I had to borrow the Nest thermostat from one of my units for another one where it was more needed, while a replacement is on the way. The unit from which I borrowed is combined HVAC (heat+cool) with an electric furnace/handler and heat pump outside. Here is how it was wired before I took the Nest out:

enter image description here

As you can see, there is only one of the R wires and it goes into Rh.

While I am waiting for a replacement, I would like to have AC accessible by hardwiring the adequate wires and turn it on and off using the handler's breaker on the main panel. I thought I would just hardwire

R (power)
Y (compressor)
G (fan)

which worked but started blowing warm air, no AC. The equipment worked fine until I removed the thermostat. In fact, that is exactly how I had it hardwired in my other property where the system does cooling only (heat coming from a radiator hydronic system). Do I need to wire differently here in order to get cool air because this system is combined, unlike in the other unit?

2 Answers 2


A heat pump system has a valve that switches the system from heat to cool. This is typically controlled by an orange wire, and based on how your nest was wired, it looks like your system follows that convention.

I'm not sure how common each type is, but some systems default to heat, and some default to cooling. You hooked up the R wire to the Y and G and you got hot air, so we can assume yours defaults to heat. Add the orange wire to your bundle and you should get cool air. You need to wait some time when switching from heat to cool, so let the system set for 15-20 minutes if you've been messing with it and got hot air recently.

  • 1
    so are you saying R+G+Y**+Orange** ?
    – amphibient
    Jul 25, 2019 at 17:19
  • 1
    R to G will turn on the fan. R to Y will turn on the compressor. R to O will switch the valve to cooling mode. So, for the unit to be "hardwired" for cooling, R needs to be attached to Y, G, and O at the same time.
    – JPhi1618
    Jul 25, 2019 at 17:21
  • Thank you, that was it: R+G+Y+O
    – amphibient
    Jul 25, 2019 at 20:49

A great way to be sure is to check out your unit's control board. Don't forget to take a picture (always good to have for future reference) and especially if the board is in an uncomfortable spot like mine was in a dirty stinky basement.

The color wires are never REALLY guaranteed to be connected in any particular way so if you can compare what you see on the control board to what's coming out of the wall for your thermostat you should be able to connect correctly.

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. Jul 25, 2019 at 20:35

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