Old Duo Therm Furnace...HELP


Senior Member
Gary, I’m picking up where I left off from the Fall. ;)

The thermostat kicked on the blower but the pilot wouldn’t light.
Removed, inspected and replaced pilot assembly but now blower doesn’t work. Cleaned combustion chamber also.
Bought a few $2.15 screwy looking fuses, replaced fuse, now both work.
Blower on…Pilot on… but main burner will not ignite???

After reading the manual for over an hour and remembering how the blower sounded I either have:
Junk in blower or air combustion chamber causing lack of airflow to open switch and allow burner to fire.
Airflow switch is dirty causing poor opening and closing.
Airflow switch is bad.

Are the both posts, that receive the thermostat wiring, on the control valve energized when trying to ignite the main burner? My friend seemed to think they should be and that a relay was bad but from what I read, I think I ruled out both relays.

If anyone has any thoughts… fire away. Talk about on the job training.

sp :cool:
Old Duo Therm Furnace...HELP


Can you get find the model number?
If it's 659 series, it's a great furnace and worth fixing. If it's the last model Duo-therm produced (can't think of the model series right now)it is a touchy furnace and very expensive to get parts for. I always recommend that my customers replace them.

Did the pilot assy. have a lot of asbestos looking stuff around it?
If so, it's the expensive one.

Wild Willy

If it ain't broke...
It will be!


Senior Member
Old Duo Therm Furnace...HELP


It is a 65930-001. I went to the local RV shop and the tech said the motor was probably shot, that made my afternoon. I went home and sawzalled the exhaust pipes off it, they were fused together by rust and wrestled it out. :bleh: The exhaust pipe attached to the blower was rotted off and there was junk and a huge hornets nest in with the combustion fan. I cleaned everything and lit it up. The bench test proved that everything was now fine. The sail switch moved and allowed power to the valve and heard the valve click open. You could actually feel the air moving at least 10 feet from the thing. So, I'll put it back together and put it back in the camper to see what happens.

Thanks for checking in.
sp :cool:
Old Duo Therm Furnace...HELP

If anyone has the Duo-them 900 of 901 series, you better just trash them as there was a recall quite a few years ago on them. They should not be used as their is a problem in the combustion chamber.