.

• Condensation of the water vapour in the flue gas is another major venting problem, corrosion of the heat-exchanger and venting is a sign of this.

The Natural Draft Gas Appliance

• To accurately define the overall purpose of venting a gas appliance is as follows;

• Removal of the products of combustion without spillage taking place at the draft diverter, and without condensation of water vapour occurring within the venting system.

The Natural Draft Gas Appliance

• It takes 10 cubic feet or cubic metres of air for the combustion of 1 cubic foot or cubic metre of natural gas.

• Approximately 11 cubic feet or cubic metres of products of combustion is formed by the combustion of 1 cubic foot of natural gas.

• All of these products must be removed by the gas venting system, of the 11 cubic feet of combustion products 2 cubic feet is water in the vapour form.

The Natural Draft Gas Appliance

• This water vapour in the flue gas is a source of condensation if the flue gas temperature falls below its dew point.

• The temperature at which water vapour would begin to condense out of the flue gas if they should be cooled to that temperature is called the dew point, which is approximately 127°F (53°C)

Carbon Monoxide • One gas that must not be found in the products of

combustion is carbon monoxide CO!

• Carbon Monoxide in the flue gas is a sign of incomplete combustion. CO is highly toxic and can be absorbed into the blood.

• CO combines with the hemoglobin (oxygen transport) in the blood and acts to reduce the oxygen carrying function of the blood suffocating the victim due to lack of oxygen.

Carbon Monoxide • The following areas should be tested for CO;

1. In the atmosphere at head height. 2. Near Gas appliances 3. Close to heating ducts 4. Near appliance draft diverters and fire doors 5. Any combustion appliance can produce CO! • You must install a Carbon Monoxide detector in

any home that has any combustion appliance.

Carbon Monoxide • The sequence of testing for CO should be;

1. In the air before the appliance is started

2. Just after the appliance is turned on

3. After the appliance has operated for about 15

minutes.

• When in doubt of the presence of carbon monoxide, call the local gas company in to test.

Venting Action of the Natural Draft Appliance

• The natural draft gas appliance is fired by a burner that is not equipped with a mechanical device, such as a burner fan for supplying the air for combustion.

• The main component of the natural draft appliance is the atmospheric burner.

• The natural draft burner is designed to fire with a zero or neutral overfire draft condition, in the combustion chamber of the appliance.

Venting Action of the Natural Draft Appliance

• This neutral overfire draft is required in order to maintain a consistent air supply to the burner flame.

• A variable draft condition over the fire would vary the amount of air supplied to the burner flame and hence effect the operating efficiency of the appliance.

• The neutral overfire draft condition is achieved by the application of a fixed draft diverter or hood.

Venting Action of the Natural Draft Appliance

• By drawing in dilution air through its relief opening, the fixed draft diverter breaks the natural draft created by the venting action of the appliance.

• A consistent air supply to the burner flame is assured regardless of variations in the natural draft produced by the appliance venting system.

Venting Action of the Natural Draft Appliance

• As the hot flue gases rise through the heat exchanger of the appliance and out through the appliance vent, the relief opening of the draft diverter allows room air to be induced into the appliance vent.

• This dilution air breaks the venting action developed by the appliance venting system, creating the zero overfire draft condition.

Venting Problems • There are two main problems that can occur in

gas venting systems spillage and condensation.

• Spillage of the flue gases at the appliance draft diverter relief opening. Spillage is caused by either a venting system being overloaded, or having a down-draft opposing the flow of the flue gases.

• A venting system can be overloaded by appliance overfiring, venting system being blocked, venting system too small, poor venting system design.

Venting Problems • A down-draft is caused by a lack of make up air or

by the outside chimney or vent location.

• If an appliance in a tight room does not receive sufficient air for combustion, then a negative pressure condition is created in the room.

• In this scenario more air is being used up by combustion than is being supplied by natural air infiltration through the building.

Venting Problems • The atmospheric pressure outside and the

negative pressure inside the building causes a flow of outside air down through the venting system.

• The appliance flue gases must generate sufficient buoyancy in the venting system to over come the down draft condition.

• If the down-draft is to severe then the flue gases cannot move up through the appliance venting.

Venting Problems • Outside chimney or vent location is another cause

of down-draft. Minimum clearance height above the highest peak is 2’

• Whether the spillage is caused by overloading the appliance venting system or by a down-draft condition, the room will have a flue gas odour.

• Using a match, lighter etc. you can determine easily which way the vent is drafting.

Venting Problems • Condensation is caused by the flue gases being

excessively cooled after their exit from the draft diverter.

• Long lateral runs through unheated areas as well as venting into an outside masonry chimney are two conditions that could cause condensation.

• An underloaded vent is the cause of these problems, the volume of flue gas is insufficient to maintain the flue gas temperature.

Combustion Air • Air required for the combustion process of the

appliance must meet the following three requirements. 1. Air for combustion at 10 ft3 air – 1ft3 gas plus

additional excess air to ensure complete combustion.

2. Air for draft diverter dilution.

3. Air for ventilation.

Combustion Air • To meet these air requirements each cubic foot of

gas may require up to 30 cubic feet of air.

• The amount of air supply required to maintain adequate appliance operation within a building or structure is a function of; 1. Total Btuh input of gas appliances. 2. Appliance design, draft control or none. 3. Tightness of the building envelope.

Draft Controls • Natural draft type burners are designed to

operate with draft hoods, by which the chimney effect is nullified.

• The primary function of the draft hood is to provide a neutral overfire draft condition in the combustion zone.

• The draft diverter’s other function is to relieve a down-draft and prevent the down-draft from entering the appliance.

Draft Controls • There are four type of draft hood designs to meet

various applications.

• Vertical Type, Horizontal Type, Horizontal to Vertical, and Vertical to Horizontal.

• It is primarily the interior design of the unit that determines the type of draft control used.

• Most domestic and small commercial appliances are of a vertical flue design using a vertical hood.

Draft Controls • The installation requirements for the draft

diverter are the following; 1. Must be installed as same room as appliance. 2. Must be installed with reference to the

horizontal or vertical plane. 3. Must be installed so that the relief opening is

accessible for testing and is not blocked. 4. Diverter can not be smaller than the flue collar. 5. Cannot be used with an appliance having a

positive overfire draft or induced draft.

Types of Gas Vents

• Type A vent = Masonry chimney, steel chimney, and factory built chimney.

• Type B vent = Double wall metal vent. • Type BW vent = Oval double wall metal vent. • Type C vent = Single wall metal vent. • Type L = Similar to B vent but with greater heat

capacity and corrosion resistance. • Type BH = Stainless steel, thermoplastic and PVC

plastic for venting high efficiency gas appliances.