I have separate heating (hot water) and air handler (cooling System). The current thermostat I'm using is a Honeywell RTH2300B and the new one I want to install is a Honeywell Wifi RTH6580WF1001.

The current system allows me to toggle between heat and cooling and so will the new one but the old thermostat didn't require a common wire but the New one does. Both systems have 24 volt power and I have an extra wire that I can use for that but the question is witch system do I take The 24 volt power from.

I would like to take it from my boiler as it is also my water heater so it is powered on all year round but I can leave power to my air handler on also.

  • The Green -G wire goes to the G on the Air Handler
  • The Yellow -Y wire goes to the Y terminal on the Air Handler.
  • The White -W wire goes to a zone valve on the Boiler.
  • The Red-R wire goes to a zone valve on the Boiler.
  • The Brown -Rc Terminal wire goes to the R terminal on the Air Handler.

Can anybody help me?

Current Thermostat wiring

  • 2
    Can you post pictures of what's going on at the air handler? Jan 25, 2018 at 12:39
  • Are there multiple cables, or is it a single cable to the thermostat that is split somewhere else?
    – Tester101
    Jan 25, 2018 at 13:43

2 Answers 2


Honeywell thermostats (also Nest, and probably others) require the C wire to come from the cooling system, in systems that have multiple transformers.


Sure fire way to make it work is an isolation relay. Sorry for the bad drawing I'm out of paper. enter image description here isolation relays look like this as well as other styles. enter image description here Air handlers often have one built in to send a signal to the boiler just for this very reason but I'd need more information the describe to you how to connect it. Another thing you can do is figure out the polarity of both transformers and ground both commons then common is the same on either one. It's a bit tricky, it involves applying line voltage to the secondary side and measuring if the voltage is additive or subtractive.

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