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
Install Hive Thermostat – C wire power
thermostatthermostat-c-wire
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I just got off the phone with Honeywell. If you only have a 4-wire older system, you'll only be able to use the fan in the "auto" position. It won't let you use the fan-only selection. You need a 5th wire for power for the wi-fi side of the thermostat. Call Honeywell and they'll walk you through it for free.
I've labeled your photo, which should help you understand what's going on.
What you describe as a "blue" wire at the thermostat, you describe as "gray" at the air handler. I can't be sure, since I'm not there, but I think they are one in the same.
If you want to install a fancy new WiFi thermostat, you're going to have to pull an additional wire from the air handler to the thermostat. Once you have that, you'll connect one end to the C terminal of the air handler, and the other end to the C terminal of the thermostat.
Best Answer
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.