furnace wont turn off

[gshinaut]

About 2 weeks ago we woke up in the middle of the night and the furnace which is typically set at 65 was 79. The only way I could turn it off was to pull the fuse. I called the dealer and they suggested testing the circuit board so I removed, tested it and found it was fine. He then suggested testing the circuit board at the AC but it is surrounded by 12" of snow. After heating for bout a week with a space heater and the fireplace I decided to try replacing the fuse - the furnace started and worked fine until yesterday when we returned after being gone most of the day and it was 89 degrees. HELP Please - is it the Tstat?

[ncitro]

When it sticks on does the thermostat indicate it's still calling for heat? Also would be good to know what model trailer, furnace, and air conditioner you have.

[Scott'n'Wendy]

My money would be on the t-stat. It tells the furnace to turn on...and off. So...what Neil said.

[AlexPeterson]

This exact problem has been reported by others and also myself. In the end I do not know if the thermostat was faulty or if the controller (accessible by removing the ceiling cover of the a/c unit) was at fault.

See post 6 of this thread and also this thread.

As others mentioned, please post details of which thermostat you have, which trailer and s/n, etc. and we can offer more help.

Edit: added this thread.

[BobinICT]

We had something similar happen to us in the middle of the night. I did some Googling at 2AM and came across something where the tstat sensor can get coated with dust on the inside area. I removed the stat from the wall and banged on it a little, put it back up and turned on the furnace. Problem solved. So now as part of check and services, I'll be hitting the stat with compressed air from time to time. That also goes with the carbon monoxide detector and the propane alarm as well.

[Scott'n'Wendy]

In the end I do not know if the thermostat was faulty or if the controller (accessible by removing the ceiling cover of the a/c unit) was at fault.

How does the controller in the AC duct relate to the furnace? My unit is one year older than yours so may be different, but why would the 'call for heat' control wire for turning on the furnace run from the t-stat to the A/C/fan control, then all the way back to the furnace? That is the length of the trailer twice in my unit.

[ncitro]

How does the controller in the AC duct relate to the furnace? My unit is one year older than yours so may be different, but why would the 'call for heat' control wire for turning on the furnace run from the t-stat to the A/C/fan control, then all the way back to the furnace? That is the length of the trailer twice in my unit.

It depends on the unit. My Coleman thermostat has dedicated great wires for the furnace, but my Dometic just has three digital wires going between the AC board and thermostat. That board then sends the signal to the furnace. I think the Furrion is similar to the Dometic (but not sure).

[AlexPeterson]

How does the controller in the AC duct relate to the furnace? My unit is one year older than yours so may be different, but why would the 'call for heat' control wire for turning on the furnace run from the t-stat to the A/C/fan control, then all the way back to the furnace? That is the length of the trailer twice in my unit.

As Neil wrote, the t-stat wires appear to go directly to the controller in the a/c unit. From there they must go to the furnace. Perhaps there are more, dunno. The t-stat when in heat mode can also be set to run the a/c circulating fan on top of the furnace fan (when 120v is available). The original thermostat also used the temperature sensor in the a/c control unit for controlling heat (there are many threads about fixing this seemingly dumb idea).

[gshinaut]

I really appreciate all of the responses!!! Since my first post the furnace continues to run past the designated temp on tstat. We pull the fuse then reinsert after awhile. The furnace is a Suburban SFV35Q, the Tstat is a Airexcel and the AC is Airexcel by Coleman Mach 10 48254C869. I like the suggestion to check for dust within the Tstate - That's an easy fix!!

[ncitro]

I really appreciate all of the responses!!! Since my first post the furnace continues to run past the designated temp on tstat. We pull the fuse then reinsert after awhile. The furnace is a Suburban SFV35Q, the Tstat is a Airexcel and the AC is Airexcel by Coleman Mach 10 48254C869. I like the suggestion to check for dust within the Tstate - That's an easy fix!!

Does the thermostat indicate it's calling for heat when the heat continues to run, or does it look like it's cycled off?

Sorry I replaced my thermostat last year and cannot remember what it looks like but I think there's an indication on it of whether it's calling for heat.

[BobinICT]

Does the thermostat indicate it's calling for heat when the heat continues to run, or does it look like it's cycled off?

Sorry I replaced my thermostat last year and cannot remember what it looks like but I think there's an indication on it of whether it's calling for heat.

Not sure what you by mean calling heat. We had ours set in the 60"s and woke in the night to a hot trailer. Indicated temp at the stat was 92 with the temp setting where we had it set in the 60"s. It wouldn't shut off even by placing the stat in the off position. I ended up going into the passthrough and killing all 12V power to the rig while figuring out what was wrong.

[ajg617]

This discussion is kind of interesting because at times, I have three heat choices on the Airxcel thermostat - elec, gas, or elec & gas (I do have a Coleman with heat pump). The elec & gas selection is very inconsistent - most times it never shows up if I manually cycle through the options and it doesn't seem to be related to external temps as we've been in the 30s and still didn't see that option. Occasionally I will cycle through and I'll see that third choice but I don't ever recall having the temp dip below effective range of the heat pump (while set to elec) and the furnace automatically kick on. It's gotten below 37 a couple of nights when we were using the heat pump and I could feel the air in the upper ducts not being warm so I'll turn the furnace on. However, when I cycle through the T-stat I will still see either gas or elec but not the 3rd choice (which was visibly displayed during our PDI and walk through). I can't figure out what controls the T-stat display. Is there a temp sensor in the AC unit that feeds the thermostat?

[ncitro]

I suspect this is going to be an issue in the thermostat. In the Coleman unit (which differs from the Furrion and Dometic) the furnace wires leave the thermostat and travel up to the air conditioner, but i believe there they are directly spliced to wires that go to the furnace (I only have a Coleman in my bedroom so it's not connected to my furnace but I'm pretty sure this is how it is setup).

I'm going to tag in howson here to confirm for me.

If I'm correct in that, then assuming the thermostat is calling for heat even though you're above the set point (I would verify this with a meter if you're comfortable, otherwise I think the thermostat has an indication on it when it's calling for heat) then there is a thermostat issue. If it's not calling for heat at the thermostat, and the furnace is running then it's probably an issue in the furnace board. There's a relay that the thermostat energizes to make it run, it's possible that relay is sticking, so that when the thermostat turns off the furnace continues to run.

The way to verify this is to find out if the thermostat is calling for heat when it shouldn't be or if the furnace is running when it shouldn't be.

[Scott'n'Wendy]

The t-stat when in heat mode can also be set to run the a/c circulating fan on top of the furnace fan (when 120v is available).

Ok..that's a different setup than I have. I guess the wires could still go to the A/C before the furnace...but would seem pretty strange to me seeing as the tstat is about 8' from the furnace in my trailer. Round trip to the A/C and back would probably be over 40'.

[howson]

The way to verify this is to find out if the thermostat is calling for heat when it shouldn't be or if the furnace is running when it shouldn't be.

ajg617 Sorry--I can't remember (or find in my notes) the wiring differences between the Coleman and Dometic.

What I do know for certain is there's an input signal to the furnace (Suburban) control board that comes from the thermostat. If the thermostat is working correctly, the control board may not be responding when the thermostat signal is removed.



Howard