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  • Craig Migliacco

What Components are in a Forced Air Gas Furnace? What do they Do?


In order to troubleshoot a gas furnace, the technician must have a thorough knowledge of each component that could be in the unit. Understanding how each component works is crucial to fixing the problem! Let's start by going over each of the major components down below!


Integrated furnace control board:

This board integrates both the ignition control with the blower control all as one board. Depending on the 24v inputs on G, W, Y terminals, it will control the function of the furnace. R is the 24v hot to the thermostat.

When the thermostat calls for heat to turn on, R and W touch in the thermostat and the W terminal on the control board is powered. This starts the sequence of operation for heat and the first thing to turn on is the inducer motor.

When the thermostat calls for fan to turn on, R and G touch in the thermostat and the G terminal on the control board is powered. This turns the blower fan on at a low fan speed. It is usually the same blower speed that the furnace runs at during heat mode.

When the thermostat calls for air conditioning to turn on, R and Y touch in the thermostat and the Y terminal on the control board is powered. This turns the blower fan on at a high fan speed. At the same time, Y powers the outdoor condenser contactor which turns on the compressor and the outdoor fan. Often, thermostats will connect the R to both Y and G to make sure that the fan will turn on, regardless of the control board type in a furnace.


Check out this video on the IFC board!


Transformer: The transformer is made up of two isolated wire coils that are both wrapped around an iron core. The primary coil is powered by 120v. Alternating current is induced into the secondary wire coil so that there is 24v present. 24v is used for the control voltage for the furnace.


Check out this video on a multi-tap transformer!


Fuse: The furnace control voltage (24v) is usually protected by a 3amp or 5amp fuse mounted by spade connectors onto the control board. As soon as the power comes into the board, it must cross through the fuse before it connects to any other terminal. This fuse protects the transformer from overcurrent and direct shorts.


Check out this video on troubleshooting if the thermostat fuse is bad!


Capacitor: If there is a PSC (permanent split capacitor) blower motor and/or a PSC inducer motor, there will be a capacitor attached to the motor. On a blower motor, the capacitor will be a round or oval cylinder but for the inducer motor, it could be oval, round or rectangular. The capacitor causes a 90-degree phase shift in the power supplied to the start winding of the blower motor which allows for initial starting torque as well as an artificial phase of electrical power to smooth out operation. Without the capacitor, installed, the motor will not start and the windings will overheat until the internal thermal limit inside the motor ups up. This will open the electrical circuit on the common wire. Make sure to replace capacitors with the same voltage rating or higher and the exact same MFD/uF rating as the existing one. Capacitors are rated by micro-farads and voltage.


Check out this video on Checking capacitors!


Blower Motor: The blower motor may be a shaded pole, PSC, ECM (Electronically Commutated Motor) multispeed motor, or ECM variable speed. The only one that has a capacitor is the PSC motor and this was the most common motor. On newer furnaces, an ECM multispeed motor is common. High efficiency gas furnaces may have a ECM variable speed blower motor. Shaded pole inducer motors may be found on some present-day furnaces whereas only older furnaces from the 1980’s and earlier used shaded pole blower motors. Shaded pole blower motors are used in low torque settings so they can be found on walk-in box, refrigerator, and freezer applications as the main fan motor.


Check out this video on distinguishing between blower motor types!


Interlock door switch: This switch cuts the main line voltage power off to the furnace if not depressed. Any time the blower door to the furnace is open, the switch is also in the off position. In order for a service technician to keep power on while servicing, a magnet is used to depress the door switch.


Check out this video exploring why power is not making it to the control board!


Inducer motor: This motor pulls the exhaust gas from the combustion chamber through the heat exchanger(s) and pushes it though the exhaust pipe to the outside air. The inducer motor is used to allow the furnace to operate in a multi-poise position and to have a much longer heat exchanger for the transfer of heat to the conditioned air within the building.

Check out this video on Inducer Motor Troubleshooting!


Hot surface ignitor: Hot surface ignitors are usually made from silicon carbide or silicon nitride and heat up when powered. The HSI will heat up so much that it turns red/orange. A gas and air mixture passes across the HSI and ignites. Common HSI’s are powered with 120v but some are made to be powered with 24v.


Check out this video on hot surface ignitors!


