Wiring in thermostat without circuit board

Looking for help wiring in my new thermostat to an old hvac unit with no circuit board.

I’ve got 4 wires at my current thermostat:
red, green, white, and blue. I found these same wires feeding to the unit, as pictured. How do I connect the adapter?

I had a similar setup with no control board. I was missing the C wire. I used a multimeter to find the 24v between two wires. For mine, the red wire was one side of the 24v transformer and I found a black wire that was on the other side of the 24v transformer. This became my C wire. You will need to look up the model number of your furnace and you should be able to find a wiring diagram.

Based on your picture, I would guess the brown wire would be the other side of the transformer. Check voltage between red and brown wires.

If you get 24v, then you would connect the red, green, white, blue wires that are bundled together to the adapter, then plug the other end of the adapter back into those original wire nuts, then the brown wire should be your C wire.

I don’t have a C wire at the thermostat. Isn’t that why I need the adapter?

I ran into the same situation with my 51 year old furnace. I was lucky though and actually had an unused 5th wire going to the thermostat. I found the other wire from the 24 VAC transformer in the furnace control box and connected it to the big red wire in this photo running up to the thermostat. So I was able to eliminate the C-wire adapter.

Yes, you need the adapter if you don’t have an extra wire going back to the thermostat to use as a C wire.

I also had some extra unused wires going back to my thermostat, so I used one for the C wire.

I want to use the adapter. My question is how do I wire it?

You will label the red, green, white, blue wires just like you labeled them behind the thermostat and remember what wire they were connected to in the wire nut. Then connect them to the included adapter. Then the pigtail end of the adapter will connect where those wires were in the wire nuts. Then if you get 24v to that brown wire, you will add the C wire on the pigtail to the wire nut with the brown wire. The adapter will basically be between the thermostat wires and the wire nuts.

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Assuming the blue wire in your thermostat bundle is actually a Y wire, C is likely to be the brown wire that feeds the red wire for the outdoor unit.

This is assuming you have an air conditioner, and that’s not a float switch for a boiler.

Also, the red wire leading to the outdoor unit looks to have a spot where the insulation has rubbed off or split, you should wrap it with electrical tape to prevent a short circuit.

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Yes. I can confirm that the blue wire would be “my” yellow wire.

And, yes…we have an A/C, not a boiler unit.

Thanks for the tip on taping the red wire. I’ll do that!

I’ve got time to work on this today. I’ll report back with how it goes!

“I get by with a little help from my friends”

Got it done! Everything works just fine. Thanks for the help!

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Glad you got it working!