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How to connect dual transformers / separate heating and cooling systems 1 Recommended Answer 4 Replies 2 Upvotes
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Last edited 7/5/19
I have been trying to properly connect separate heating and cooling systems with dual transformers since 2013.  I have had 1st gen Nest thermostats up to the more recent 3rd gen Thermostats.  I've been able to get this to work back in 2014 but likely due to software updates, etc it stopped working and none of the Nest support staff were able to help.  Now that dual transformer systems are more common, I was able to see more posts about this topic and more people facing the same issues.

I have 3 Nest thermostats for 3 separate heating zones connected to a boiler.  I have 1 cooling zone where the AC is connected to one of the 3 nest thermostats.

The AC has Y1 Y2 Rc C G
The 3 zone boiler relay has W Rh C

The issue is connecting the Nest that has both AC and heating.  Here is the issue...

With Y1 Y2 G Rc Rh W all connected...

- if I use the C from the AC unit, the boiler 3 zone relay chatters because the nest for some strange reason is trying to draw power from Rh to charge the battery even though Rc and C from the AC unit is connected.

- if I use the C from the boiler 3 zone relay, the chatter stops but the Nest reports that there is now power from Rc

To make matters worse, there is a lot of unexpected behaviour that also happens such as the Nest will sometimes NOT report that the there is no power and sometimes the Nest goes for days without drawing power from the boiler 3 zone relay and sometimes it's drawing power all the time and sometimes the Nest reports no power from Rc essentially disabling AC altogether.  Here is an interesting one...  to resolve this, I disconnect Rc and the C from the AC in the winter but was able to activate the AC unit??!!  but it would stop for an hour or so and resume for an hour.

While I have heard from other users in the community that connecting the AC C wire is the right way, how do we stop the Nest from trying to draw power from Rh when Rc and C wire from the AC unit is connected?

IMHO the best way to handle this on the back end is to simply assign Ch and Cc and the Nest can handle the dual transformers properly.  Allow the assignment of the * for another common.... or ensure that the Nest does not try to draw power from Rh when Rc is connected.
All Replies (4)
I see that there is an old thread about Dual Transformers and youtube 

It seems that the way to correct this is to use the C wire from the AC or Rc transformer.  My problem is that my boiler relay has a C wire for heat or Rh transformer.  

If I connect the C wire from the AC / Rc transformer and leave the C wire from the boiler or Heat / Rh transformer unconnected, the Nest started to try to steal power from Rh even though Rc and the C wire was connected.  This caused the Rh boiler relay to chatter and essentially flicker on and off many times a second.

If I connect the C wire from the Heat / Rh transformer and leave the C wire from the AC / Rc transformer unconnected, the Nest complains about no power from Rc and the AC will not run.

I still can't believe that it has been 5 years, 3 generations of Nest thermostats and I still need to disconnect Rc / C and reconnect Rh / C in the winter and vice versa in the summer.

Is there a PRO setting that will help address this that either NEST or someone knowledgeable can shed light on?
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Thanks for sharing your thoughts, and I am sorry to hear that you are having this issue on your system.
A dual fuel heat and cooling system (also called a hybrid or packaged system) is a combination of a heat pump for primary heating and cooling and a furnace (usually gas or oil) that provides a second and/or third stage of heating. The Nest thermostat is compatible with these systems and supports up to 2 alternate heat sources. And you also have 3 separate heating zones connected to a boiler. Usually, using a common wire, whether it is from an AC or heater, would be fine to use on your Nest thermostat. 
With the strange behavior of your furnace that keeps on kicking off and on should be checked by a pro. The best option in your case is to find a Nest Pro installer in your area with our online Pro finder. This way, the tech will be able to physically check your system wiring (since you have a 3 separate heating zones) and will be able to properly install your thermostat to work with your kind of system.
Best regards,
marked this as an answer
Good to hear that it is now working as intended. 
Please get back to us if you have further questions and the team will be more than happy to assist you.
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