Central heating thermostat

Lucien Nunes · Jan 1, 2023

That connection has been like that since the heating was installed nearly 30 years ago, the issue now is there is no power in the cables.

Badly made connections can eventually go open circuit, as the surfaces where the wire is just barely touching oxidise over time. If you were checking for presence of a feed by touching your meter probe on the screw, loss of continuity would cause a zero reading, as that is probably the incoming feed from the programmer. (I expect the stat output is the yellow core oversleeved in red)

We seem to have three different symptoms with no certainty of how they relate to each other.

Room continues heating when it shouldn't
Pump runs when it shouldn't
No feed to room stat

Definitely need to see inside the wiring centre and, if it's not obvious from that, the configuration of motorised valves (Y-plan, S-plan etc.)

timhoward · Jan 1, 2023

Alan Stirrup said:
The issue that prompted me to check the thermo was the temperature upstairs was intolerable and the pump was running continuously even when the heating had been off for several hours.

I have a feeling this could be two faults.
Pure speculation but my guess would be that (as already mentioned above) initially the zone valve head stuck open. That could lead to boiler and pump being on 24/7 with only the overheat stat on the boiler to possibly interrupt proceedings.
In turn being on perpetually could have then helped an old time clock to give up the ghost.
Complete guesswork though.

The first test is whether when you ask the time clock to call for heat, a terminal in the wiring centre becomes live.
If nothing changes in the wiring centre then the time clock itself is the first suspect.

Also, do you have one or two zone valves?
With everything off, does the little lever move freely on one of them (or the only one)?

Alan Stirrup · Jan 1, 2023

timhoward said:
I have a feeling this could be two faults.
Pure speculation but my guess would be that (as already mentioned above) initially the zone valve head stuck open. That could lead to boiler and pump being on 24/7 with only the overheat stat on the boiler to possibly interrupt proceedings.
In turn being on perpetually could have then helped an old time clock to give up the ghost.
Complete guesswork though.

The first test is whether when you ask the time clock to call for heat, a terminal in the wiring centre becomes live.
If nothing changes in the wiring centre then the time clock itself is the first suspect.

Also, do you have one or two zone valves?
With everything off, does the little lever move freely on one of them (or the only one)?

I'll run a test on the time clock, Tim, hopefully this will give us a better indication of what's going on. I have one zone valve and the lever moves freely without resistance.

timhoward · Jan 1, 2023

Alan Stirrup said:
I have one zone valve and the lever moves freely without resistance.

If this is the case without calling for hot water or central heating AND it’s the Honeywell style unit I’m imagining then I think you have a problem there as the lever should be quite tough to move with everything off. It should feel as though you are overcoming a strong spring. It sounds as though it’s stuck.

With power off, the cover comes off with a small screw near the lever. If you take the cover off I’d imagine you could then see if it’s sticking or not. The gear teeth may look shiny and worn.

If required, the head is a fairly easy replacement as long as you note where the wires go, part is available at Screwfix. Sadly you need the more expensive 3 port head.

(I appreciate you are not looking for extra expense but if it were my house I’d get a plumber to change it to 2 x 2 port valves. The heads are much cheaper for future replacement and 3 port valves can leave the motor running perpetually for hours on end after one permutation of actions. )

Lucien Nunes · Jan 1, 2023

If this is the case without calling for hot water or central heating AND it’s the Honeywell style unit I’m imagining then I think you have a problem there as the lever should be quite tough to move with everything off.

Assuimng this is a 3-port valve, (did we confirm that explictly?) this is true only if the last demand was for water, which would leave the valve idle in the spring refurn (B) position if the last demand was for heating the valve will be parked in the A position with the motor energised and the lever will be free to move. Briefly demanding hot water only, should release it back to the B position.

timhoward · Jan 1, 2023

Lucien Nunes said:
Assuimng this is a 3-port valve, (did we confirm that explictly?) this is true only if the last demand was for water, which would leave the valve idle in the spring refurn (B) position if the last demand was for heating the valve will be parked in the A position with the motor energised and the lever will be free to move. Briefly demanding hot water only, should release it back to the B position.

Thank you - that brings up three things!
1 - you are right, he said he had one zone valve but we don't know for sure it's a 3 port.
2 - when I said "everything off" in my head that meant the fused spur to the boiler and associated control wiring but I should have been clearer.
3 - it's somewhat ironic that I even mentioned this behaviour of last call for heating without considering the system could still be powered....
Some photo's will hopefully add some clarity in due course.

Alan Stirrup · Jan 2, 2023

timhoward said:
If this is the case without calling for hot water or central heating AND it’s the Honeywell style unit I’m imagining then I think you have a problem there as the lever should be quite tough to move with everything off. It should feel as though you are overcoming a strong spring. It sounds as though it’s stuck.

With power off, the cover comes off with a small screw near the lever. If you take the cover off I’d imagine you could then see if it’s sticking or not. The gear teeth may look shiny and worn.

