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Question about wiring "ecobee3 lite" to a zone-valve hydronic boiler

Matty_Nativeseed Member Posts: 3
edited November 2021 in Thermostats and Controls
Hi all! Nice to finally meet your acquaintance. I have been lurking around here for a while, using this forum very successfully to solve some previous issues. The amount of knowledge here is incredible. Now that I’ve hit an issue that I can’t solve by lurking, I figured I’d start by posting here!

My electricity/gas company recently offered us some free smart thermostats to try to save energy, so I snagged two “ecobee 3 lites” and wired them up. One of them has had no issues thus far, but the other keeps rebooting. I read online that this is a common issue, often caused by faulty wiring in the R or C wires that prevent constant power, or sometimes by the furnace intermittently shutting off power to the thermostat as part of an over-temperature protection.

Some background: My heating system is a hydronic baseboard, natural gas boiler (Burnham Series 2). I have three zones, each controlled by a Honeywell V8043E1012 valve, which are powered by a Honeywell AT72D1683 24VAC transformer. There is also a separate AC system that is driven by one of the thermostats.

The ecobee that has not had any issues is the one that also controls the AC. There was a C wire associated with the AC system which I wired up to the ecobee and it seems to work fine. However, the ecobee which has been having issues is the one that controls heat only. Previously, that thermostat was 2-wire only (R and W). However, the thermostat wire did have an unused third wire, so I attached that to the neutral part of the 24VAC and used it as a common.

You can see my system’s “before” wiring here:

And here is the “after” wiring that I implemented:

I thought the wiring solution was pretty straightforward, yet it seems to be having issues consistently power the ecobee. I’ve double checked all the connections/wire-nutting and don’t see any things that I think would cause a bad connection. I’ve also double-checked the load calculations on the transformer, and it doesn’t seem like it should be an issues. Each zone valve draws 7.7 VA, and the ecobee3 is supposed to draw 3.5. That’s a total of 26.6 VA, which I thought would have left me enough headroom to the transformers rated 40 VA. However, do these thing wane in capacity as they age? How would I test that?

The other common culprit – the heater intermittently shutting off power as part of a over-temperature protection – I suppose could be possible. However, most of the things I’ve read online talk about this in the context of a furnace, with recommendations of cleaning filters and ductwork. Obviously that is not applicable to me. How would I go about testing whether this is the issue, and if it is, how would I go about fixing it?

In summary, my three main questions are this:
1) Does anything look incorrect about how I’ve chosen to wire up the system?
2) Could the 26.6 VA load be too close to the rated 40 VA if the transformer is aging? How would I test this?
3) How would I test whether my boiler is shutting off power as part of an over-temperature protection? If it is, how would I solve this underlying problem?

Thank you so much for any help, and I’m happy to answer any other questions that pop up!



  • Matty_Nativeseed
    After poking around some more, should the t-stat's (R) be the wire that is continually connected to the 24VAC, and the (W) be the one that is attached to the yellow zone valve wire? I can see that the way it was maybe wouldnt affect valve operation, but it would affect the ecobee's ability to get constant 24V
  • Jamie Hall
    Jamie Hall Member Posts: 23,433
    Unless I am mistaken -- quite possible! -- The transformer should feed C and R. W then goes back to one terminal of the zone valve, and the other terminal of the zone valve goes back to C W vs. R doesn't matter on a two wire thermostat -- but it does on a three wire one.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Alan (California Radiant) Forbes
    @Jamie Hall said:

    Unless I am mistaken -- quite possible! -- The transformer should feed C and R. W then goes back to one terminal of the zone valve, and the other terminal of the zone valve goes back to C W vs. R doesn't matter on a two wire thermostat -- but it does on a three wire one.
    I agree.

    "R" from the transformer should go to "R" on the thermostat, otherwise it's like connecting a lightbulb to only the hot wire; it needs a neutral as well. "W" should then go to open the zone valve.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
    EBEBRATT-Ed Member Posts: 15,616

    Something like this
  • Erin Holohan Haskell
    Erin Holohan Haskell Member, Moderator, Administrator Posts: 2,326
    Welcome to Heating Help, @Matty_Nativeseed. I've merged your two posts into one here.


  • Matty_Nativeseed
    Thank you Erin! Sorry for reposting. I thought the first one got deleted when I tried to edit some typos too quickly after posting. But I guess it was just awaiting moderation.

    And thank you everyone else for the clear explanation -- that was indeed the issue and it is now working great!
    Erin Holohan Haskell
  • Zman
    Zman Member Posts: 7,573


    Something like this

    This is your solution. Your valves were originally wired backwards and it did not matter until you introduced the need for constant power and a "C" at the t-stat.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein