SF-35F Blowing Air but no heat
4-22-2014 I have a SF-35F furnace in my RV trailer. Whenever I turn the thermostat to heat, I get airflow, but no heat. It will blow air for about 4 minutes and then shut off. I can repeat it by turn...
Forum Discussion
Furnace cycle:
T-stat contacts make up calling for heat
Fan comes on to purge combustion chamber
Air flow causes 'sail switch' to make up allowing power thru 'high limit switch' on to control board
Control board sends high voltage via high tension wire to spark electrode
Control board sends power to gas vlave opening it
Spark electrode lights main flame
Main flame generates a milivolt signal that goes back to board (via spark electrode/wire) proving main flame lit.
Control board keeps power on gas valve until t-stat contacts open
Sail switch has a paddle that sticks into path of air flow.
IF air flow is low (motor not up to speed---low 12V DC voltage) sail switch won't make up
IF sail switch is dirty....it won't make up
NOW IF you hear the 'click, click, click' of spark electrode then sail switch is not the problem.
Then it could be bad electrode position,dirty burner, bad gas valve, bad control board.
First place to start...sail switch. Should have 12V DC to/thru it when fan is running
T-stat contacts make up calling for heat
Fan comes on to purge combustion chamber
Air flow causes 'sail switch' to make up allowing power thru 'high limit switch' on to control board
Control board sends high voltage via high tension wire to spark electrode
Control board sends power to gas vlave opening it
Spark electrode lights main flame
Main flame generates a milivolt signal that goes back to board (via spark electrode/wire) proving main flame lit.
Control board keeps power on gas valve until t-stat contacts open
Sail switch has a paddle that sticks into path of air flow.
IF air flow is low (motor not up to speed---low 12V DC voltage) sail switch won't make up
IF sail switch is dirty....it won't make up
NOW IF you hear the 'click, click, click' of spark electrode then sail switch is not the problem.
Then it could be bad electrode position,dirty burner, bad gas valve, bad control board.
First place to start...sail switch. Should have 12V DC to/thru it when fan is running
