Table of Contents
Gas Furnace Components 2 - Gas Burners - 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 where an induced draft exists. For sealed combustion chambers a forced or induced draft will exist.
There are burners with precisely drilled holes in them. Then there are slotted gas burners that extend into the combustion chamber and generally have induced draft combustion chambers. However, 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 the altitude and capacity of the furnace.
Altitudes over 1800 feet will need burners with smaller holes. That is because there is less air at higher altitudes. Therefore 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. Hot combustion gases are forced into a heat exchanger. Power burners are not typical in residential applications but used in commercial applications.
Gas Furnace Components 2 - Heat Exchangers
The gas furnace heat exchangers are constructed of heavy gauge metal with special alloys to resist the 2000° F+ temperatures. These high temperatures occur in the combustion chamber. The heat exchanger must receive an annual inspection to ensure it has complete integrity. Cracks in heat exchangers do develop and these cracks can leak carbon monoxide gasses into the air stream which provides conditioned air to the spaces.
It is also imperative that any dwelling with an appliance that burns fossil fuels and solid fuels have a carbon monoxide detector installed in the zones which are served by these appliances. Furnace heat exchangers are safe. However, accidents do happen, so it is a good idea to be safe and have carbon monoxide detectors. In some states, it is the law to have detectors installed near sleeping areas.
Gas Furnace Components 2: Venting and Ventilation
Single wall flue vent used in an atmospheric boiler.
Gas Furnaces need to be properly ventilated for proper combustion. They need fresh air for the combustion process, including combustion air. They need to ventilate the combustion gasses after the combustion process properly is complete.
Depending on the type of gas furnace and where it is installed will depend on the type of ventilation. The ventilation used for both combustion air and for venting the combustion gasses. High-efficiency furnaces have vents for both the fresh combustion air and for venting the combustion gasses.
These vents are usually constructed of PVC or of stainless steel. PVC is the preferred material used for this because it is cheaper. It is also 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.
This way, 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.
Codes and Regulations
The Mechanical Code and the Fuel Gas Code cover 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 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.
Gas Furnace Components 2 - Ventilation of Combustion Gases
The ventilation of the combustion gasses is another important area. If the furnace is rated at less than 90% efficiency, then the ventilation of the combustion gasses will usually be done with the standard vent pipe. It could be the 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 gasses is forced by a forced draft or induced draft blower. The combustion gasses get a little help this way flushed into the outside air. There is the table of venting materials and the temperature ratings for those materials.
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 proper support and structural integrity. This requirement prevents the vent pipe from damage or falling.
Depending on the codebook or local municipality will depend on the requirements for running a vent pipe. Also, for the termination point of the vent flue. However, it is common sense that these vents do not run near combustible materials. Additionally, they do not terminate near any windows or fresh air vents. Any opening that leads to inside spaces.
Gas Furnace Components 2 - Forced or Induced Draft Motors
Part of the combustion gas ventilation system on many gas furnaces is a forced draft or an induced draft blower. That includes a pressure switch. Very few have centrifugal switches, but most use pressure switches. The forced draft or induced draft blower on modern gas furnaces pulls combustion air into the combustion chamber. That is for proper combustion.
It forces the combustion gases up the flue and out to the atmosphere. Problems occurring with the earlier 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. The furnace needs a proper draft that is forced or induced in the combustion chamber and for combustion air. The forced draft or induced draft blowers require little maintenance. However, there are some which require oil for the bearings regularly. 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.
Also, see Gas Furnace Components 1
and Gas Furnace Components 3 to complete this series.
Video showing a gas furnace starting up.
Gas Furnace Components 2
