I have dual transformers, separate heating and AC.
I've recently installed a nest learning thermostat (3rd gen). This is my wiring. The c wire comes from the the transformer on the AC side (pairs with Rc). The system works and is able to call for both heat and cooling.
However the problem I noticed is that when AC is off, the outside condenser unit has a chattering noise. I removed the side panel on the unit and found that it's the contactor trying to actuate.
I actually have another nest thermostat E but it also causes the same chattering.
I tried my old thermostat - no chattering.
It's like the nest is leaking a small voltage to the Y1 wire even if it's not calling for cooling.
After a long chat with the nest support, they asked me to remove the c wire and it did stop the chattering and they are asking me to go without the c wire. However based on what I've researched, not having a c wire could cause a lot of potential issues down the road.
I've read a reddit post that suggests adding an isolation relay (though the post added the relay on the heating side). I have not purchased a relay yet but did sketch the wiring that I think should work .
I'm wondering if the relay is going to work or there is other solutions. If the underlying issue is indeed leaked voltage, then will having a relay simply hide the problem, e.g. the chattering may occur at the relay?
Any suggestion is greatly appreciated! Thank you!