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How to connect dual transformers / separate heating and cooling systems 1 Recommended Answer 4 Replies 2 Upvotes
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.
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