Hey so I know this is annoying and asked way too often but the wife wants a WiFi thermostat and of course I don’t have a “c” wire hooked up and there is no “c” terminal on my control board. We have two zones and as of now I’m only taking care of the one. My question is can anyone help me find a place to hook the c wire to? It’s a 2008 Weil McLain (spelling) pictures are below enter image description here enter image description hereenter image description hereenter image description hereenter image description hereenter image description hereenter image description here enter image description here

So here is the wire diagram for the zone controls Sorry it’s sideways enter image description hereenter image description hereenter image description hereenter image description hereenter image description here

  • 1
    The fourth photo, "boiler wiring" is partial and poorly focused. Please upload a photo that shows that label clearly and completely. Please also upload photos of the thermostat and the wiring on the thermostat mounting plate if it has one. Include if possible unused wires that are part of the cable to the thermostat, if any.
    – jay613
    Oct 21 '21 at 3:47
  • Guessing you intended to upload several photos but accidentally uploaded the same one several times? Also, note, the zone valves each have their own internal transformer and their external wiring does not provide access to 24V Common. That's unfortunate. Still not clear what the voltage is on the boiler's own circuitry. EG what is the T-T voltage when open?
    – jay613
    Oct 21 '21 at 15:12

This looks like it uses millivolt style signaling to turn on, and thats not compatible with many of the fancy, popular brands of smart thermostats (Nest, Ecobee).

A typical HVAC unit sends 24 volts to the thermostat and the thermostat switches that voltage to the various wires (R, G, W, Y) to control different parts of the system. These old heaters don't supply any voltage (very little voltage) and rely on the thermostat to just be a basic contact closure.

If you are electrically inclined, you can make any thermostat work with the addition of an external relay, but it's probably best to look for a wifi thermostat that is compatible with millivolt. These units typically have a separate power adapter that would need to be plugged in because WiFi is a bit much for a few AA batteries to handle.

  • Why do you think this is millivolt?
    – jay613
    Oct 21 '21 at 3:50
  • @jay613 The bits of the wiring diagrams uploaded don't show a 24 transformer, and the thermostat is connected to two "T" terminals. It might not be a "traditional" millivolt system that is powered by a thermopile, but it seems to just need a contact closure style thermostat that doesn't receive 24v. If the asker wants to verify, they could measure the voltage between the two thermostat wires, but systems like this usually don't use 24v.
    – JPhi1618
    Oct 21 '21 at 3:59
  • Agreed we don't see a transformer but the pictures are partial and fuzzy. I do see 120V supply and an LED display so there may be a transformer. If there isn't a thermopile there must be something .... a contact closure has to have some voltage. I'm waiting for more info!
    – jay613
    Oct 21 '21 at 4:48
  • So I found a diagram sorry it’s sideways for the zone controls Honeywell R845A1030 Honeywell Switching Relay amazon.com/dp/B000979EWE/… sorry it’s sideways
    – Bri mac
    Oct 21 '21 at 11:29
  • @Brimac -- can you post photos showing the controls of your boiler and the posted wiring diagram more completely please? Oct 21 '21 at 12:05

Adding to @JPhi1618, here is a guide on converting millivolt to 24v https://hvacrschool.com/milivolt-systems-w-modern-thermostats/. You’ll need a 24v spst normally open relay, a 120vac to 24vac transformer, and at least 3 wires running to the thermostat. Here is an overview image:

enter image description here

Finally, I must state that I am not a professional, and make sure to read the article and confirm that what you are doing will work with your setup. If you do not feel confident, then I would recommend you get a millivolt thermostat as JPhi1618 recommended. Even if you do feel confident, proceed at your own risk.

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.