• 2 lugs on boiler control, one I'll call "red" or "R" that comes from the built-in transformer with ~14v and one I'll call "white" or "W" which, when jumpered to R, turns on the boiler. Very typical old system, except weird voltage (maybe?)

  • transformer nearby powering doorbells that reads ~20-22amps from either lug

  • old thermostat with 2 wires, red and white.

What I've tried: - run new wire (I'll call "common" or "C") from doorbell transformer to thermostat location.

  • connect R from boiler to Rh on thermostat, W from boiler to W1 on thermostat, C from doorbell transformer to C on thermostat. NO POWER UP of thermostat

  • connect R from boiler to Rc on thermostat, W from boiler to W1 on thermostat, C from doorbell transformer to C on thermostat. NO POWER UP of thermostat

I am going to buy a 24v transformer locally and try the same as above swapping it for the doorbell transformer (which is cryptically marked, but may be 16v, since apparently that is a common voltage with doorbells).

My concern is that if the idea is that the 24v transformer is simply serving as the common, the 14v coming from the R lug on the boiler is not enough power to power the ecobee. On the other hand, I could:

  • run both wires from the separate transformer to the ecobee call them "R from separate transformer" and "C"

  • connect R from separate transformer to Rh on thermostat, connect W to W1 on thermostat, and C from separate transformer to C on thermostat

The concern I have is that the boiler W lug is currently only getting 14v from its own transformer when the thermostat turns on, would upping that to 24v from the external transformer be a problem?

This feels like a weird issue where my boiler switch requires one voltage and the thermostat requires a different voltage. Is it possible that the boiler's internal transformer is just old and weak? That using a new 24v transformer to signal the boiler to start would be ok?

Hope I've explained clearly. I can take and add pictures if needed.

edit: I've sifted through many similar questions, but couldn't figure out if this particular question had already been answered

  • Can you provide a make/model and photos of the boiler and its control unit? Wiring diagrams would be even better Dec 22, 2016 at 23:24
  • @ThreePhaseEel I'll get that and add it--after Christmas :)
    – snuggles
    Dec 23, 2016 at 14:55

2 Answers 2


Install both a separate 24v transformer and an isolation relay with a 24vac coil at the boiler. Use the separate transformer to power the Ecobee3 thermostat and use the Ecobee3 to energize the new 24vac relay coil. Use the new relay to provide a set of dry contacts for switching the existing 12v controls (nominal) on the boiler whenever the Ecobee3 energizes the relay. Keep the new 24v system electrically isolated from the existing 12v system.

  • Sigh. Yeah that's what I was afraid of. Just to clarify: does it actually make any sense that the boiler controls is using 12v? I have never heard of it, but my experience is limited. I'm just wondering if either I'm mis-reading the hot lead (unlikely, but possible), or something like that. Anyways, I'll try to track down a 24v to 12v isolation relay.
    – snuggles
    Dec 23, 2016 at 14:59
  • I do not know anything about your particular boiler, so I cannot comment on the 12v control for that. Relays are readily available and easy to install. A fan relay with a 24v coil or anything similar will do. They are stock items at HVAC supply houses. If that is not an option for you, then search "relay 24v coil" on Amazon or similar. Relays are at least as easy to find as a new 24v control transformer (PM44450 on Amazon or similar).
    – user39367
    Dec 23, 2016 at 15:27
  • On second thought, the PM44450 has a fuse instead of a circuit breaker. I recommend a transformer with a built-in circuit breaker instead. They are more convenient. (Honeywell AT175F1023 or similar)
    – user39367
    Dec 23, 2016 at 15:49
  • just to clarify: I ended up switching the boiler control to a new Carlin one, which has 24v coming from it's T/T lugs. But this answer would have worked with the old setup, too. A point of clarity: even if you have 24v coming from your boiler, you want to electrically isolate the ecobee from the boiler. A relay does that. They are cheap (<$10), and extremely easy to get working.
    – snuggles
    Dec 30, 2016 at 16:51

I was having the same problem...until I switched my multimeter to ACV!!!

I had purchased a fast stat common maker. I was measuring 12V across the thermostat terminals and R and C with my multimeter set to DCV, and wondering where I was going to get 24V from. I found a 24V DC transformer and hooked that up to R and C thinking it would work but of course, no. Honestly, I played with it for hours, then it dawned on me that I must be doing something wrong.

Nothing was working, ecobee wasn't happy.

Switched multimeter to ACV, and found that lo and behold, R and C on the boiler does give 24V. rewired everything and from then on everything worked perfectly.

good luck!

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