High Performance HVAC Gas Furnace Components Page 2

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Gas Furnace Components: Venting and Ventilation

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Gas Furnace Components Page 2

Gas Furnace Components: Gas Manifold

High Efficiency Trane Gas Furnace

The gas furnace manifold connects the gas valve with the main burners and it is important that the proper pressure be present inside the gas manifold for proper combustion. Manifold pressure is different from natural gas to liquid propane and this pressure needs to be set with a special instrument which the HVAC installer or HVAC start-up technician should have. The gas manifold pressure should be set according to the gas furnace manufacturer’s instructions and is done by an adjustment on the gas valve as noted in the gas valve section. The main burners are connected to the gas manifold usually by little brass fitting generally referred to as spuds. These spuds have precisely drilled holes in them and the size of these holes will depend on the type of fuel, propane or natural gas, and the amount of gas the engineer wanted to send into the gas burners for the right size flame for the exchanger. Most gas furnaces come equipped with spuds for natural gas but liquid propane spuds are available. It is very important that the gas manifold pressure be set according to gas furnace manufacturer’s instructions otherwise an improper flame may be present in the combustion chamber. Little maintenance is required of the gas manifold but if the unit is exposed to the elements as some gas packs are it is possible for tiny insects to get into the gas panel these insects can impede the gas flow from the spuds and the gas furnace may not operate or may not operate properly. If an insect problem exists contact an exterminator. Never modify the gas furnace cabinet or block any vents which allow ventilation for the gas furnace. This could result in improper and dangerous combustion inside the gas furnace heat exchanger.

Gas Furnace Components: Gas Burners

Combustion Chamber with Manifold (Black Pipe w/bends) Connecting to the Gas Valve

Depending on the type of gas furnace and how the gas furnace was engineered will depend on the type of gas burners. Inshot burners generally shoot a flame into the combustion chamber were an induced draft exists or for sealed combustion chambers a forced drafts will exist. There are burners with precisely holes in them and then there are slotted gas burners which extend into the combustion chamber and generally have induced draft combustion chambers but these can also have forced draft combustion chambers if the combustion chamber is sealed. The size of these holes and slots in gas burners will also depend on altitude. Generally, altitudes over 1800 feet will need burners with smaller holes. This is because there is less air at higher altitudes so less fuel must be fed into the combustion process for proper combustion. It is important for proper combustion and the longevity of the gas burners that these gas burners receive regular maintenance to keep them clean and free of rust.

Power burners use a blower to blow a mixture of gas and air into a combustion chamber for combustion and the hot combustion gases are forced into a heat exchanger. Power burners are not common in residential applications but used in commercial applications.

Gas Furnace Components: Heat Exchangers

The gas furnace heat exchangers are constructed of heavy gage metal with special alloys to resist the 2000° F+ temperatures which can occur in the combustion chamber. It is very important that the heat exchanger receive an annual inspection to ensure it has complete integrity. Cracks in heat exchangers do develop and these cracks can leak carbon monoxide gases into the air stream which provides conditioned air to the spaces. It is also imperative that any dwelling with an appliance which burns fossil fuels and solid fuels have a carbon monoxide detector installed in the zones which are served by these appliances. Gas furnace heat exchangers are safe but accidents do happen so it is a good idea to be safe and have carbon monoxide detectors.

Gas Furnace Components: Venting and Ventilation

Induced Draft Blower On Trane Gas Furnace

Gas furnaces need to be properly ventilated for proper combustion. They need fresh air for the combustion process, combustion air, and they need to properly ventilate the combustion gases after the combustion process is complete. Depending on the type of gas furnace and where it is installed will depend on the type of ventilation used for both combustion air and for venting the combustion gases. High efficiency furnaces have vents for both the fresh combustion air and for venting the combustion gases. These vents are usually constructed of PVC or of stainless steel. PVC is the preferred material used for this because it is cheaper and considered by some to be more durable than stainless steel pipes. Stainless looks better and the durability between the two different materials of vent piping can be debated. This method of providing combustion air from the exterior of the dwelling served by the gas furnace is the best way of providing combustion air as combustion air is not dependent on the air inside the structure for adequate combustion purposes. If the structure has a tight seal and allows little infiltration air a gas furnace (or any other type of appliance which consumes air including exhaust fans) can use up much of the air in the structure or dwelling when it is running for prolonged periods. The Mechanical Code covers this and the HVAC Contractor who installs the gas furnace will be fully aware of this issue and take measures to ensure an adequate amount of combustion air exists for the gas furnace to operate properly. It is also important that any type of venting system, whether it be intake or exhaust, that these vents have screens over them to prevent birds and other critters from getting inside these pipes. They will cause mechanical problems if they get inside and get stuck.

The ventilation of the combustion gases is another important area. If the furnace is rated at less than 90% efficient then the ventilation of the combustion gases will usually be done with standard double wall B-vent or single wall ventilation pipe depending on its proximity to combustible materials. On most modern furnaces the venting of combustion gases is forced by a forced draft or induced draft blower. The combustion gases get a little help this way in finding their way out of the flue and into the outside air. There are codes about where these combustion gas vents run and where they terminate to the outside air. All combustion gas ventilation piping must have the proper support and structural integrity to prevent the vent pipe from damage or falling. Depending on the code book or local municipality will depend on the requirements for running vent pipe and for this termination but it is common sense that these vents do not run near combustible materials and do not terminate near any windows or fresh air vents that lead to inside spaces.

Part of the combustion gas ventilation system on a many gas furnaces is a forced draft or an induced draft blower and pressure switch. The forced draft or induced draft blower on modern gas furnaces pulls combustion air into the combustion chamber for proper combustion and it forces the combustion gases up the flue and out to the atmosphere. Problems occurring with older forced draft or induced draft blowers are that metal blower wheels rust and fall apart. When this happens the pressure switch won’t make and complete the safety circuit. The gas furnace will not fire until the blower wheel is replaced so that proper draft can be forced or induced in the combustion chamber and for combustion air. The forced draft or induced draft blowers require little maintenance although there are some which require oil for the bearings on a regular basis. This should be done by the maintenance technician when you have your gas furnace checked on a maintenance schedule. The pressure switch requires no maintenance however if moisture gets into the switch it probably will fail and need to be replaced.

Gas Furnace Components Page 3.

Video showing a York gas furnace with hot surface ignition starting up.