Boiler kept running

EricPeterson

My wife was working late on a project (making masks) and woke me after midnight to report that the heating system was acting strangely.

Our boiler has three zones and in one of the zones it was 76 degrees and the boiler was running. I went to the basement and saw that the corresponding zone valve was open. I then checked the thermostat for this zone, it was on setback at 64 degrees yet the reported temperature was 76. I switched the thermostat from Heat to Off and went back down to the basement.
The boiler was still firing and the zone valve was still open even though the thermostat was set to Off.
I then disconnected one of the thermostat wires from the zone valve and then the zone valve started to close and then the boiler shut off. I removed the second wire and did a continuity test but the thermostat was not calling for heat according to my multimeter.

So I am mystified as to what was going on here. I have hooked the thermostat back up and things are working normally. I did a test by having the thermostat call for heat and the system performed correctly.

The zone valves are White-Rodgers "1361 Two Wire Zone Valve" and wired according to their specification. This particular valve is probably almost 30 years old.
The thermostat is an Emerson that was installed this fall by an HVAC contractor (it also controls A/C in summer).
This is the first time I have ever had a problem like this. Obviously I would like to know the root cause. I was not able to reproduce the problem.

The way these zone valves work is that when the thermostat calls for heat, this call goes to the zone valve which opens, after which it then calls the system for heat.
So either the thermostat or the zone valve was "stuck" somehow. I can't attribute this to just the zone valve though since it behaved as expected after I removed one of the thermostat wires. So that leaves the thermostat.

Any theories on this?

Thanks,
Eric Peterson

Jamie Hall

Intermittent short in the wires from the thermostat to the control valve.

woodrow

had one last year when the customer turned the fan switch on the stat to on it powered the zone valve open and heat would not stop also check and make sure the wires come direct from the stat and dont have any isolation relays wired in

STEVEusaPA

Short somewhere, hanging zone valve end switch, bad batteries in thermostat (if it has batteries).

EBEBRATT-Ed

I would suspect a sticking valve. When you bumped it working on the wires it unstuck

HVACNUT

Also check/tighten all Common wires.

EricPeterson

Thanks for the responses. I plan to replace the thermostat with a smart model which requires power. Currently the wiring is 2-wire cable so will have to replace it with a 3-wire cable to add power.
I will also replace the zone valve once the heating season is over.

@Jamie Hall - if intermittent, has occurred exactly once this heating season, and nothing has disturbed the current wiring in that time. New cable will eliminate this possibility.
@woodrow - no the fan switch on the thermostat was not changed.
@STEVEusaPA - new thermostat with fresh batteries last fall. not sure how a zone valve can hang then free up when the thrermo wire is disoconnected.
@EBEBRATT-Ed - the thermo wires are connected with a screw terminal, I did not bump the valve unscrewing the first wire, and zone valve opened shortly after I removed that first wire.
@HVACNUT - always good advice, but a loose wire would not explain this behavior.

My conclusion is that the thermostat locked up somehow because that best explains the behavior: zone valve remained open and system running until one of the thermo wires was removed. Why that would clear up the condition I have no idea.
So replacing the thermo and thermo wires will address that possibility. Had it not been 2AM I might have investigated further at the time but I guess I will never know for sure.