4 wires at thermostat: R G B W (red green blue white)
5 screws at furnace: G C W Y R
Single-stage gas furnace with A/C unit

The home's previous owners kind of did everything in a very make-shift way.

I'm trying to replace an old, analog thermostat (4 conductor) with a new wi-fi thermostat (needs 5 conductors). I added a new single wire between the thermostat and furnace. I moved the existing 4 conductors from the old thermostat into the new one. (No problem.)

Here's a picture of the board at the furnace. Where should I attach my newly added single wire?

I have a voltmeter. Can I test for 24 volts? How and where?

I've already watched countless videos on using various methods to solve the common "c-wire missing" problem. But I do NOT want to do any of these, since I (now) have the 5 wires I need:

  1. Use the fan wire for the C-wire and just forget about ever using the fan-only switch again.
  2. Use more complexity with that add-a-wire circuit kludge.
  3. Buy a different thermostat that eats batteries constantly.
  4. Install a new 24-volt transformer.

Where's the simple "I've already added a new wire" video????

enter image description here

Also, how can I measure the total load on the 24 VAC transformer using my multimeter?

1 Answer 1


You simply have to connect the new wire you ran between the "C" terminal on your thermostat and the "C" terminal on the furnace board.

You can test with your multimeter, you'll get 24VAC between R and C terminals.

Judging from your photo you have an air conditioner (it will be connected to Y and C), and there is already something else on C -- perhaps a humdifier (likely two wires C+W?).

The only concern here is potentially the load on your transformer. You get maximum of 0.5A at 24VDC (written on the right side in your picture). You'll have to find the current draw of everything, and if the sum exceeds 0.5A, you could run into some troubles (voltage dropping, causing anything from random malfunctions, to everything not working, to burning out your transformer).

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