My house which was purchased from a bank (foreclosure) was originally wired with both central air and heater (forced air). But when the previous owner left, he took the air conditioning unit. The heating unit (a furnace forced air in the attic) stayed. No modification was made to the wiring as far as I know. Currently the existing thermostat has difficulty with some settings especially cool/fan (ends up blowing hot air) and heating up the house.

I am ready to switch the existing thermostat to Ecobee 3 lite and wonder if there is anything I should change in the thermostat wiring now that the AC unit is no longer connected.

  • Can you post photos of the existing wiring at both ends? Feb 7 '19 at 12:39
  • So the prior owner removed the condensing unit (compressor unit outside), did he also remove the evaporator unit inside (aka A-coil, refrigerator coils? What I am wondering about is how costly would it be to restore the a/c function. If you circulate air with the blower fan through the ducting, you will be heating the house in summer because ducting in the hot attic will allow heat flow into the air as it passes through them. Feb 7 '19 at 12:49
  • The previous owner removed both the condenser and the evaporator units. There is a copper coil hanging outside the house where the evaporator (i magine) used to be. So you are right, the house gets heated in the summer. So far we switch off the thermostat as we have no need for it in the summer time, but I realize that in the winter time when ever we set the the thermostat to “fan or cool” by error, it heats the house instead. Feb 8 '19 at 13:36

For now the only wires that are needed are Rh, W and C. The G terminal operates the fan. It sounds like you don't want to be operating the fan. You can simply connect the G terminal and use the fan at your discretion. Leave the Y terminal disconnected. If and when you want A/C you simply pull the stat off the wall connect Y and reinitialize the ecobee. When they start they check what wires are connected, and if you have a wire connected to it that doesn't go anywhere it may cause minor problems for the ecobee.

In regards to the A/C replacement referring to Jim Stewart's comment. A new air conditioner will be using 410a refrigerant. You don't want to reuse the old copper lines because

  1. Lines used on r22 are not compatible with 410a

  2. If the old unit was 410a, the lines need to be sealed from atmosphere when disconnected.

  3. Flush kits are available to clean the lines (I would never use one, I always replace the lines)

So the cost of replacing it would be very close to the cost of a new installation. The electrical has been run. But it may not be done right.


I would setup the Ecobee as if you had the AC unit. It will try to turn on the AC unit and operate as if it was there. I have mine (Ecobee 4) hooked up this way. My AC unit has died and I am not ready to buy a new system, so in the summer, I just turn off the thermostat and run my portable AC units. If the wiring is there, I would jsut hook it up as if the AC unit was installed, that way when or if you replace the AC unit, its ready to go and no re-wiring needs to be done. Even if you forget to turn the thermostat off, the unit will turn it on, but nothing will work, except maybe your furnace blower. Not a bad thing unless it is blowing warm air around the house!

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.