Showing results for 
Search instead for 
Did you mean: 

My ecobee thermostat works fine, but new nest learning has trouble. Could wire voltages be wrong?

Community Specialist
Community Specialist

Disclaimer: This thread was migrated from our previous version of the Google Nest Community. You can continue to receive updates on your thread issue here or simply ask, browse or more in the new Google Nest Community.

Original Poster: Evan Einstein 

I am upgrading an old Ecobee thermostat with a Nest learning thermostat. The old thermostat worked perfectly, but the Nest is behaving strangely. The baseplate buzzes even without the thermostat attached (it stops when I disconnect the Rc wire). And at first the heat wouldn't turn on and now it is stuck on even after power cycling both the thermostat and the boiler. The air conditioning seemed to be operating properly. 
I measured the voltages of the wires at the thermostat and they seem too high. My C to Rh is 27v, but C to Rc is 56v. Is this a problem? 

My system uses C, Rh, Rc, G, W, Y. It is AC and heat by boiler. I ended up putting back the old thermostat for now so I don't have the heat stuck on. Any help is appreciated.

Community Specialist
Community Specialist
Hi Evan, 
Sorry to hear about your Learning thermostat's bizarre behavior! That's certainly odd. Let's see what's going on here. 
Due to the presence of both an Rh and Rc wire, you might have a dual-transformer system. These are notoriously difficult to wire with Nest thermostats due to 1) the Rh and Rc wires having separate power sources and 2) the thermostat alloting one wire per terminal. However, those might be irrelevant due to the high voltage you're reporting--unfortunately, our thermostats function well with 20V-30V systems, but are incompatible with higher voltage.
If you do wish to use your Learning thermostat, we'd strongly suggest a Nest pro come out and take a look at rewiring/possibly installing a step-down transformer to prevent any damage to your HVAC system or your home. 
Hope this helps! Let me know if you have any other questions for us.
-From Ai, Community Specialist.