Furnace/AC Operation

[c.apple]

Hi all. Have a question about furnace and ac operation. Have a 2022 transcend xplor 321bh, suburban furnace, Coleman Mach AC and Coleman Mach thermostat. When set to heat system works as it should. With it getting a little warmer, I’ve switch the ac on during the day and while the ac is running, the furnace is also running. Is there something I need to do so that the furnace does not blow hot air while the thermostat is set to cool and the ac running?

[ncitro]

Hi all. Have a question about furnace and ac operation. Have a 2022 transcend xplor 321bh, suburban furnace, Coleman Mach AC and Coleman Mach thermostat. When set to heat system works as it should. With it getting a little warmer, I’ve switch the ac on during the day and while the ac is running, the furnace is also running. Is there something I need to do so that the furnace does not blow hot air while the thermostat is set to cool and the ac running?

I'd suspect a bad or stuck thermostat. I'm 90% sure that on the Coleman the furnace is wired directly to the thermostat. For it do act as you describe the thermostats heat relay would be stuck and those contacts are energized at the wrong time. In the short term you could pull the thermostat and remove the heat wire, that should disable the furnace and allow just the AC to run. Another option would be to pull the fuse for the furnace from the fuse panel, but I can't promise there's not other things on that fuse, so I'd pull the thermostat wire. It should be the "W" wire.

[c.apple]

Thanks for the fast response. Sounds like the permanent fix is to replace thermostat. Is it recommended to upgrade the manual Coleman Mach?

[ncitro]

Thanks for the fast response. Sounds like the permanent fix is to replace thermostat. Is it recommended to upgrade the manual Coleman Mach?

I've not had issue with mine, but to be fair it only runs my bedroom air, my furnace is run on my living room thermostat which is aDometic AC unit. I'd pull the wire first to be sure that's the issue, but I'm almost sure the Coleman controls all that in the thermostat itself. My Dometic uses a control board in the AC unit to run the furnace which makes it more difficult to troubleshoot.

[c.apple]

Should have mentioned that even when running just the fan the furnace runs. I only have access to 15 amp while at home so pulling furnace wire while running fan should indicate faulty thermostat correct?

[ncitro]

Should have mentioned that even when running just the fan the furnace runs. I only have access to 15 amp while at home so pulling furnace wire while running fan should indicate faulty thermostat correct?

Yes that's right.

[c.apple]

Pulled the white wire and switched to fan mode and the furnace still kicked on and ignited. Some sort of relay?

[ncitro]

I don't know that it would be the white wire (Grand Design colors don't mean much) you're looking for the "W" terminal on the thermostat. Failing that I guess the thermostat wire could be pinched or shorted somewhere? Possibly up in the air conditioner where the two wires to the furnace split out?

[Jlawles2]

Just did a brief scan of the manual. You may need to drop the inside cover on the ceiling unit and try disconnecting the furnace wire there. Look for anything that may be bridging the terminals like stray wires from other connections.