Burner tubes: Burner tubes allow the fuel gas to mix with the air to prepare the mixture for combustion. After ignition, the beginning of the flame rests on the front burner retention head. There is a flame channel at the front of each burner tube that connects a pathway for the gas from one burner tube to the next. This ensures that the gas exiting each of the burner tubes within the furnace are ignited.


Primary heat exchanger: This is the first heat exchanger that the exhaust gas enters. While the exhaust gas is passing through this heat exchanger, the temperature of the gas remains high so that no condensation occurs within the heat exchanger. Remember that a byproduct of the combustion of a gas is water. However, if the exhaust gas remains high in temperature, condensation will not occur, and the exhaust will carry the water vapor outside to reject it. Furnaces with only a primary heat exchanger can only attain up to 80% efficient energy conversion from gas to heat.


Secondary heat exchanger: Furnaces that are 90% efficient conversion from gas to heat include a secondary heat exchanger to transfer more heat energy from the exhaust to the conditioned air. While the exhaust gas travels through the secondary heat exchanger, the water vapor within the exhaust gas condenses. The secondary heat exchanger must be made of a material that can handle both heat and moisture. This secondary heat exchanger will often reduce the temperature of the exhaust gas below 100 degrees F before exiting the furnace.


Check out this video on a clogged secondary heat exchanger!


Combustion Chamber: This is the box in which the combustion of the fuel gas takes place. The box allows air to mix with the fuel and permits a mounting location for the flame rollout switch. This is also a protective shield for some of the other closely positioned components within the furnace.

Flame rollout switch: This is a thermal limit switch that is designed and positioned to open up the 24v electrical safety circuit any time a flame pops out of its normal position. The flame is usually pulled forward due to the inducer motor pulling the exhaust gasses. However, in the case of a cracked heat exchanger, the blower motor which is used to push the conditioned air within the building will push air through the crack in the heat exchanger and into the combustion chamber area. This pressurized air will push the flame away from the heat exchanger and toward the flame rollout switch. When this occurs, the flame rollout switch will trip and shut off power to the electrical gas valve. The flame rollout switch will not reset automatically and must be manually reset by a service technician. When a flame rollout switch opens, the technician must verify if there is problem such as a crack in the heat exchanger which would cause a serious safety problem with carbon monoxide inside the building.

Thermal limit safety switch: This switch protects the furnace from overheating in case the blower motor is not turning on or is at too low of a blower speed. The switch will be rated by two temperatures. The high temp is when the switch opens the low voltage electrical circuit and the lower temp is when the switch will automatically close to allow the 24v circuit to be complete. This will allow the furnace to automatically turn back on after it has cooled down.


This video includes the flame rollout switch and various thermal limit switches!


Electrical gas valve: There are several versions of the electrical gas valve. One is a combination gas valve that has the pilot valve powered by a 30mv thermocouple. The main valve is powered by 24v. Another type is a 750mv gas valve with both the pilot and the main valve powered by 750mv. Another version may have both the pilot and the main gas valve powered by 24v. Another is the direct ignition gas valve that only has a main solenoid powered by 24v and no pilot. Another version is a Smart Valve where the ignition control module is found mounted on the top of the valve. Another version is the modulating valve that has a communicating control board built on the valve.


This video is on a direct ignition gas valve!

Pressure switch: The pressure switch makes sure that the inducer motor is working and pulling the correct WC (water column) pressure from the combustion area. If there is a problem with the exhaust gas not escaping or the inducer motor not turning on, the pressure switch will not close the electrical safety circuit. In 90% efficient gas furnaces, the pressure switch will also stay open if the furnace condensate is clogged.


Check out this video on pressure switch problems!


Flame rod: The flame rod is just a stainless steel rod that has a ceramic base to isolate it from the furnace cabinet. This rod is mounted in a position where the flame will envelope the rod. The flame rod is also referred to as the flame sensor but the rod does not sense anything. The control board provides power to the rod and the voltage gets rectified by the flame which completes the circuit from the rod to the ground frame. The direct current that is drawn during the rectification process is measured by the control board via a ground wire attached to the combustion area. The direct current measured is usually between 1 to 10 microamps.


Check out this video on the furnace flame sensor!


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Published: 10/21/2020 Author: Craig Migliaccio

About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 15 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel & https://www.facebook.com/acservicetech/

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