I have a Laars gas boiler that has 3 heating zones. I want to replace the old mercury honeywell tstat on the 1st floor (heat only) with a newer honeywell RTH9580 wifi model. I've taken pictures of my entire system, labeled the wires in the pics to indicate the associated connections, and included the Laars wiring diagram. There is a transformer mounted on the wall near the boiler that is wired in-line to the system (power from the panel --> into the emergency boiler switch --> into the wall transformer --> then to the boiler (low water cut off and boiler circuitry). There is also a transformer within the boiler circuitry itself. Can someone tell me the best method (proper preferred, not necessarily easiest) to connect the C wire? 1 - Power, Switch, and Wall Transformer 2 - Low Water Cut Off 3 - Boiler 4 - Boiler Circuitry 5 - Terminal Strip 6 - Zone Valve  and Thermostat Wiring 7 - 1st Floor thermostat wiring

  • Possible duplicate How do I identify the C terminal on my HVAC?. I'm leaving this open because you have two separate transformers.
    – BMitch
    Commented Sep 16, 2014 at 18:44
  • The "How do I identify the C terminal on my HVAC?" was the post I made with a question for connecting/configuring my AC unit. This one here is for my 1st floor thermostat on my boiler only. Thanks BMitch.
    – Evan
    Commented Sep 16, 2014 at 23:18
  • When you say "24 Volts when tested" and "0 Volts when tested". How are you testing? The Y lead is grounded out in the schematic, so you will get 0 Volts from Y to ground. You should get 24 Volts between Y and R.
    – Tester101
    Commented Sep 20, 2014 at 20:24
  • Ok the way we wired this up, with the doodad and all…has the wire going to the C terminal coming from the Y, and this leaves the thermostat unpowered.
    – Evan
    Commented Sep 20, 2014 at 20:36
  • Googling around I found this person had the same issue with the same attempts:justanswer.com/hvac/…
    – Evan
    Commented Sep 20, 2014 at 21:04

2 Answers 2


Okay... After reviewing your new photos, I think I have it figured out.

This is what your system looks like

Boiler wiring
You'll have to excuse the glitter pens, it's the only thing I could find on my wife's desk.

It's pretty simple actually. When a thermostat calls for heat, the valve for that zone opens, and the boiler is also signaled via the auxiliary switch in the valve.

To get a C wire to one or more of the thermostats, you'll simply run a wire from the other side of the transformer on the wall.

New boiler wiring

NOTE: If this diagram is accurate, the white wire to the thermostat is R, while the red wire is W.

The original wire from the transformer should be connected to the R terminal on the thermostat, while the new C wire should be connected to the C terminal on the new thermostat. This should supply power to the thermostat, and allow it to operate as normal.

The way your thermostat is currently wired, you'll have to swap the white and red wires to get it to work. So white will go to R, and red will go to W.

To make this fit a more common wiring style, I'd probably rewire it a bit. So that the red wires going to the thermostat, connected the transformer and the R terminals. And the white wires connected from the zone valves to the W terminals on the thermostats.

Rewired to fit a more common style.

Then you'll run the blue wire from the white transformer lead, up to the C terminal on the thermostat.

Rewired with C

  • 1
    HVAC control circuits are usually fairly simple.
    – Tester101
    Commented Sep 20, 2014 at 22:40
  • no dice. The wiring at the thermostat is normal - red to the R, and white to the W. Should I try swapping them? What about connect the C wire from the thermostat to the red wire coming off the wall transformer? It seems as though the wall transformer wires should be switched, no?
    – Evan
    Commented Sep 20, 2014 at 22:40
  • 1
    I'd try swapping them. White to R, red to W, and your new wire to `C. Do you get 24 Volts between the white wire and the new wire at the thermostat?
    – Tester101
    Commented Sep 20, 2014 at 23:05
  • 1
    ok will do and will get back with you…have to put this down for a bit or my wife won't put out tonight.
    – Evan
    Commented Sep 20, 2014 at 23:28
  • 1
    That was it! I switched the R and W wires and it works perfectly. I appreciate all of your help. In this case it looks like poor workmanship on the installer's behalf was the problem. I can't express how grateful I am for your time and effort with this Tester101.
    – Evan
    Commented Sep 21, 2014 at 20:36

For others looking at this post years later...I just spent a night researching this. Seems like this worked for the OP, but watch out on steam or other gravity fed systems. For a simple single zone steam system at least, the thermostat switch is just another switch in line (in series) with all the other safety cutoff switches and the pressuretrol as well. What that means is that

1) your R wire might not always be hot for this to work. On my Utica boiler, for example, the pressuretrol is between the 24vac power and the R wire. So my R is only hot when the heat is on (not helpful to power a thermostat to turn it on..but if a catch 22)

2) Even if it is, it could draw current through other things such as an electric damper or even the gas valve if it's really stupidly wired in order to power your fancy Wi-Fi thermostat. That causes problems for those devices AND causes a voltage drop at the thermostat (although I suspect it can handle a bit as it likely regulates that down anyway).

All that said, the right way on these systems where the thermostat switch is hard wired is to install an isolation relay. Ecobee had a good FAQ on that here: https://support.ecobee.com/hc/en-us/articles/227874527-ecobee3-with-add-in-Isolation-relay

You also may be able to run a 4th wire with the constant power if you're heat only and split Rh and Rc. Emerson has an article about that using a wall wart, but same concept would apply to running a fourth wire straight to the transformer on the boiler. Not sure if this is common for most Wi-Fi thermostats or not...it relies on then sourcing power from Rc only and Rh just being a dumb switch. https://sensicomfort.com/support/article/adding-a-24-vac-external-transformer

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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