2010 house and thermostat. White Rogers 1F80-361 controlling a lennox furnace. We have a fan switch located above our Tstat that will turn the furnace blower on and will also energize a make up air fan tied into the cold air return. The issue is that we had AC installed and now turning this switch energizes the AC unit as well. Fan switch is wired into G but looking at the wiring diagram it looks like when I energize the G contact it's running through the board and energizes the Y contact. Is there a way around this easily as if I run the furnace fan with the Tstat options it doesn't energize the make up air? Furnace wiring diagram

Update: looks like the tstat auto position and the y share a home. I was able To switch the fan mode to on which obviously runs the furnace fan but I could then use the external switch which also energizes the make up air without the ac turning on. As soon as the tstat switch goes back to auto the ac contractor pulls in.

Furnace model is g43uf-48c-090-08


enter image description hereenter image description hereenter image description hereSo fan only mode will operate the fan as it should. Using my fan switch will turn on the ac if it’s in heat or cool mode. It looks like the fan switch of the thermostat can be on power or cool according the the above schematic. What I think is happening is energizing G separately is also energizing Y as that’s where the switch is sitting when in auto mode. AC is on y and C My external switch goes through a 120/24V contact and then comes into the furnace and is on R and G I’ll try get a picture of the terminations but I’ll have to try shrink the file size.

Added the switch and make up setup. There is 120V going to the switch above the Tstat. It comes back down to the yellow hoyme box and gets swapped to 24V. From there out to the furnace and is terminated on R and G for fan.

I assume it’s what was mentioned and the tstat only has an auto or on for the fan. Being in auto means it’s common with the Y terminal.

  • hang in there, reading
    – Traveler
    Commented Nov 17, 2022 at 20:22
  • 1
    Looking at the schematic, it does not show the G and C are connected. G is always for Fan Only function. You mentioned to have a switch. Show me. I believe the AC installer messed up
    – Traveler
    Commented Nov 17, 2022 at 20:29
  • Can you post the model number of your furnace please? Commented Nov 18, 2022 at 5:05

2 Answers 2


If the room thermostat is in Cool mode, it will activate the Fan (normal function)

However if you activate the Fan only mode, only the Fan will run.

It looks like the AC installer messed up.

fan only


Most HVAC systems have "bang-bang" controls, meaning at a setpoint BANG they turn on and run full blast until they hit the desired setpoint then BANG they turn off. In these systems, the furnace does not know about the A/C unit and they don't talk to each other.

To control the furnace, the thermostat

  • shunts R(h) to W to "call for heat". The furnace takes the fan under local control, running it when the heat exchanger is warm enough.
  • shunts R(h) to G to "call for fan". This overrides the furnace's automatic fan control and runs the fan alone.

The A/C evaporator is installed in the furnace's air handling stack, so the furnace's blower is needed to circulate the cold air. Again in these bang-bang systems, the furnace has no idea the A/C unit even exists. So when the thermostat wants the A/C to run, it must simultaneously

  • shunt R(c) to Y to call for A/C compressor to run, and
  • shunt R(h) to G to call for fan.

It's common for this to be physically wired as a 4-wire (Red White Green Yellow) thermostat cable from furnace to thermostat with wire functions on the same letter as the first letter of the color name.... and then a 2-wire thermostat cable (Red White) carrying C and Y to the A/C unit. (thus, the A/C uses the furnace's 24V transformer).

Sometimes the A/C has its own transformer, that's why there is R(h) and R(c), in which case the A/C can run direct to the thermostat with R(c) and Y. Probably on a 2-wire cable that is red and white.

When people retrofit smart thermostats, they typically need a 5th wire, "C". (typically on the Cyan wire or blue). Often the thermostat was run on 4-wire cable and compromises must be made. Typically the "fan only" functionality is deleted, so G is empty on the thermostat... and Y on the thermostat is wired to both G on the furnace and Y on the A/C unit.

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