Removed furnace, now air conditioner won't work.

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JordanGrabe

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May 3, 2020
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As the title says, I removed my furnace since it didn't work which isn't a problem we have other sources of heat. But now the air conditioner doesn't kick on.

I assume the thermostat was getting power from the furnace. I know I have a POS and a NEG that powered the furnace and a thermostat wire down where the furnace was. Is there a way to power the thermostat from there without the furnace? Like wire nutting the POS to one of the little wires from the thermostat wire? Or did the furnace reduce the voltage lower for the thermostat? I don't want to fry my thermostat.
 

Rene T

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I?m sure You know that the A/C runs off 110  volt AC. I?d start there. Do you have power at your entrance panel? Use a volt meter to check. Did the breaker trip? Shut it off and back on to reset it to make sure.
 

Gary RV_Wizard

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You can use any 12v source to power the thermostat; just find some source that is easy to wire.  That should get the a/c running.

One of the wires at the furnace will be the "Heat Needed" signal wire. That's what triggers the furnace ignition cycle.  That's a separate wire from the 12v+ that powers the thermostat itself, but if you can't find the power wires, you could re-purpose the signal wire by connecting each end (one end to 12v+ and the other end to the 12v terminal on the thermostat. Since you don't have a furnace, you don't need that wire.
 

Lou Schneider

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What kind of furnace, A/C and thermostat do you have?  The way power is routed to the thermostat depends on the manufacturer.

For example, the DuoTherm Comfort Center thermostat in my RV only has a data cable connecting the thermostat to the control unit inside the A/C.  The control unit supplies DC power to the thermostat and the on/off contact that controls the furnace. 

Other makes have a separate cable going from the thermostat to the furnace and gets DC power from it.

Knowing what you have will make it easier to find the correct connections.
 

Stayputguy

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Jun 13, 2021
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Minnesota
You can use any 12v source to power the thermostat; just find some source that is easy to wire. That should get the a/c running.

One of the wires at the furnace will be the "Heat Needed" signal wire. That's what triggers the furnace ignition cycle. That's a separate wire from the 12v+ that powers the thermostat itself, but if you can't find the power wires, you could re-purpose the signal wire by connecting each end (one end to 12v+ and the other end to the 12v terminal on the thermostat. Since you don't have a furnace, you don't need that wire.
Gary Rv wizard. I did the same thing. Scrapped the furnace in 2010 keystone Springdale that is a permanent fixture on a lake lot. I don’t use the furnace, it had a mouse problem, and I like the added space I got from removing the furnace. But now ac won’t turn on. I know you are correct about power to the thermostat as the cause of the problem. But, how do I fix it? Where the furnace had been I have 1 red, 1 yellow, 2 blue wires. Red has voltage…I touched red to yellow and blew fuse at converter box. Can I just work with these 4 wires to get thermostat and ac working? Or do I need to tap into another power source?
 
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