I have a tri-level home, 3 zones with 3 individual thermostats. This is a pretty old school system. The home was built in 1969.
The mid level seems to run constant, even when the thermostat is turned completely down where it shouldn't be running. Temps have exceeded 80. I thought it might be time to switch out the old mercury t-stat to a digital one. I purchased a honeywell programable one that I thought would work.
Behind the old switch were 2 green wires twisted together and around one terminal (I think W) and a green and white wire twisted together and around the R terminal enter image description here What I am getting from the instructions is that I need to connect to the Rh and W terminals. Since I don't have air, fan and it's not a smart t-stat, where do I connect the remaining 2 wires? I assume they don't stay twisted together and in the same terminals. Any help would be appreciated.
Also, if this isn't the solution to maintain a regular temperature on our main floor zone, what can I do to fix it? Thank you!

  • Did removing the thermostat make the heat stop running? Commented Mar 13 at 3:31
  • Weirdly, it did not. It only stops when we flip the switch at the unit
    – Debi
    Commented Mar 13 at 6:04
  • I'd troubleshoot at the unit then before replacing the thermostat Commented Mar 13 at 11:46
  • Is the pump running when the thermostat is off and the other zones are not calling for heat? If so, you have an electrical problem - if not, it's thermosyphoning.
    – Ecnerwal
    Commented Mar 13 at 14:23

2 Answers 2


Most likely (since removing the thermostat didn't stop it) you have thermosyphoning causing heat delivery with no pump running. That's a pretty common installation defect for boiler systems. Hot water from the boiler in the basement rises into whichever zone is "easiest" and cold water falls from there back to the boiler.

Typically this means you need some sort of heat trap to stop the thermosyphoning while permitting pumped flow. Spring check valves and deep U bends are two common types of heat traps.

Alternatively, you have a pump running and/or a zone valve open when they should not be, but that's more unusual and requires an electrical defect.

  • Thank you very much. This is officially over my head...lol. I think I am going to just hook the old t-stat back up with the 2 green wires on top and the white and green on the bottom since I don't know where to put the 2 extra wires on the new t-stat. Attaching a pic of new t-stat available connections for extra 2 wires if anyone knows what to do with them.
    – Debi
    Commented Mar 13 at 22:35

new t-stat connections

Pic is of the available connections on the new t-stat. I think that I only need the R and W, but have 4 wires.

  • This should be edited into the Question, it does not belong as an Answer. Commented Mar 14 at 13:47

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