SF-35F Blowing Air but no heat

The most common problem on the SF series is a bad Hi limit switch. If the Hi limit is stuck open, then the control board will not start the Ignition cycle. Then it shuts off. The Hi limit is the first thing I check and the easiest to access on the inside of the RV. Doug

Moved from Forum Technical Support

There is a lot of information if you google search SF35 furnace repair. Have a look there, and tell us tomorrow what you found.

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

bleed the propane line by starting the stove, turn tank on, just a though.