Furnace comes on even when Thermo Switch in off

[OldPilot]

On our 2007 Roadtrek the furnace will come on as soon as I turn the Battery SW on. It is cold weather now. I checked the Thermostat and it is turned OFF.
I am sure this problem is temperature related because it won't do this when we have warm weather.

Do you think it is just the thermostat malfunctioning or is there some other switch in the furnace I need to investigate?

Is there some adjustment within the thermostat that I can make?

[hbn7hj]

Comes on as in blower or comes on as in heat? I doubt that the burner is coming on but if it is you need to fix it. Probably the thermostat. Just the blower coming on would be a defective over temp thermal switch. A cheap part but you have to remove the furnace to replace it, I think.

Just a guess.

[OldPilot]

It comes on with heat. The strange thing is the whole unit operated superbly last weekend when we camped. As it got cold I set the thermostat up from off. The furnace and heat came, then shut off as it reached the set temp, then on again as need be. It cycled just the right way until I turned it off.

Now temp here is 31-deg. I went to check van, turned battery on, and furnace lit up. I moved thermostat from off to heat and then back to off. The furnace ran with heat.

A week ago I saw this happen. We were not as cold as now. The furnace ran for only a short period (with heat) then shut down. I guess when the thermostat is off, it really thinks I mean very low heat. I'll have to research a manual that talks to thermostat operation and adjustment. I think the fan thermo switch is working okay because I have observed where the fan continues for a short period after the burner goes off.

[booster]

Originally Posted by OldPilot

It comes on with heat. The strange thing is the whole unit operated superbly last weekend when we camped. As it got cold I set the thermostat up from off. The furnace and heat came, then shut off as it reached the set temp, then on again as need be. It cycled just the right way until I turned it off.

Now temp here is 31-deg. I went to check van, turned battery on, and furnace lit up. I moved thermostat from off to heat and then back to off. The furnace ran with heat.

A week ago I saw this happen. We were not as cold as now. The furnace ran for only a short period (with heat) then shut down. I guess when the thermostat is off, it really thinks I mean very low heat. I'll have to research a manual that talks to thermostat operation and adjustment. I think the fan thermo switch is working okay because I have observed where the fan continues for a short period after the burner goes off.

It sounds like the slide switch for the mode selector on the thermostat is stuck on the heat selection so when you turn it off it doesn't go off. Pull of the cover on the stat and see what is going on and if the mechanical contact is welded or the linkage broke that is made to move it.

[Bud]

I live in a humid climate. I had similar problems. It was Roadtrek years ago that suggested the problem might be corrosion, it was. I sprayed carb cleaner (all I had at the time) on the slide and moved the thing up and down a bunch. All was well. Then maybe 6-8 years later I had to do the same.

[mloganusda]

Same thing has happened to me on my 2017 Trend in 2021 & 2022. I tried several solutions, but they did not correct the problem. I now pull the 5-amp fuse to keep it from running. Others have replaced with a digital thermostat to correct the problem.

[TX-Trek]

If the control board itself is failing, I would suggest replacement with a Dinosaur Electronics unit.