DuoTherm 901 Series Furnace

Monaco Montclair wrote:
mine does the same thing, since new, I think its a normal thing. its pre programed to clear air in the fire box for a sec or so. been doing good since new.now its all happy-camping

You do NOT have the same model Furnace. ALL furnaces have the air chamber purge before ignition. Doug

mine does the same thing, since new, I think its a normal thing. its pre programed to clear air in the fire box for a sec or so. been doing good since new.now its all happy-camping

1. Duotherm took that model off the market after a few years, because they had the exact same problem you have
2. They will work on the bench but when installed they do just what you experience. We do NOT even work on them and there have been no parts available for over 20 years. If we have a trade in (rare to keep a unit this old for sale), in years past, we replaced the furnace because we KNEW you could not be positive that we could not guarantee that the unit would run correctly
3. The problem was the burner design and the static (case) pressure was the culprit. You could leave the inside access door OFF and the unit would fire and run, install the door to force the air thru the ducts and it would misfire or just quit heating. You can play with it all you want, but it will never work correctly. Doug

That furnace uses a spark electrode for ignition but also uses a 'sensor' (thermocouple) to prove main flame.
Dirty/defective thermocouple......no main flame proven/control module closes gas valve.

It could be a defective thermostat with the anticipator gone bad (if equipped)
If you have to wait a little while after turning the thermostat down before it will restart I would look at the overtemp switch.

Lastly the motor could be slowing down as heat builds thereby allowing the sail switch to break the circuit because of low airflow.