Faulty cylinder stat?

[stem]

A faulty pump overrun wouldn't explain why, the pump could still be switched off by turning the cylinder thermostat down. Unless it's a coincidence, which is virtually impossible because the OP said it has happened a few times. What are the chances of a faulty pump overrun switching the pump off at exactly the same time the thermostat is adjusted?....

Can't say I have found an overrun that has failed intermittently either. They seem to stay on 24/7 or don't work at all. But I guess it's possible in theory.

 

[stem]

I don't have a frost thermostat that I know of Stem where would it most likely be located?

It was a long shot, they are usually fitted if the boiler is in a garage or other unheated area to stop it (or exposed pipes) freezing overnight. Most domestic properties don't have them. Sometimes they are built into the boiler, but as you say.....

Besides I have a wife so the house is never really that cold.

So, if it did have an integral frost stat it would be unlikely to operate.

 

[Agile]

A faulty pump overrun wouldn't explain why, the pump could still be switched off by turning the cylinder thermostat down.

I have to disagree with that.

The pump over run stat changes the pump from the switched live to the permanent live.

So if that stat erroneously operated the pump would run from the permanent live supply irrespective of any other control settings.

Tony

 

[stem]

If the boiler provides the pump overrun, the pump is not wired to the cylinder thermostat. It's the boiler that controls it. Adjusting the cylinder thermostat won't effect it during the overrun period. And remember the Nest has (should have) turned off the power supply to the cylinder thermostat at this point anyway, so it won't have any power to switch.

 

[Dan Robinson]

Nest has (should have) turned off the power supply to the cylinder thermostat at this point anyway, so it won't have any power to switch.

Except for the hot water off terminal. The circuit of which is often miswired.

Those cyliner stats are pants too.

 

[stem]

Except for the hot water off terminal. The circuit of which is often miswired.

Yes, that is was what I was thinking a bit back when I asked about the possibility of the wires being transposed at the Nest. The cylinder thermostat end wiring shouldn't have been changed though.

The odd part about it is that the cylinder thermostat seems to do what it should when operated manually.

Oilyragg

Can you check that your wiring corresponds with this link. Click on the 'Y Plan diagrams' and you will be able to see what should be live and when.

You say that the pump is running when it shouldn't be. But, is the boiler operating too? If the boiler is running, what is being heated? Radiators / hot water / both

Was the system working correctly before the Nest was installed?

Have you changed the position of any wires at the cylinder thermostat?

 

[Oilyragg]

You say that the pump is running when it shouldn't be. But, is the boiler operating too? If the boiler is running, what is being heated? Radiators / hot water / both

Was the system working correctly before the Nest was installed?

Have you changed the position of any wires at the cylinder thermostat?

The radiators are not heating up as for the water I'm not sure I can't hear the boiler from the bedroom and have been turning off the pump from the stat before checking to see if the boiler is firing but in my sleepy state I don't remember thinking the tank was hot but that's no guarantee.

Before and a few days after the install it all worked fine.

No changing of the cylinder thermostat wires. The only wires I've touched are the heat link wires.

The next time it happens I'll find out what (if any) is being heated.
I've gone and walked out of work without my multi meter so won't be able to check it tonight but will try again tomorrow.

Thanks for everyone's input on this I keep you informed.

 

[Oilyragg]

So I've had a multi meter across the terminals on the heat link and this is the result:
HW CH off 2,4,5 240v
3,6 no power

HW on 6-240v 3 no power
CH on 3,6 240v

So I'm guessing the Nest is working as it should, now to trace the wires.

Pump didn't cut in last night so I couldn't see if the boiler fired.

Something else I've noticed is the boiler will run for 10 minutes then switch off for 30sec then switch on again even though the room is not up to temperature, not sure if this meant to happen or not but I'm sure the old timer kept the boiler running until room stat cut in, could the two be related?

 

[stem]

I've noticed is the boiler will run for 10 minutes then switch off for 30sec then switch on again even though the room is not up to temperature.

Boilers always produce more heat than can be dissipated by the radiators, especially when they have been on for a while, so it's normal for the burner to cut in and out, so that it doesn't actually boil. So if that's what is happening that's normal.

 

[Oilyragg]

Last night the pump was running again but the radiators and water tank were cold, I turned down the tank stat and the pump stopped but after I turned it back up the pump started again and the boiler fired for a few seconds and then stopped.

So basically the pump is running without calling for heat?

 

[stem]

Well, I'm running out of ideas on this one. It's certainly very odd. I still think it's an external control fault, because of the fact that the cylinder thermostat which (if wired correctly) shouldn't have a live supply from anywhere, is switching the pump / boiler when you adjust it.

The absurdity of this could be illustrated like this. If you turned off the main electrical supply to your house at the consumer unit, then when into the kitchen and switched on the kitchen light would it come on? No, it wouldn't happen (unless it was wired incorrectly). What you have is the same situation. The electrical supply from the Heatlink to the cylinder thermostat (terminal 6) is off, yet, you turn on the thermostat and the pump runs. Shouldn't happen.

So, to me I still believe the wiring is wrong somewhere. The crux of the matter is, where is the cylinder thermostat getting a live supply from when it shouldn't be. So I would start with checking the cylinder thermostat is wired as it should be, according to the link to the diagrams I sent earlier. Not just that the correct three switching wires are going there, but that they are corrected to the right terminals.

Then there is the other matter of:

So basically the pump is running without calling for heat?

Is the pump wired to the terminals provided for it at the boiler? (this is different to the diagrams because your boiler requires the pump to run for a while at the end of the heating sequence to cool the boiler, and so is wired directly to special terminals for it at the boiler)

 

[Oilyragg]

I have to agree stem it must be something I've done wrong somewhere. I'll do as you say and start at the cylinder stat to see how it's wired and check a few other bits that I can but I'm thinking it's going to be a call to British Gas as I really don't want to start going into the boiler internals. It just that I know how hard it can be diagnosing a problem before it's completely failed.

 

[Agile]

We need to know where the pump is connected as that is the crux of the matter.

Unless it is connected to the boiler then that is plain wrong.

Surely you can follow the wire from the pump?

Tony

 

[Oilyragg]

We need to know where the pump is connected as that is the crux of the matter.

Unless it is connected to the boiler then that is plain wrong.

Surely you can follow the wire from the pump?

Tony

The wire from the pump goes straight into the wall so it will take a bit of investigation to find out where it goes after that but I'll do my best.

 

[stem]

When you do find the solution, please let us know. It will help for next time