Glow worm Ultimate 50ff boiler on constantly

[skim123]

Hi,
I have a Glow Worm Ultimate 50FF which is on constantly.
This setup has been running ok for the last 26 years

With the Programmer on:-
Both motorised valves move on/off when thermostat for both water and heating are adjusted - so appears ok
The House/hot water heats up & then when at temp, the motorised valves shut off, and the boiler overheat thermostat trips the boiler out.

If the programmer is switched off, the boiler still fires up and trips out due to overheat.

There doesn't appear to be any leaks anywhere that may have caused a short circuit.

I've had a couple of heating engineers around and neither appear to know the cause.
Any ideas?

Thanks in advance.

 

[terryplumb]

Motorised valve would be a prime suspect.

 

[skim123]

Thanks Terry, they do move when the thermostat is adjusted though, but I understand where you are coming from as it's
the motorised valve that sends the signal for the boiler to fire up isn't it?

Are you saying that it can be sending the signal even though the valve is in the off position and moves?
They are Danfoss units with removable heads, would it need the whole valve changing or just the motorised head part?

Edit
Just had both the heads off, the valves move with just fingers, and the levers move ok?
When the motorised valve is in the on position the pump starts, when off the pump goes off after a delay.

 

[CBW]

It can still be a stuck microswitch.

 

[terryplumb]

Testing voltages at the wiring centre would confirm,but it sounds like one of the valves is permanently sending voltage to the boiler via a microswitch that's closed circuit .

 

[ianmcd]

As the others have said you have a sticking end switch on one of your zone valves

 

[skim123]

Thanks everyone, certainly makes sense, I'll check it out.

 

[dilalio]

I've had a couple of heating engineers around and neither appear to know the cause.

For real?

 

[skim123]

Yes, I can only think they were either clueless or didn't want the job. One said it must be wired up wrong, but if that was the case it's been wired wrong for 26 years??

Looks like there are good clued up people here though.
Thanks everyone.

 

[stem]

Based on your description, I can see why they might think it is wired up wrongly, because motorised valves are responsible for controlling both the boiler & pump, However, you say the pump goes off but the boiler doesn't.

[Sometimes the pump is controlled directly from the motorised valves, and sometimes where a 'run on' time is required it's controlled indirectly via the boiler]

when at temp, the motorised valves shut off, and the boiler overheat thermostat trips the boiler out.

If the programmer is switched off, the boiler still fires up and trips out due to overheat.

When the motorised valve is in the on position the pump starts, when off the pump goes off after a delay.

However, you only mention above "the motorised valve" and earlier on you mention having two motorised valves. So, perhaps the other is the culprit? Also, these faults can be intermittent so it may work sometimes and not others. Which may be the case here because if you were testing a good valve and the other had the faulty switch and was stuck on, then thank would keep the pump and boiler running.

As @terryplumb to be sure a test with a multimeter when the fault is present and both valves are closed (off) would confirm.

 

[skim123]

I checked the pump again today on my return from work, it actually wasn't going off, so a little red herring in my description. It had been going off before when the problem was more intermittent.
Thanks everyone for the valuable info, I tested & replaced the head on the heating zone valve and it appears to be behaving itself now.