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Old 05-13-2012, 10:03 PM   #1
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Help an HVAC noobie out... can I combine the furnace and A/C t-stat?

I have the original Armstrong and Suburban A/C and furnace. I bought two digital thermostats to replace the originals, but thought I may be able to return one if I can join the two into one functioning unit.

Is it as simple as I think it is or should I leave them separate?
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Old 05-14-2012, 06:37 AM   #2
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I am not sure about how the Armstrong is wired, but both the Comfort Control Center (CCC) and CCC2 are set up to control both. Our 1997 came with a CCC which controlled the A/C, Heat Pump, and Furnace.
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Old 05-14-2012, 12:48 PM   #3
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Yeah, I'm just not 100% on it... I don't know enough about them. The Suburban is only two wires and it has an electric start. I'm not how that affects the situation, if at all. Thanks for the feedback though!
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Old 05-16-2012, 12:17 AM   #4
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Probably not. The furnace t-stat is just a thermo-mechanical switch that lets 12 VDC pass through it while A/C units use 24 VAC to control relays inside the unit. I don't know of any t-stats that will let you use both 24VAC and 12VDC.

Gimme a year or so and I'll have a custom circuit that will do that. It's a little ways down on my priority list.

-Jason
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Old 05-16-2012, 05:12 AM   #5
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Probably not. The furnace t-stat is just a thermo-mechanical switch that lets 12 VDC pass through it while A/C units use 24 VAC to control relays inside the unit. I don't know of any t-stats that will let you use both 24VAC and 12VDC.

Gimme a year or so and I'll have a custom circuit that will do that. It's a little ways down on my priority list.

-Jason
Rats, ok, guess I'll have to keep the old ugly one as well.
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Old 05-16-2012, 06:24 AM   #6
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Originally Posted by jmpratt View Post
Probably not. The furnace t-stat is just a thermo-mechanical switch that lets 12 VDC pass through it while A/C units use 24 VAC to control relays inside the unit. I don't know of any t-stats that will let you use both 24VAC and 12VDC.

Gimme a year or so and I'll have a custom circuit that will do that. It's a little ways down on my priority list.

-Jason
Really? My Interstate came straight from Jackson Center with a single thermostat that controls both the rooftop A/C unit and the furnace. The thermostat is 12v.

I couldn't tell you off the top of my head what the make and model of the thermostat is, but the air conditioner is made by Carrier, and the furnace is made by Hydro Flame Corp.
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Old 05-16-2012, 07:09 AM   #7
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12volt vs 120volt

If the furnace and A/C are as old as your AS, the furnace and it's wall thermostat runs on 12volts. The A/C and it's built in thermostat runs on 120volts. I would be surprised if a 1978 rooftop A/C had any 12volt components. Therefore without doing some kind of major modification, they can not share a wall thermostat like your home units do.
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Old 05-16-2012, 08:13 AM   #8
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Really? My Interstate came straight from Jackson Center with a single thermostat that controls both the rooftop A/C unit and the furnace. The thermostat is 12v.

I couldn't tell you off the top of my head what the make and model of the thermostat is, but the air conditioner is made by Carrier, and the furnace is made by Hydro Flame Corp.
I didn't think about the newer thermostats in RVs and TTs. I was only thinking about household t-stats. Considering that, maybe a new RV thermostat will work.

Just checked PPL. Give them a call to see if this will work:
RV Thermostat - PPL Motor Homes

-Jason
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Old 05-16-2012, 09:23 AM   #9
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Another thing to consider is that the two thermostats are in different areas of the trailer and the wires are between the skins.Moving the furnace wires to the AC locations would be the most logical way to go, but it would still require fishing wires from behind the walls or removing the inner walls.
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Old 05-16-2012, 12:11 PM   #10
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I did that with our Dometic AC and an Atwood Furnace. Our AC had no 12 V circuit at all.

Dometic makes a single zone digital t'stat and a control box about $109. The control box is kind of large but Dometic told me to put it in the return air area of our Penguin ac. It has standard plugs that the AC plugs into (along with an electric heat strip if you have one). Then you have to run 7 wires to the T'stat location (2 for the furnace, 2 for 12 V AC, and three to the new T'stat). I got lucky and was able to run the wire between the ceiling and the roof over to the cabinet. Then thru the cabinet to behind the refrigerator. I picked up the 12 V power from the refrigerator.

I can now digitally control the AC, heat strip, and furnace. I can't run the heat strip and the furnace at the same time though (I used to do that).

Something is still not correct with the T'stat though. I may be getting too much heat from behind as it backs up to the refrigerator vent stack. Or the T'stat may be defective. I am going to to insulating first.
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Old 05-16-2012, 03:13 PM   #11
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My t-stats are side by side, so the distance isn't an issue, but what I may do is leave the furnace thermostat where it is in the bedroom and relocate the A/C t-stat to another area in the living room. I didn't realize the furnace was 12V but it makes total sense. I'll return the second digital thermostat and be on to the next project. Thanks!
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