OK so here is the scenario.

Upgrading thermostat from dumb to smart (Ecobee). I've done 2 others in my house that required re-wiring so I have a good sense for what should be done but I'm stuck.

My furnace is a gas fired boiler with 2 zones (honeywell synchron) valve system that is connected to two separate thermostats. Upper level is Heat + AC so stat uses the common wire coming from the A/C unit. Lower level is heat only, so I need to run a C from furnace to thermostat to provide 24V continuity power. I've run all the wiring but so far can not get the ecobee to power up, tested the old dumb thermo and it works so my issue is C wire.

The wiring is: Board R, W -> split before zone valves -> R, W into zone valves and then back out -> into the 24v transformer where it ties into R,W out to the thermostat.

I opened the board for the furnace, located the C, tested the voltage at the board (read 27v between C and R), connected a spare 18 awg from the existing thermostat wiring to the C and ran up to the Stat. The Ecobee did not power on and the voltage between C and R at the stat is only ~10v.

I did a few voltage tests along the way and it looks like pre-syncron valves the voltage is 27v between C and R, after the white and red go into the valves and come back out the voltage measures ~10v between C and R.

Wondering if 1. anyone has experienced this before and 2. if anyone has any suggestions as how to proceed? Wondering if somewhere on the valves maybe I need to ground the C as right now i'm just bypassing the valves and taking the C straight from the board to the thermostat?

Any help would be greatly appreciated.


  • I casually looked at a wiring diagram for what I think is a Synchron valve, and I don't see where a transformer wire would go "in and then out". There are two screws that the transformer wire goes to and anything else that needs 24vac would connect to the transformer in parallel - not be connected to any "output" of the valve. Maybe some part numbers or pictures would help. – JPhi1618 Mar 18 '19 at 14:43
  • Also, it's not clear from your description that you know this, but the "C wire" is just "the other side of the transformer". The transformer has two wires coming out of it (on the 24v side). One of those turns into the "red" wire for the control circuits and the other wire is the C or Common wire. – JPhi1618 Mar 18 '19 at 14:45
  • Can you post the model numbers of the parts involved, as well as photos of the wiring? – ThreePhaseEel Mar 18 '19 at 23:23
  • Just following up on my original post. The root cause was the valves were wired backward originally, i.e. yellow wires out of valves went to the R on transformer and R from Stat.. I swapped them to the C on the transformer and the W from the stat, spliced a C wire also to the C on the transformer and connected the R from the stat to the R on the transformer. Everything now works great and there is power to the stat... Thanks to google and this link that helped me figure it out doityourself.com/forum/thermostatic-controls/… – Jack Mar 20 '19 at 12:55

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