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.
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.
If there's a C terminal on your board (can't tell from picture), use that.
If you have an air conditioner, it will have two wires that connect back to the thermostat (usually via the furnace): so long as one is going to
R, the other will be
Cand you can splice into the same spot.
Lastly, you can splice into a wire from the transformer. You'll need a multi-meter to figure out which one it is (measure 24V between the
Cside of the transformer and the
- It does look like common is grounded to the chassis, so you can also use that to identify the side of the transformer -- find the side that has a wire connected to the chassis.
It'll be one of the bottom two terminals:
You should use the multi-meter to test -- no matter which method you use -- before you actually splice into anything.
With the furnace powered on but not actually running (fan off, heat off), you should measure
24V AC between the
R and the place you're hooking into.