Sequence of operation gas furnace

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Are the limits and roll out switches closed

furnace sets code for roll our switch or limit open. Furnace locks out until switch closes or if furnace locks out on set amount of tries during 1 heat cycle

Furnace control board starts inducer

are the pressure switches closed

after a set amount of time the furnace locks out on pressure switch fault. usually the inducer will continue to run for either set amount of time or will continue to run until switch closes.

ignition board closes relay in board to energize the ignitor.

after a set amount of time the ignition board closes a relay in the board and sends 24v to the gas valve.

the ignition board send usually between 90v -110v through the flame sensor and looks for a set amount of micro amps through the flame sensor. usually 3-5 U.A.

did the board sense the required micro amps?

after a set amount of time the ignition control board locks out on flame failure

furnace continues to run until thermostat is satisfied

Sequence of operation and likely causes of faults.

W call from thermostat

is the pressure switch closed before inducer is on. (Some furnaces check t see if the pressure switch is closed and locks out on pressure switch closed before inducer call but some do not monitor this.)

no

yes

if pressure switch is closed need to replace pressure switch if inducer is not not running. If inducer is running need to check for reason inducer running before heat call.