Dead furnace and thermostat feed

[Iraqvet05]

Late last year, I swapped out a mechanical Coleman Mach thermostat for a Honeywell digital. The A/C worked fine on the Honeywell but the furnace would not kick on and I ignored the issue believing it was a programming issue with the new thermostat.

This weekend we camped and the A/C would not come on. Thinking I had an issue with the Honeywell, I swapped the Coleman back in but the A/C and furnace were still dead. After some troubleshooting, I determined the 12V POS feed to the thermostat was dead. I tapped an adjacent light for 12V and ran it to the thermostat and got the A/C running again but the furnace is still dead.

The furnace and fridge are on the same fuse and the fridge runs fine on 12V/gas. I believe the Atwood furnace supplies 12V to the thermostat (red/white) wire but I need to remove my sofa to gain access to the furnace. I also saw in the furnace manual that some Atwood models have a reset button but I have never seen one in my TT since I bought it 3 years ago. Any suggestions or do I need to pull the sofa and remove the lauan covering the furnace to get a shot at it first?

[Iraqvet05]

Problem has been fixed. Apparently my Atwood furnace has a "reset" switch that had tripped. I had to remove the sofa and some of the plywood around the furnace to get to the switch. Turning it off then back on restored 12V to my thermostat.

[westend]

Old Biscuit knows his HVAC stuff. Confirm that your thermostat is wired as he posted.

[Iraqvet05]

I'm certain the Coleman is wired correctly based on the notes and pictures I took as a reference before I removed it. I don't know the furnace model number. Jayco did a good job at burying the furnace so it's hard to even see.

[Old-Biscuit]

Coleman Mach/Atwood to Honeywell wiring should have been connected like this:

Red wire to “Rh” terminal (Rh and Rc utilize a jumper) (12V DC power)
White wire to “W” terminal (12V To furnace from t-stat)
Yellow wire to “Y” terminal (12V to A/C relays)
Green wire to “G” terminal (A/C High Fan)
Gray (not connected) (A/C Low Fan) Unless a hi/lo switch is installed
Blue (not connected) (12V Neg)

HG/HE on backside of t-stat set to HG


What Atwood Model do you have?Atwood Furnace Manual

[BFL13]

Pretty sure it is all about the wiring to the new thermostat. I have seen various schematics for which wire goes where when swapping thermostats. Don't have info on your particular swap, sorry.

[westend]

I'd suggest to interrupt the expected power feed from the furnace to the thermostat to confirm that it is, indeed, the power source for the thermostat. That ought to save any unnecessary wallboard removal. I think you'll find that the 12V to the thermostat is parallel to the control board wiring for the furnace.