IS it an 'electric heater' OR an AC propane heater'
IF it's an electric heater than it would have just a fan and a heater grid (like a space heater).......fan blows/electric current flow heats grid/warm air comes out. Grid may be bad or an internal fuse blown for grid circuit.
IF it's an AC furnace with propane heat....
It will use 120V AC to motor and 24V AC thru transformer for the rest f furnace circuit.......relay to sail switch to limit switch to control board to ignitor/gas valve.
Blower motor running......then relay has been energized
Next would be sail switch Is it making up?
Then limit switch Is it closed?
Then power to board Pull connector/clean contacts.
Then power to ignitor/gas valve?
Just like a DC furnace except 24V AC control power
Atwood 8900 Series can be DC or AC
