Hello plouie10, this is a bit of a tricky one. The Heat pump is the primary cooling and heating. Since you also have gas heat you have a choice. So, you can set up the heat pump to be a cooling only unit, and set the heat source as gas. This means you have a split system as well. You will a thermostat wire cable going to the air handler and a cable going for the air handler to the outdoor unit.
if you choose gas heating as primary heat, your nest will be set up as AC with Gas Heat. The colors of the wires used depends on your original installation, so make sure you make a copy of each wire that’s on your old thermostat back plate what color wire is connected to which screw terminal. The typical wires needed will be R, G, Y, W, C and possibly O/B. R is for the incoming power, typically RED wire. The Y wire is used to call for Cooling. It is typically Yellow. The W wire is for calling for Heat, in your case Gas Heat and is typically White wire. The G is for the Blower fan. It is typically Green. The C wire is the Common wire and is the complement of the R terminal. This is not always present in old systems. If it is in use on you old thermostat, we will definitely want it use it with the Nest. Now the O/B wire is what activate the 4-way reversing valve in the outdoor unit. This is what changes heat pump from cool to heat or heat to cool. And here is the sort of hard part. Some systems need to energize that reversing valve to be cooling and some systems only need to energize it to be heating. In other words the system either defaults to be a cooling system, or default to be a heating system, and the wire is used to change modes.
if you choose gas as your primary heat, we need to decide if we need to make sure that the reversing valve is energized in order to get cooling in your home. If your system defaults to cooling, we will not need whatever wire is connected to the O/B in your old system.
Now, let’s say you want to use your heat pump for heating and cooling and you want to use your gas heat as your auxiliary or emergency heat. you will still need all of those very same wires only your nest will be set up as a heat pump system, and you will have an auxiliary or emergency heat, and that’s where you will put the W wire. now what is auxiliary or emergency heat. Durning winter the heat pump is taking the heat from outside and brings it inside, on very cold days/nights the outdoor unit will start icing up, it will freeze, and it must go into a defrost mode. now, during defrost mode the auxiliary heat/emergency heat might be turned on. Now in this system configuration when that happens your gas heat will turn on when your heat pump is defrosting.
The decision is going to be yours, but it looks like chances are, the previous owner was set up to use AC cooling from the heat pump, and gas for the primary heat of the house.
I hope this helps, if you have questions, please simply ask.
Ken, The Cooling Wizard
NestPro, Google Pro, Mechanical Engineer and HVAC service company owner.
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