If required, the head is a fairly easy replacement as long as you note where the wires go, part is available at Screwfix. Sadly you need the more expensive 3 port head.

(I appreciate you are not looking for extra expense but if it were my house I’d get a plumber to change it to 2 x 2 port valves. The heads are much cheaper for future replacement and 3 port valves can leave the motor running perpetually for hours on end after one permutation of actions. )

Thanks Tim,
I've just taken the head off and the lever is free moving (I can't see the gubbins as it's pretty well sealed,) I can see the female "D" slot that the valve goes into and I've moved it with a screwdriver which presents with substantial resistance. The unit is an American made Danfoss, which I doubt will be available these days although the motor is a Drayton Synchron. What would be a suitable like for like replacement in your opinion?

timhoward · Jan 2, 2023

@Alan Stirrup Danfoss heads and valves are very much still available. Does yours have 2 or 3 pipes (the 3 port version sits on a T )
While I have a strong suspicion that the zone valve is part of the problem (based on experience of what is most likely to break) we haven't done enough to confirm this, or prove that it's the only problem. (Also, if the valve itself is stiff, a new head will burn out very quickly, so it might need an entirely new valve fitting)

So before going any further you do need to do some electrical tests. Did you get anywhere regarding some photographs? (e.g. the wiring centre). You need to be sure that the controller is calling for heat, the wall stat is receiving power, and in turn energising the supply to the zone valve.

It wouldn't be expensive to get a sparks in to diagnose this by the way - I'd expect to have an understanding of what is going on in under an hour.

littlespark · Jan 2, 2023

OP is in Liverpool if a nearby member is up for a look.

I don't think Tel will be sober enough yet.

Alan Stirrup · Jan 2, 2023

timhoward said:
Thank you - that brings up three things!
1 - you are right, he said he had one zone valve but we don't know for sure it's a 3 port.
2 - when I said "everything off" in my head that meant the fused spur to the boiler and associated control wiring but I should have been clearer.
3 - it's somewhat ironic that I even mentioned this behaviour of last call for heating without considering the system could still be powered....
Some photo's will hopefully add some clarity in due course.

timhoward said:
@Alan Stirrup Danfoss heads and valves are very much still available. Does yours have 2 or 3 pipes (the 3 port version sits on a T )
While I have a strong suspicion that the zone valve is part of the problem (based on experience of what is most likely to break) we haven't done enough to confirm this, or prove that it's the only problem. (Also, if the valve itself is stiff, a new head will burn out very quickly, so it might need an entirely new valve fitting)

So before going any further you do need to do some electrical tests. Did you get anywhere regarding some photographs? (e.g. the wiring centre). You need to be sure that the controller is calling for heat, the wall stat is receiving power, and in turn energising the supply to the zone valve.

It wouldn't be expensive to get a sparks in to diagnose this by the way - I'd expect to have an understanding of what is going on in under an hour.

I have taken pics of the wiring centre I just have to convert them on my laptop so they'll load. Mine is the three pot sitting on the "T" I did a hand test on the valve and it moves freely with some resistance, probably just a little more than say, a gate valve. I've set it manually to heating and water and the downstairs heating has balanced out correctly so I'm of the opinion that the head unit has died mid position hence the imbalance and no signal to the thermostat. I'll try and load the pics tonight and thank you very much for your help so far, we seem to be heading in the right direction with your guidance.

123 · Jan 2, 2023

The 3 port valve head should have no affect on the room thermostat getting voltage or not, it should go from the heating programmer to the thermostat.

brianmoooore · Jan 2, 2023

123 said:
The 3 port valve head should have no affect on the room thermostat getting voltage or not, it should go from the heating programmer to the thermostat.

And then to the 3 port valve, so don't condemn the valve just yet.

Alan Stirrup · Jan 2, 2023

Here's the pic of the junction box in the airing cupboard. I've done a visual check at the back of the timer and all the wires are located.

brianmoooore · Jan 2, 2023

Hmmm, a surface pattress box and strip connector, rather than a purpose made heating connector box, where all the connections are labelled.
I think the yellow with the red sleeve, near the middle, is the heating wire from the room stat, so the red in the same cable is probably the feed to the room stat, and the wire joined to it on the strip will be the heating on wire from the programmer.

Alan Stirrup · Jan 2, 2023

brianmoooore said:
Hmmm, a surface pattress box and strip connector, rather than a purpose made heating connector box, where all the connections are labelled.
I think the yellow with the red sleeve, near the middle, is the heating wire from the room stat, so the red in the same cable is probably the feed to the room stat, and the wire joined to it on the strip will be the heating on wire from the programmer.

That's the quality you got with new build houses 28 years ago, Brian and now it's coming home to roost!

Central heating thermostat

Lucien Nunes

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timhoward

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Alan Stirrup

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timhoward

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Lucien Nunes

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timhoward

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Alan Stirrup

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timhoward

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littlespark

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Alan Stirrup

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123

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brianmoooore

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Alan Stirrup

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