thermostatthermostat-c-wirewiring
Got a wifi thermostat. Behind the old thermostat was a blue "C" wire. The other side of the blue wire in the equipment room isn't plugged into anything. I need to get the 24v common hooked up to this blue but don't know where it goes. Any help is greatly appreciated.
There is a set of connections in the air handler that usually includes the "C" terminal, and it should be labelled (though it may be hard to read depending on the location of the control board and/or wiring/etc in the way).
One way to tell for sure is that while the furnace is not running, using a multi-meter you can measure between the "C" and "R" (or Rh or Rc terminals) and you should see 24VAC. R (or Rh and Rc) will already be going to the thermostat.
As an example, on your furnace, from the manual:
The colors may match, but don't count on it. The sure-fire way is to check using a multi-meter as I said above.
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.
Best Answer
The wiring diagram you posted is quite clear on what it wants you to do; namely, it wants the C wire connected to the furnace chassis ground. To do that, simply drill/tap a 10-32 hole in a suitable spot on the furnace chassis if there isn't one present already, screw a premade ground pigtail into it, and nut the C wire to that pigtail.