I am wiring up a new thermostat for an old boiler, and the thermostat I am using needs a common wire (Proliphix NT130h -- I wanted an NT1X0e series, which use PoE and don't need C, but they are no longer available).

The system uses a Honeywell "aquastat" controller that has an integrated transformer hooked up to Rh and W, so there's no way to add a common C to that transformer (short of what this guy did with taking apart the control unit and soldering a wire to the back of the board).

Here's what it looks like (though this is a much newer model, it looks pretty much the same): control unit

Luckily, I have an additional 24VAC transformer right next to the boiler that I can use (it was meant to be used to power the thermostat for an air conditioning unit, but that unit was never installed, only the thermostat power and wires. I know that I can theoretically use that transformer to power the thermostat by hooking it up to Rc and C. However, the installation manual notes that C must be in phase with Rh, which is not the case in my installation which has C in phase with Rc.

Since I don't think it will work to use only a single wire from the transformer (even though the manual says "in phase", I assume that it actually means "complete circuit"), I am trying to figure out another way to do this.

What I would like to do is hook up the heat through a relay. That way, I can send power to the thermostat without needing separate Rh and Rc (in fact, since there is no AC, without needing Rc at all).

As I envision it, when the thermostat calls for heat, it will connect Rh to W which will connect the trigger side of the relay, which will close the circuit on the heater wires that used to go to Rh and W.

I have two concerns:

  1. Is there any special relay I need, or will anything I find designed for low voltage work?
  2. Do I need to worry that either side of the relay won't have enough of a load on it, possibly causing the equivalent of a short circuit on either of the transformers?

1 Answer 1


I talked to a local HVAC repairman, and he said it should work, so I did it, and it works.

Here's what the circuit looks like:

circuit diagram

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