Furnace coming on when using the A/C

[MAVetter]

Hello, We have a 3791RD (2017) bought new and are full-time rvers. In Florida using the air conditioning and yesterday the furnace comes on at the same time. Not knowing what to do I ran out and shut off both propane tanks. The blower still comes on intermittently. So this morning I pulled the 15 amp furnace fuse and have stopped the blower from coming on. We have 2 A/C units and Dometic thermostats. Has anyone experienced this. Is my problem the thermostats or something else? Thanks

[K0LCB]

That sounds like a wiring problem

[Roadtravelers]

I believe you have the thermostat set to AUTO

[jeffba]

Originally Posted by jimandpat

I believe you have the thermostat set to AUTO

[DQDick]

I agree with KOLCB. Not having the thermostat set on auto will cause the AC fans to run when the furnace comes on, but it should not work the other way around.

[MAVetter]

Originally Posted by DQDick

I agree with KOLCB. Not having the thermostat set on auto will cause the AC fans to run when the furnace comes on, but it should not work the other way around.

DQDICK is correct. JimandPat and jeffba are not correct. We run the a/c on auto for 3 years and never had this happen...

Problem is either the thermostat or another problem. I don't know how it would be wiring after 3 years of operation just fine. Any other ideas my fellow rvers?

BTW: By pulling the 15amp fuse for the furnace, it also stops heating the water (even electrically). So, we've put the 15 amp fuse back in; turned off the propane tanks; turned on the electric water heating switch. The furnace blower runs a couple of minutes and then stops b/c the furnace is not getting propane. Can live with this short-term, but will be searching for a repair in the next day or two.

Thanks.
Mark

[K0LCB]

Originally Posted by MAVetter

DQDICK is correct. JimandPat and jeffba are not correct. We run the a/c on auto for 3 years and never had this happen...

Problem is either the thermostat or another problem. I don't know how it would be wiring after 3 years of operation just fine. Any other ideas my fellow rvers?

BTW: By pulling the 15amp fuse for the furnace, it also stops heating the water (even electrically). So, we've put the 15 amp fuse back in; turned off the propane tanks; turned on the electric water heating switch. The furnace blower runs a couple of minutes and then stops b/c the furnace is not getting propane. Can live with this short-term, but will be searching for a repair in the next day or two.
R
Thanks.
Mark

I suspect one or more of your thermostat wires have managed to get together. Heat and AC are 2 separate circuits except near thermostat.

[MAVetter]

Thank you K0lCB. Is this something I should be able to track down? Or should I contact a repairman. I'm pretty handy and have done other small repairs on the rv.
Thanks again!

[jimcol]

If pulling the 15amp fuse disables other devices there should be an on/off rocker switch on the top of the furnace. Leave your fuse in and disable the furnace from this switch until you can get permanent repairs.

[MAVetter]

Thanks to everyone for your responses.

[MAVetter]

Originally Posted by K0LCB

I suspect one or more of your thermostat wires have managed to get together. Heat and AC are 2 separate circuits except near thermostat.

So. I've turned off the switch at the furnace and that has stopped the blower from cycling on/off. And I can now leave the propane tank open.

I unscrewed the stat from the wall and pulled the wiring out to view it. There are only 3 wires connected to the stat: white (heat); yellow (cool); and blue. The red wire has been trimmed shorter and is not connected to anything, although the red cover has been trimmed back 1/2 inch. From all the videos I've watched I thought the red was the power ???

So I do not find the white, yellow or blue wires are touching anywhere they are stripped.

So, what's the probability the Dometic Stat has gone bad? Or should I be calling an RV repairman?

Thanks again for your help/guidance/suggestions.

[Jimmer]

I had that problem. Not sure if it is the same problem you had. I looked at the owners manual and they said to reprogram or reset. I followed the instructions and it solved my problem. You have to hold 2 buttons at the same time. If your owners manual is not help contact the factory. Good luck.

Jim Bonhote
2014 Montana 3582RL; 2015 Ford F350 Platinum 4X4 SWB

[rmthelen]

Our unit did the same identical thing when it was new, it was finally tracked down to a faulty thermostat. Thermostat mode is Dometic CCC2. I suspect that yours is the same.

[dfb]

Fan on auto, Zone 2 off... on, mode, mode mode til it shows Furnace...i hate the thermostat on these..

[biznsa]

I had a similar problem with the ac/furnace operation. There's also a system board located within the unit in the living area that controls the operation. The board has something to do with coordinating the heat/ac between both units. Sorry I couldn't remember more.

[Rsmarlow82]

Not sure but we had issues with the Dometic thermostat in our 2018 unit. I'd be suspicious of the thermostat, especially if it is the newer style "touch screen type. Our dealer told us to "really" fix our issues we should go back to the old style nit with push buttons Good luck

[John Shasky]

Suggest resetting the system thermostat and reprograming it again.

[Tomg52]

Originally Posted by MAVetter

Hello, We have a 3791RD (2017) bought new and are full-time rvers. In Florida using the air conditioning and yesterday the furnace comes on at the same time. Not knowing what to do I ran out and shut off both propane tanks. The blower still comes on intermittently. So this morning I pulled the 15 amp furnace fuse and have stopped the blower from coming on. We have 2 A/C units and Dometic thermostats. Has anyone experienced this. Is my problem the thermostats or something else? Thanks

I had the same problem on my 2018 3121 RL. I set the fan to auto, then I pressed both arrows to toggle between Metric and English. That solved the problem for me.

[Lack of HP]

Originally Posted by MAVetter

Hello, We have a 3791RD (2017) bought new and are full-time rvers. In Florida using the air conditioning and yesterday the furnace comes on at the same time. Not knowing what to do I ran out and shut off both propane tanks. The blower still comes on intermittently. So this morning I pulled the 15 amp furnace fuse and have stopped the blower from coming on. We have 2 A/C units and Dometic thermostats. Has anyone experienced this. Is my problem the thermostats or something else? Thanks

I had same problem. CW replaced the stat control board and board on furnace. Still did same thing at times. I took the wall panel off in the basement and started moving wires. Saw a spark and the furnace came on. Loose wires touching at a connection. Re did them no further problems

[KJT]

If you have a dual zone system you need to turn off one of the zones that shows your furnace mode.
Had same problem with my 3750FL.