Zone valve not working - heating stays on tho timer and thermostat 'off'

[Min]

I have a gravity fed central heating system with a new condensing boiler that was fitted a few months ago and the whole system was power flushed. Since then the rads have been much hotter (toasty ).

But now the heating sometimes stays on, even though the room stat is satisfied, and/or when the timer dictates the heating should be off.

There's a Drayton Mid Position Accuator MA1.

The heating stays on when it shouldn't maybe once every few two or three weeks, but each time it's when I've had the heating on all day. My impression is that the motor is getting too hot. If I drop the motor unit off the pipes (when the timer and the stat are 'off'), the heating goes off after about 5 - 10 minutes.

A plumber had a look, dropped the motor off, sprayed some WD40 on the copper spindle, and said if it carried on sticking the whole system would need to be drained down and the valve replaced.

It is possible to tell whether it's the motor that needs replacing, or the valve itself?

Thank you, and happy new year!

 

[Min]

Hmm, having read some other posts, think it is part gravity and part pumped (not sure if this makes a difference)

 

[stem]

It will be a fully pumped 'Y-Plan system' if you have a mid position valve. Water is fed from the pump to the valve where it is diverted to either the hot water cylinder, the radiators or both. You might have tanks in the loft for feed and expansion, but the circulating water is still fully pumped.

My first guess is that you have a frost thermostat somewhere that is overriding all of the other controls and turning the heating on when the temperature falls below its set temperature.

It's unlikely to be the valve sticking, because even if it stuck open, the radiators would only get hot when the Hot Water Cylinder Thermostat was calling for heat to operate the boiler. When the programmer is selected to 'heating off' and the room thermostat is 'satisfied' there would not be a live on the white wire feeding the motorised valve and it is this live that is fed through the motorised valves microswitch to operate the boiler when the heating is required. So would disappear when the the programmer was selected to 'heating off' and the room thermostat was 'satisfied'

Edit.
Having just reread your post and I as you say the heating goes off after 5-10 minutes, (I thought you had said it stayed on all day, but I see that refers to something else now) then it probably is the valve sticking open, and the few minutes that it stays on for, is whilst the hot water cylinder thermostat running the boiler whilst it is heating up.

 

[Terry lambert]

Easy enough to check with a multimeter and spindle by removing head

Fairly common fault with brass section sticking keeping the end switch on, WD40 doesn’t lubricate, with that valve you would remove the circlip on the spindle and use some three in one oil or similar use grips to move the spindle up and down and rotate to get the oil to the O ring below

It is double sealed so no need to drain down, be careful not to drop spindle down into valve.

If the problem persists then change valve

If the spindle is fine it could be the end switch sticking/faulty replace whole valve as spindle had previous issue as well

 

[Terry lambert]

Of course replace circlip after, if I’m doubt call a professional

 

[Min]

Sorry Stem - when CH on all day it was because I advanced the timer. CH should've gone off at 10pm and/or when room stat not calling for heat, but I woke up half cooked at midnight!

Thanks Terry - will try 3 in 1 on spindle - but it is pointing down so relying on capillary action not gravity for oil to soak in. Can move spindle back and forth (about 20 degrees) but no play up and down.

Trouble with multimeter is the problem is intermittent (and I don't know how to use a multimeter) so heating engineer v unlikely to be here when problem is actually happening.

If 3 in 1 doesn't work, I'm calling an engineer.

Thanks for your help

 

[Terry lambert]

When you remove the circlip the spindle moves up and down, put circlip back after, fiddly so don’t do it if not confident

Just coating the spindle is unlikely to improve it