I have a 3 zone boiler heat system, one t-stat for each floor. It's worked fine for years except twice in the past few months. The first time I was out of town and my wife called and said the upstairs was very hot. I can't remember how I got it straightened out over the phone but I did. I figured the old mercury t-stat on that floor was sticking on so I replaced it with an electronic version.

Then today we had the same problem, t-stat set on 65, temp is 75. I switched that t-stat from heat to off and then took a look at the boiler and valves. The manual override on that zone valve is in the 'auto' position, making me think it's not stuck open but that's the only cause I can think of.

Can the valve be stuck open without the manual lever moving from the auto position? Doesn't the lever move when the valve opens, or does the lever stay in the 'auto' position whether the valve is open or closed? The only idea I have is to rebuild or replace the zone valve.

The control for the system looks like it's just a junction box, no relays. Do the t-stats just control the power to the valves directly, not provide a control signal? There must be a relay to activate the pump then, right? Or am I completely confused?


  • 2
    Without more information (like schematics or wiring diagrams), regarding your particular system, it will be difficult for someone to help you. Answers are very likely to be just as vague as your question.
    – ArchonOSX
    Commented Mar 29, 2016 at 7:16

2 Answers 2


Well, we know water's getting through & the circulator's pumping. So, I'd say get an HVAC guy out to inspect, test & replace whatever. The valve actuator motor could be failing & getting stuck. Meaning, you're periodically getting heat when another valve opens & pumps. Or, the valve's circuitry has an intermittent problem with not signaling the valve to close or the circulator to stop. I don't think there's a rebuild option here, sorry. Plus, you'll know what's what when the other valve's go.

  • I aim to please. It should be a quick & permanent fix for an HVAC guy.
    – Iggy
    Commented Mar 29, 2016 at 13:29
  • I don't have anything like an electronic control, it looks just like it's a box full of spring loaded connections. I think the 24v from the t-stats controls the valves and a relay for the pump. I picked up a new valve and installed it. Should be able to test it soon. If my soldering is any good.
    – Lockhart
    Commented Mar 29, 2016 at 20:03
  • No leaks and the valve opens and closes when it should. I think I'm good to go. I think the old valve was stuck open and that zone was getting heat when any zone called for it.
    – Lockhart
    Commented Mar 29, 2016 at 22:17
  • I hope it does the trick for a good long time. Good work!
    – Iggy
    Commented Mar 30, 2016 at 1:15
  • One more question. Once the system has all of the air out of it and is running well, is there any reason to leave the valve open that supplies cold water to the system?
    – Lockhart
    Commented Mar 30, 2016 at 15:00

A unit called a zone controller receives the signal from your thermostats and kicks on the appropriate valves/pumps. Assuming your problem is that the upstairs zone actually turns on when it shouldn't be -- as opposed to the upstairs getting hot simply because heat is rising from the lower floors -- then you must have a miswiring or failure in your controller, valves, or pumps. Or, less plausibly, an incorrect piping connection across zones.

Locate your zone controller - it may be a plain-looking box, but it will have electronics inside, not just junctions. This should be wired to your thermostats and also to your zone valves, circulation pumps, and boiler. It will look something like this:

zone controller example

Once you've found that, locate a wiring diagram. There may be one on the inside cover of the controller. Otherwise, you should be able to locate one online if you can get the brand and model number. Check your home's wiring against the diagram. If it looks right, identify which valves/pumps should operate for each zone and test the zones, one by one, by turning on their thermostat and then checking if the valves and pumps are properly engaged. Once you've narrowed down which components are behaving incorrectly, you should be able to either identify the miswiring (e.g. turning on zone 2 always kicks on zones 2 and 3) or identify which components are not working correctly (e.g. circulation pump 3 is stuck on).

  • I recently installed the 4 zone version of this, and it works great. It is much easier to troubleshoot a problem such as a bad zone valve because of the indicator lights. The original installer made a mess of my system and it was wired completely wrong. It was a rat's nest of wires that was hooked up to 2 transformers that were attached to the side of my boiler. It was wired completely improperly, which caused the circulator pump to never shut off. A zone valve controller makes it much harder to mess things up. Commented Jul 28, 2016 at 18:27

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