Furnace works fine, but won't stay warm all night


New Member
In my 2007 Georgetown 391, I've got a suburban gas furnace (not sure which model, but one of: SF-20F to SF-35F) controlled by a DuoTherm thermostat - this one has three switches - (cool, off, furnace) (fan on - auto) (fan hi-lo) and a sliding temperature control.

We've just set out fulltiming and have encountered our first nippy weather - at night we use only the front furnace or A/C unit and that keeps the bedroom temp fairly constant while the heater or A/C cycles on up front. For the last two nights, it's gotten down to about 50 degrees, and with the thermo set to 65 degrees, we hear it working as we go to bed, but wake up to a pretty cold 58 degree temp inside. Moving the switch from "furnace" to "off" and back to "furnace" gets it going again, and all seems OK. The fact that it comes on immediately indicates that the set temp is indeed above room temp, and the thermostat knows it. So my question is: what might explain the system seeming to work fine, but not coming back on again after cycling off because the set temp is reached? Since it happens during the night I really don't know whether it happens after the first cycle, or just some time later...

We really need to figure this out because we plan to be in Colorado in a few weeks and will need to leave the unit for a couple of days with the furnace(s) running to keep it warm.

As far as I know, the rear furnace works correctly, and the front furnace has seemed to work correctly in the past.


Senior Member
Re: Furnace works fine, but won't stay warm all night

There are a couple of important factors not mentioned. Were you on shore power, or depending upon the batteries? Most furnaces are not reliable once the battery voltage drops to 10.5V or below and can be doubtful at 11V. If you were on shore power, that probably means that this is not the issue.

When you reset it, did you wait to make sure that the blower did supply heat, and was not just blowing cold air? If battery voltage was too low, that would probably be a symptom. I think that if I were you I would also test the rear furnace if that happens again. Whether or not it effects both would tell us quite a bit.

The fact that turning it off and back on, tells me that your furnace was in what is called "lock-out." Turning it off, then on is the way to clear that condition. Lock-out will happen any time that the furnace attempts to light and fails. It really don't tell us the reason. Does this happen every time that you use it, or just occasionally? Intermittent problems can be the most difficult to figure out.


New Member
Re: Furnace works fine, but won't stay warm all night

Arewe Did you get the heat fixed i had the same problem it was my DuoTherm thermostat replaced it all was great