Central Heating stays on all the time

[kaithavallapil]

My CH system has an (old) Ideal Classic heat-only RS boiler, Grundfoss pump, hot water tank, two Honeywell zone valves, Honeywell ST6400A timer and Honeywell CM67 room thermostat.
(The zone valves were recommended and fitted after problems with a three-way valve.)

Everything worked fine until recently when the heating just stayed on regardless of changes I made to the thermostat. The pump runs all the time and the only way to turn the heat off is with the boiler thermostat. Switching off at the timer doesn't do anything. A few days before this happened the heating wouldn't come on at all. but eventually did after I switched the room stat on and off a couple of times, which made me think the thermostat was the problem.

I called Honeywell and explained the circumstances, telling them that the thermostat is about six years old. I was told that the thermostat might be the problem, so I changed it to the replacement for the CM67, which is the CM907.

Same problem. Now I am wondering if the zone valve is stuck open. Any ideas, anyone?

I also have some questions about how the components work with each other.

I mean, when the thermostat calls for heat, where exactly does the signal go: to the zone valve?... boiler?... or pump?... or combination of these? How does the signal from the thermostat(s) travel to start or stop the system.

Elementay questions, I know... probably several variations to the theme... but my system set-up is over 20 years old... can someone give me a clue so that I can understand how the components communicate with each other? (Or recommend a book)

Thanks

 

[flameport]

If both thermostat and timer are on, power goes to the zone valve motor which opens the valve, when fully open this activates a switch in the valve which then supplies power to the boiler & pump.
When power to the valve is disconnected, a spring inside pulls the valve closed and causes the switch inside to open.

If the valve sticks open or the switch inside fails, the boiler & pump will run all the time regardless of other controls.

 

[45yearsagasman]

Good answer flameport. I'll second that.

 

[kaithavallapil]

Thank for this. I looked at a wiring diagram (S plan, I guessed as I have 2 zone valves) and figured out, but I appreciate the confirmation. Looks like I changed the theremostat for nowt.

Is it a case of replacing the valve altogether? Or can anything else be done?

Thanks again

 

[Echo the husky]

Are both valves closed when nothing is on? The valves little manual lever will be very loose if the valve is still open.

If the valve is faulty electrically you need only replace the actuator so no need to drain down the pipework. Honeywell call the actuator a powerhead.

It is unlikely to be the programmer or thermostat as both would have to be faulty and stuck on to keep the heating on, unless they are grossly miswired.
The other possibility is if there is a frost stat wired into the system and it is calling for heat. A frost stat will bypass the programmer and room stat, and open the heating valve.

If the heating valve is open when the programmer and room thermostat are switched off, (manual lever loose), turn off all the power at the main isolator. If the valve springs closed when the main power is cut, there is something external powering the valve, and the valve will likely be fine.

If it is still open with the main power cut, the valve actuator is likely faulty, there was a bad batch not long ago where the little moving parts all sort of fall apart and jam. If this is the case remove the actuator from the valve body, (there are instructions everywhere on how to do this) if the actuator springs closed as soon as you remove it from the valve body, the valve body is likely jammed, this is highly unlikely though.

 

[kaithavallapil]

Thanks a lot for this. Turning the power off and the lever is loose and I can compare with the zone valve for hot water, which had tension. Thanks also for the diagnostic help, especially to know that the least likely thing is the valve body jammed and I hope it's just a change of actuator rather than the whole unit.

I also found a few great youtube videos that explain the stuff and links are below if it helps anyone