Nest thermostat wiring

[Robert-123]

We have a boiler heating system and a separate AC system, both about 10 years old. The two systems are controlled by the same thermostat which I am looking to replace with the Nest thermostat. It's currently an old Honeywell Q539A. The wires are labeled "Rh", "G", "R", "Y", and "G". According to the compatibility tool, the system is not compatible, which seems to be due to the extra "R". However, if that "R" is really an "Rc" (for cooling), then the Nest would be compatible. 

Am I correct in assuming that the wire labeled on the old thermostat as "R" is really an "Rc" and that the system is therefore compatible?

[aperezdeleon]

Hi Robert-123,

Thanks for posting in our community.

Getting the right wiring configuration for your Nest Thermostat can be confusing at times; don’t worry though; help is here!

Since you’ve mentioned you have two pieces of equipment that control the temperature in your home (a boiler and an AC), your system would have to be wired in the Nest Thermostat as a conventional system; however, for heating to kick in, the Nest Thermostat would need a dedicated wire for triggering the boiler; in this scenario, a W wire is needed, as the Y wire in your current setup would only turn on cooling.

Some systems, called dual transformer systems, use separate power sources for heating and cooling (Rc and Rh). If you have both an Rh and an Rc wire, my suggestion is to check this article to get installation help from a professional. They'll find out if a W wire can be added to your system as well, to make it Nest Learning Thermostat compatible.

Let me know if you have any other questions.

Regards,

Angel.