Where is my common wire on the unit?

Question

I have 2 systems in my house, my heat is gas hot water radiator heating and A/C is a separate newer system. I'm trying to figure out if I can run a C wire off air handler from the A/C system to run a wifi thermostat. When I hook up the new thermostat with my current wiring it pulses my boiler for power and causes it to just turn on and off making a horrible noise. I have 3 unused wires behind my thermostat, red, white and brown. Based on the wiring diagram can somebody shed some light on where I would connect the common wire to?

Here's my current thermostat wiring

Here's the wiring diagram

Wiring mess in the air handler

Answer 1

You'll want to focus your attention to the wiring at the bottom of your last photo, that's where all the control wiring is.

Don't touch the wiring on the primary side of the transformer, as it's at line voltage and could cause a nasty shock.

If you clip the zip tie holding the bundle of wires together, you should be able to get a better look at what's going on. I can't tell exactly what's going on from the photo, so I'll explain what would typically be seen.

You should see a cable with a red, blue, green, yellow, white, and possibly brown wires. This is the cable that runs between the air handler and the thermostat. You should see another cable with a red and white wire (it may include other colored wires as well), which runs between the air handler and the condensing unit. According to the schematic, there should be a red and brown wire coming from the secondary side of the transformer.

The red wire from the transformer should be connected to the red wire from the thermostat, this is your Rc wire. The yellow wire (Y) from the thermostat cable, should be connected to a the red wire from the condensing unit cable. The blue wire (C) from the thermostat cable, should be connected to the white wire from the condensing unit, and the brown wire from the transformer. The green wire (G) from the thermostat cable, should be connected to the black and green wire from the air handler.

At the new thermostat you should have two cables, one from the air handler and one from the boiler.

• Remove the jumper between Rh and Rc, if one exists.
• Connect the red wire from the boiler cable to Rh.
• Connect the white wire from the boiler cable to W.
• Connect the red wire from the air handler cable to Rc.
• Connect the yellow wire form the air handler cable to Y.
• Connect the green wire from the air handler cable to G.
• Connect the blue wire form the air handler cable to C.

NOTE: This is all based on typical wiring, your wiring may vary.

NOTE: Some thermostats may only use the Rh and C wires for power, so supplying an Rc/C combination may not work.