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Nest looses power when heat is called

Community Member
  • My old TStat had Red and White wires only. I added a wire for a C connection on the nest and connected to the common of the 26vac transformer. The system has no control board. It’s a hydronic with Honeywell control valves. All is well until the thermostat calls for heat then the thermostat immediately loses power until I turn the thermostat down. Looking closer at the transformer wiring the valve motor is wired directly to red on the transformer and the white common is switched through the thermostat. With the switch open I have a difference of potential between C and R on the thermostat. Switch closes I lose difference of potential thus no power. Any Ideas on how to fix this?


Use the red wire to go from nest Rh to 24 volt side of transformer. Use the wire you added from transformer to C on nest and Common side of valve. Use the white wire to go from nest W or W1 to 24v. side of valve. Then configure your heat properly. Good luck 

Community Specialist
Community Specialist

Hi there,


Thanks for reaching out. A few questions: what type of Nest Thermostat do you have? You can visit this link to check what type of thermostat you have. Do you notice any messages or codes on your thermostat or in the app? Please provide your thermostat power readout.


Nest Learning Thermostat and Nest Thermostat E:


  1. Go to your thermostat Quick View menu.
  2. Go to Settings.
  3. Look for Technical Info.
  4. Select Power.


Nest Thermostat 2020:

  1. Open the Google Home app and then tap the thermostat and then tap Settings .
  2. Select the Device Information.
  3. Choose Technical Information.


I appreciate your input, @HVAC.