I'm trying to connect an ac compressor to an old Lennox G12Q3-110-3 furnace. Tried it a couple of ways and can either get the compressor to turn on or the furnace fan. Not both. Any ideas on how to connect it correctlyenter image description here

this is the schematics on the furnace.

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wiring at the thermostat.

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Inside the furnace. Bottom two left wires, white and red are from the ac compressor. Yellow goes to Y on thermostat.

  • That green pigtailed into the 2 yellows seems wrong based on the diagram.
    – MonkeyZeus
    Jun 11 at 16:36
  • yeah, that's what i'm thinking too. The green is just a ground. Going to a screw in metal on the right of the transformer.
    – mike
    Jun 11 at 16:40
  • You need to add the A/C schematics. Why didn't your installer wire all of this for you? Note: your furnace seems to be non-communicating but if your A/C is communicating then the wiring could get even trickier. I had a journeyman install and braze my line sets from the furnace to the A/C. He did t-stat wiring for me since I wasn't sure about what I was doing. Sadly, it didn't work. I had to read the manuals a few times, do some trial-and-error, blow a furnace fuse, read the manuals a few more times, and then I finally got it working. I do not blame the guy one bit, fyi.
    – MonkeyZeus
    Jun 11 at 16:45

Green should be the fan in the air handler. the yellow is for the outside unit.

If no separate control board and it is properly wired you may have to put the green and yellow on the yellow this will turn the air handler on when the compressor is energized.

You don’t want to jumper green to yellow because that would turn the compressor on when the fan only was activated at the thermostat.

I have done this in the past for “mutt” systems guys use other words but mixed components systems mutt or Hines57 are common not so offensive names.

  • if i separate the two yellow wires, nothing works. only thing i can do is get the fan to turn on, or ac compressor. Depending which wires i connect together.
    – mike
    Jun 15 at 15:06

The W, R, G from the thermostat appear in your photo to be correct according to the furnace’s diagram.

Compressor Red goes to Thermostat Yellow. Compressor White goes to Furnace Yellow.

Thermostat yellow is the thin one bottom right.

It’s unclear which is “Furnace Yellow” as there are three. Two in your hand and a thicker one nearby. The diagram implies there is a terminal block with 5 terminals. But I don’t see one and the blue wire comes straight out of the transformer coils, so it's hard to identify which wires are the ones referred to by the diagram. I cannot tell what the other two yellows, and the green in your hand are for. They are not in the diagram. That green is not the thermostat G wire.

More photos might help. If you follow the Blue, White, and Brown wires that are (correctly) connected to the thermostat, do they go to a terminal block or to a place where they can be clearly identified and grouped with a particular yellow wire?

Here's another Question (that one about C wires) with a better photo of what I think is a similar furnace, and a more complete diagram. It looks like the correct yellow for the compressor white is the one in your hand. It's shown in the upper right part of the diagram that is missing from your photo.

However I still can't tell what the green wire in your hand is!

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