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Gas Furnace Sequence of Operation is Important to Know!

This excellent Youtube Video made by AC Service Tech LLC that explains why it is important to know the Gas furnace sequence of operation when troubleshooting furnaces.  If one sequence of operation does not occur then as a troubleshooter you can focus on the sequence that is missing and fix the problem.  Thanks so very much to Craig Migliaccio for taking the time to make this excellent video! I have the sequence of operation below. The YouTube video illustrates the sequence of operation on a Bryant Carrier 92% condensing furnace.  The proper sequence of operation is:

  1. The thermostat calls for heat by the home owner (you) turning the thermostat up.
  2. The thermostat provides 24 volts to the W terminal on the control board. When you check with a voltmeter you should have 24 volts AC (alternating current) between W and C (com) on the control board.
  3. The control board sends 110-125 volts AC to the draft inducer to start the draft inducer.
  4. If the vent and combustion chamber are open then the pressure switch closes the connection between the two wires that go into the pressure switch. If the furnace pressure switch, limit and rollout switches are all closed (allowing current to run through them) then the control board starts the furnace ignition sequence.
  5. The control board sends power to the furnace ignitor which should glow bright orange for a set number of seconds. Usually 15 to 30 seconds. During the set time that the ignitor is glowing the control board sends 24 to 28 volts AC to the gas valve to open the gas valve allowing gas to flow and the gas burners to ignite (start burning). If the gas burners do not ignite in 5 to 8 seconds the flame sensor does not sense the flame and the control board turns the gas valve off. FYI-If this happens you might need to clean your flame sensor or replace your flame sensor. Please click here if you are interested in the flame sensors we sell. If the flame sensor does sense the flame the control board sends 24 volts to the gas valve and the gas burners continue to burn until the thermostat is satisfied.
  6. After the furnace gas burners ignite the furnace, the furnace’s heat exchanger heats up for approximately a minute or so then the furnace blower comes on to distribute the heat throughout your home.
  7. The furnace blower motor and gas continue to burn until the thermostat on your wall satisfies the need for heat (reaches the set temperature) then the thermostat breaks the voltage in the W (white) wire and cuts the gas burners off.
  8. The furnace blower motor continues to run for a set amount of time that is determined by the furnace’s control board until the heat is removed from the furnace’s heat exchanger. Then the blower motor goes off until the next heating cycle from the thermostat on the wall is needed.  If you have any questions please contact us anytime. Our email address is: support@arnoldservice.com
  9. We Would Love to Help You Out and Earn Your Business! Steve & Barbara Arnold

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