I came down to the kitchen this morning and noticed that the heat in that room was on. I checked the thermostat, and it was set to 68 (there is no off switch on this one) but was registering about 80 degrees. My smart thermometer in that room agreed - 78 degrees. So I went down to the basement and of the 7 or 8 zones, it was the only one on. At first, I thought I would just turn the valve for that one off, but then worried that it might result in a buildup of pressure (since there was no where else for it to go), so I turned off the big red "emergency off" switch to power the whole thing off. I guess I would hope the boiler has a pressure limit auto-off mechanisms, but it was summer and so I thought there was little cost to turning it all off and it eliminates any potential risk.

Here are pictures of the various zones, the boiler unit and a close up of the zone valve for the malfunctioning zone. Edit 1a: I've added some photos of the thermostat itself - it's an old model.

Our seven zones with valves

Our boiler

The zone valve for the malfunctioning zone

The thermostat in question

thermostat with cover off

another pic of thermostat with cover off

What would cause this? A broken thermostat? Or a broken zone valve? and if it's the zone valve, do I need to replace it with the same model (Honeywell 40004850-001) or can I use a newer model?

Edit 1b: Note: the thermostat is an old-school round one. All the other ones in our house are the more modern digital ones with the standard plug-adapter on the back. But this one looks more attached to the wall, so not sure how easy it is to remove and/or test the wires. Is it easy to remove this kind and replace with the new digital model?

Update: it turns out it was the thermostat. I took it off the wall, and things worked fine. I replaced it with a newer simple two-wire digital heater thermostat, and everything seems back to normal.

  • That was a prior problem in a different room of our house. WE had the zone valve replaced for that to fix it. But that one had a proper digital thermostat, while this one has an old-school analog one.
    – Doug
    Jul 29, 2023 at 17:36
  • thermostats are pretty simple and rarely break - they just connect two wires together. you can easily replace the thermo with a new one - just don't landfill the mercury one ( I dropped mine off at a mercury recovery company for free ). Jul 29, 2023 at 17:39

1 Answer 1


My best guess would be a bad t-stat. to diagnose: Pull the t-stat off the wall and using a meter measure across the W and C wire terminals. If you get zero voltage it means the t--stat is closed IE: on. That's assuming pulling the stat doesn't disconnect anything. Some models do.

Next: Even if you had a failing zone valve, unless some other zone was calling for heat, the boiler wouldn't be firing.

Please don't throw parts at this. While it looks like complex system (I have something similar), it's just a lot of replication to feed the various zone. It shouldn't be hard to diagnose

  • I added some info about the Thermostat. It's an old-school analog one.
    – Doug
    Jul 29, 2023 at 16:10
  • boiler might have an indirect tank hookup as well in which case it cycles for domestic - can't see the whole mechanical room from the pic Jul 29, 2023 at 17:25
  • I'm not sure I follow that last comment about the indirect tank hookup. What additional image would help you diagnose that issue?
    – Doug
    Jul 29, 2023 at 17:39
  • @Doug Sorry, but I don't understand your question, if it was directed at me. I didn't mention anything about an indirect hookup. ....just maybe a means to diagnose the problem., I still stand by my post: If no other zone is calling for heat, even you had a stuck zone valve, the boiler shouldn't be firing....it's the t-stats job to turn on and off the boiler and zones. Jul 29, 2023 at 21:56
  • 1
    @GeorgeAnderson sorry George, that comment was an answer to FreshCodemonger's comment above.
    – Doug
    Jul 30, 2023 at 17:11

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