2 common wires, 1 thermostat


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Old 01-22-15, 02:31 PM
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2 common wires, 1 thermostat

My home has a separate heating and cooling system; I have hot water baseboard heating and central air conditioning. There is an R and W wire coming from my heating system, and RC, Y, G and C come from my air conditioning. The ecobee3 thermostat (along with the Nest and the Lyric) needs a constant 24v to run the thermostat, which it gets from the RC terminal, and the C terminal completes the circuit. I have a unique problem that I canít seem to find a good solution to. I lose power in my area sometimes during October/November because of hurricanes and I have the most essential circuits in my home hooked up to a transfer switch which is ran by a generatorÖ one of which is my heating system. The problem I run into is that if I were to lose power there would be no power going to the air conditioning transformer anymore because I donít have the A/C as an essential circuit, so despite me having the heating system on generator, the thermostat wouldnít work. I was going to just disconnect the common from the A/C and run a common wire from the heating system, but ecobee support informed me that when there is power from the RC wire and the RH wire, it will always use the RC wire. Being that I have 2 separate transformers for the heating and A/C units, during the summer when the A/C is on it would try and pull power from the RC wire and return it via the common wire now going to the heating system. That wonít work, I can't take 24v from one transformer and return it to another.

Iíve spoken with ecobee support and after throwing some ideas back and forth the only 2 options he came up with were to use 2 thermostats, or hook up a backup UPS to the A/C system (which would cost way too much and is unreasonable to just run a thermostat). I asked both my HVAC contractors that installed my boiler and my A/C but they didn't really know a good answer.

I have come up with an idea, but I wanted to bounce it off someone who knows more than I do.

Note: The thermostat termination block in the below pic is my current Honeywell thermostat, I just used it as a reference.


I want to run the common from the A/C and the common from the heat to a toggle switch. Itís a lot of work, but I would be able to have a common wire to both systems, and I could choose to use the common from either one. During normal operation the toggle switch could be positioned to have the common wire run back to the A/C, but when the power goes out and the A/C has no power the backup generator can supply power to the heating system. Now the thermostat would use the RH terminal and the return common could go to the heating system. I have to switch over the circuits on my transfer switch manually anyway when the power goes out, so this would just be another step.

What do you think?
Would this work, is there is better way, or is it ridiculous? Or should I just stick with a battery powered thermostat?
 
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Old 01-22-15, 05:03 PM
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I believe that circuit would work, it's an interesting situation you would have to leave that switched to AC through out the cooling season,and switched back for heating.
Geo
 
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Old 01-22-15, 05:15 PM
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Welcome to the forums.

Well.... actually.... it would half way work. You are showing switching the Commons but you would also have to switch the R's too.

Right now.... you could switch the commons but there would not be any 24vac on the Rc terminal if the A/C system was dead. You have to switch your Rh to Rc at the same time you switch the C's.


You have to remember you're working with two transformers and they cannot meet.
When the switch is in the bypass mode..... getting power to run the thermostat from the boiler.... the A/C system nor FAN can be selected at the thermostat or you could potentially be connecting 48v to your system or creating a short based on the phasing of the transformers.
 
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Old 01-22-15, 05:49 PM
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Geochurchi,

Thanks, I plan on always letting the A/C provide the power. I would only need to switch the common to the heating system if I were to lose power since the heating system can be ran by generator and the A/C can't.

PJmax,
Would I need 24v on the RC terminal in this situation? When I spoke with tech support they said that the RC is always used for power if both RC and RH are a avaliable. It will use RH to C when RC is not available.

I would turn the thermostat off, then use the bypass toggle just to be safe.

I don't mind wiring this, but honestly all I really wanted was a thermostat that I can control remotely. The downside to wifi thermostats is that wifi will kill 2 AA batteries relatively quick. What I was originally looking for was a smart thermostat with an Internet connection that I could hook up with a separate power source to just avoid this whole problem. Than I could just add that separate transformer powering the thermostat to a circuit that can run on my generator and everything would be great. After talking with ecobee, Nest and Honeywell support apparently that's not an option. I don't see why not, I think it's a good idea.
 
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Old 01-22-15, 05:56 PM
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It will use RH to C when RC is not available.
If they told you that then you should be ok.
 
 

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