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
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.