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I have hydronic radiator heat powered by a gas furnace situated in one mechanical room of my house and a currently being installed central AC whose air handler is situated in another mechanical room (separate locations).

I would like to control both of these units from a single thermostat with separate thermostat wires going from the thermostat to each respective unit.

  1. Is this possible? Do I need a special thermostat or can any modern thermostat support this split functionality?

  2. Product recommendations?

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2 Answers 2

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You can use any heat/cool thermostat on the market to do this. All is required that you run 2 separate 18/4 wires from each system to the thermostat.

R is what carries continuous 24v power to the thermostat. RH is continuous power for heating, and RC is contentious power for cooling. As long as you have constant 24v to these terminals, your system will work. Many modern thermostats have these built in to one terminal on the thermostats (will only show R). All the thermostat does is split the 24v continuous power to RC and RH at there thermostats control board.

From Furnace:

Connect R to R - If your thermostat has RH and RC you can either keep the jumper in, or attach it to just RH for heating. This does not matter as long as you have 24v constant from the unit.

Connect W to W For Heating

Connect G to G for the Fan

From A/c Air handler:

Connect R to R - you can use both R wires at same connection on thermostat if there is only an R terminal. If there is RH and RC like above, you can either leave the jumper in, or connect it to RC.

Connect Y to Y for Cooling

Connect G to G for the Fan

The reason for connecting the G terminal for only the air handler is because you will only really want to have the fan continuously running for the cooling season. Additionally, depending on the type of air handler you have, some control boards require the Y and G terminals to be energized together to run the Cooling and the Fan speed on high. If older units with older style control boards have just Y energized on a call for cooling, the unit will only run the outdoor condenser, which will cause issues with the system. Almost all thermostats will energize G and Y together to avoid this happening if there thermostat is used on a older system.

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This shouldn't be a problem, as long as you get a thermostat that supports separate heating and cooling power supplies. Any thermostat with both R, and Rc, and/or Rh should be able to handle what you want.

R = power (usually 24 VAC).
Rh = power from heating system.
Rc = power from cooling system.

The purpose of the separate Rh and Rc terminals, is so that the heating and cooling systems can each have their own separate transformer. In some cases there is only one transformer, so the Rc, Rh, and R are all connected together. When separate heating and cooling systems are used, Rc should come from the cooling system, and Rh should come from the heating system. Y Would be the cool call back to the cooling system, and W would be the heat call back to the heating system.

Installation

Pull 18/4 or 16/4 cable from the thermostat to the heater, and the same cable from the thermostat to the cooling system.

Cooling

  • Connect the red wire to the R terminal at the cooling system, and the Rc terminal on the thermostat.
  • Connect the white wire to the Y terminal at the cooling system, and the Y terminal on the thermostat.
  • (optional) Connect the green wire to the G terminal at the cooling system, and the G terminal on the thermostat.
  • Leave the black or blue wire disconnected.

Heating

  • Connect the red wire to the R terminal at the cooling system, and the Rh terminal on the thermostat.
  • Connect the white wire to the W terminal at the cooling system, and the W terminal on the thermostat.
  • (optional) Connect the green wire to the G terminal at the cooling system, and the G terminal on the thermostat.
  • Leave the black or blue wire disconnected.
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I hope this can clarify a little bit. Even if the thermostat has only an R terminal on it, you can still use it in this case. RH and RC serve no purpose but to deliver 24v to there thermostat. That is why most new thermostats on the marked will only have an R terminal on them. –  Mnc123 Dec 29 '13 at 22:21

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