I am installing a Hive Active thermostat. My previous thermostat was battery operated and did not use a C wire. However, I found a 5th, blue wire, behind the thermostat so I used that as the C wire and connected it to the new thermostat and to the c terminal on the furnace(there is also another wire in the c terminal already, not positive what for). When testing voltage all wires are at 27 volts testing at the furnace. I also tested at the thermostat and all are at 27 except for the Rh-C which is not getting power. This needs 24 volts for the thermostat to power on...what am I missing? Thanks

  • The thermostat won’t power on though. I did shut the furnace door and confirmed that it is receiving power but nothing on the thermostat end. That is why I was questioning why there is no voltage when putting the multimeter on the Rh and C terminals. Thank you Dec 16, 2018 at 2:59

1 Answer 1


Loren, it is normal and acceptable for 24-volt controlled appliances to have a transformer that delivers 27-28 volts. Then when the circuit is under load the voltage can drop a couple of points and still deliver the required voltage. The fifth blue wire should work fine as a common and make your new stat work fine. Don't get too hung up on those voltages you are seeing.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.