2

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?
2

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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.