Faulty cylinder stat?

[Oilyragg]

Hi diynot

Last week I did a diy install of the 3rd gen Nest thermostat replacing my programmer with the Nest heat link and from my understanding I have a Y plan system which is how I wired it in following the manual instructions.

Everything is working fine you will be pleased to hear but the other night I was woken up by the Grunfoss pump running. I checked the the HW and CH and they were both turned off so I then turned down the cylinder stat and the pump stopped running? This is an intermittent problem and has happened a few times at night.

I noticed when I wired the heat link up it didn't have a CH off wire to attach but as my old programmer didn't have it I left it as is.

So my question is do you think I've made a mess of the wiring or are these symptoms likely to be a failing cylinder stat or even a failing PCB?

I have a Prima F boiler and the PCB was replaced about 2 years ago and the 3 way motorised valve was replaced about a year ago.

Any advice is much appreciated

 

[stem]

If turning down the cylinder thermostat stops the boiler, the boiler will be OK. As far as the boiler is concerned it is switched 'on' or 'off' depending upon the demand from the external controls.

So it sounds as if you have a wiring fault somewhere.

From your photo it looks like you have an 'S Plan' system. This has '2 Port' motorised valves. Is this correct?

If so, you won't need the 'hot water off' wire from the programmer. [There should be another motorised valve somewhere else for the central heating control]

A 'Y Plan' has a single 3 port valve. I don't see this, but it is difficult to tell from the angle of your photo.

However, the hot water shouldn't be running if the Nest isn't asking it to. So, if the Nest isn't calling for hot water, but it is being heated, I would check carefully the wiring between the Nest Heatlink and the Cylinder thermostat. The wire from the Heatlink terminal 6, 'Hot Water call for heat' should go to the cylinder thermostat.

 

[Oilyragg]

Thank you for the quick response stem, I have 3 connections leading to the motorised valve which lead me to believe is was a Y plan system plus I can't see another motor anywhere else on the system but if it is a s plan system I'll definitely check the wiring again.

 

[stem]

Sorry yes, it is a Y Plan, I can see that now. [Should have gone to spec savers] So you will need to connect the 'hot water satisfied' (Nest terminal 4) to the grey wire of the motorised valve. It may not have been there if your previous programmer didn't have the facility to have the central heating on independent of the hot water.

Non the less, it still appears that the cylinder thermostat is getting a live from somewhere when it shouldn't be, so check the connections between it and the Nest terminal 6.

 

[Agile]

But, it would seem only intermittently!

Perhaps that only occurs when the Nest turns of HW on its time off setting but the cylinder stat is still calling for heat?

 

[Oilyragg]

I'll check that now thanks stem. Just to clarify there shouldn't be a permanent live to the cylinder stat until the water is on or is it different from home to home?

Yes Agile the pump doesn't run continuously it just starts up of its own accord?

 

[stem]

Just to clarify there shouldn't be a permanent live to the cylinder stat until the water is on or is it different from home to home?

No. Not in any installation. If there were a permanent live, the hot water would come on anytime day or night when the water in the cylinder cools. The Nest would have no control over it whatsoever.

The hot water thermostat should be powered from Heatlink terminal 6. This is only live when the hot water is selected 'on'.

 

[Oilyragg]

Sods law I've left my multi meter at work and I'm wrong about the HW off wire it's the CH off wire that I don't have sorry for confusion. I'll check the wire feeds when I get back from work.

 

[stem]

Thanks. You don't need a central heating off wire, so that's OK.

If the thermostat is starting up the system when the Nest is selected 'off' the cylinder thermostat is getting a live supply from somewhere it shouldn't be and starting up the system.

Is there a possibility that the wires in the Heatlink 4 and 6 are transposed? They are both yellow. If they were, the hot water would be 'on' when selected 'off' and vice versa. if you don't use a lot of hot water during the day you might not have noticed if it was still being heated up during the night.

 

[Oilyragg]

It is a possibility but when I turn the hot water on the boiler fires no problem and it's the same with the heating, it's weird everything works as expected, from the phone app to the room stat and the manual control all works perfectly?

 

[Agile]

Is your pump wired to the boiler to provide pump over run?

What model of boiler?

Where is boiler installed?

I am wondering if it is the first stage of frost protection but need answers to those questions to see if that may be the problem.

Tony

 

[stem]

If its working as it should when operated manually, then the wiring would seem OK.

At the end of the day. If the cylinder thermostat is turning 'on' the hot water when the Nest is set 'off' it is getting a live supply from somewhere, so maybe the Nest is switching on terminal 6 for some reason (fault?) or a live is fed to the thermostat from somewhere else?

If after checking the wiring, the problem still persists, you could try disconnecting the wire from terminal 6 overnight and seeing if it still comes on, that would show if it was the nest switching on when it shouldn't.

As a bit of a curve ball, I don't suppose you have a frost thermostat anywhere do you? They are powered 24/7 and turn the heating 'on' when the temperature falls below their set level. This wouldn't be effected by the cylinder thermostat, but if the frost stat switched 'off' at the exact time you turned the cylinder thermostat down..... Not very likely I know but....

 

[Oilyragg]

Hi Tony the boiler is in the kitchen and it's a Potterton Prima F but I'm no heating engineer so I don't know how the pump is wired to the boiler. Sorry.

I don't have a frost thermostat that I know of Stem where would it most likely be located? Besides I have a wife so the house is never really that cold.

 

[Dan Robinson]

have a Prima F boiler and the PCB was replaced about 2 years ago and the 3 way motorised valve was replaced about a year ago.

Is your pump wired to the boiler to provide pump over run?

What model of boiler?

Where is boiler installed?

I am wondering if it is the first stage of frost protection but need answers to those questions to see if that may be the problem.

Tony

Nothing like reading the opening post Tony.

 

[Agile]

The Prima F has no frost protection but it does have a SPST stat to provide pump over run.

That could be intermittently faulty, particularly after 20 years of use.

The OP has stated the pump comes on but has not mentioned the boiler firing up. That would mean that this stat is about the only part which could normally cause the pump to run on its own.

BUT to progress this possibility the OP needs to follow the pump wire to confirm that it is correctly wired to the boiler pump connection block.

Tony