Boiler replaced not turning off after CH demand from nest has ceased

[madnessx9]

Hey,

Had our old boiler replaced which had a fully working nest setup for years, currently with new boiler when the nest ends the heating or hot water demand the boiler carries on heating and I've left this for 20 minutes with no change, I've had to switch it off to make it stop and then after reboot as soon as there is demand it comes on and then does not turn off. I've checked that the 24v link has been removed.

For now I've installed the old programmer and everything turns off and on as intended and it's left me really confused, its exactly the same setup with the new boiler vs the old, any ideas spring to mind?

The boiler is a Vaillant Sustain 418, cannot find much in the way of this model being mentioned online, many ecotanks etc.

May have to call out an electrician at some point as I've wasted hours on this today but hoping it might be something simple I've overlooked.

 

[kidgreen61]

Sounds like you have a sticking end switch in one of your zone valves

 

[madnessx9]

Sounds like you have a sticking end switch in one of your zone valves

Which does not stick with the old programmer installed?

 

[MNW67]

Possibly an intermittent fault? and the fact it works with the old programmer is just a coincidence? Just a thought!

Is there a manual override on the Nest?

 

[madnessx9]

Possibly an intermittent fault? and the fact it works with the old programmer is just a coincidence? Just a thought!

Is there a manual override on the Nest?

It could be intermittent but I have tried several times with the same result, I was trying to enable hot water only and then CH etc and then the first time the old programmer is on it works flawlessly.

Its a 3rd generation Nest so has a button on the face to manually turn on and off but the nest is showing as off when the boiler is continuing to heat, I never tried to turn it on and off via this button though.

 

[MNW67]

Which does not stick with the old programmer installed?

Do you know what sort of set up you have e.g. Y-Plan or S-Plan? How many zone valves do you have? How many CH zones do you have?

 

[CBW]

Could be sticking relay in Nest if not motorised valve, especially if all works well with old programmer. You’d be testing for voltage present.

 

[Echo the husky]

Can you post some photos of the wiring?

 

[madnessx9]

Hey, sorry, had notification of first 2 replies and then nothing, thought I'd check back and many replies.

Do you know what sort of set up you have e.g. Y-Plan or S-Plan? How many zone valves do you have? How many CH zones do you have?

I believe it is an Y - Plan

Could be sticking relay in Nest if not motorised valve, especially if all works well with old programmer. You’d be testing for voltage present.

Will be interesting to try and test that relay in the nest, will investigate, I know the valve has been looked at twice by engineers as the boiler was done and they said it was working fine. Nest makes the clicking sounds when it turns on and off, although sometimes delayed, I blame the connectivity, doorbell is bloody slow to respond over wifi too.

Can you post some photos of the wiring?

Its messy, I have an original nest wiring that the engineers removed (chopped off wires), the wiring for the programmer and then my cleaned up nest install, with ferrules.

Cannot install into a nice position, need to extend the wiring, not enough slack in them.

 

[Echo the husky]

That is not wired for Y plan, it looks like S plan.

Y plan has a single valve, S plan has 2 separate valves.

Assuming the system works properly with the old controller and all the wires in the loose connector blocks haven't changed, the nest looks like it is wired correctly. The black wire switching on heating and brown for hot water.

You will need to do some tests to check the nest relays are switching properly. They can be damaged and the contacts weld together if they wired wrong. It sounds like one of the relays is stuck on if the old controller works, If there was a valve problem I would expect it to be a problem with the old controller too.

Tests to make at nest receiver:
With the heating set to on terminal: 3 should become live, heating off it should be dead (and terminal 1 should then become live),

with hot water set to on: terminal 6 should become live, hot water off it should be dead (and terminal 4 should then become live).

Take care when testing, ensure you are able to use the tester safely on mains voltages before proceeding.

 

[madnessx9]

That is not wired for Y plan, it looks like S plan.

Y plan has a single valve, S plan has 2 separate valves.

Assuming the system works properly with the old controller and all the wires in the loose connector blocks haven't changed, the nest looks like it is wired correctly. The black wire switching on heating and brown for hot water.

You will need to do some tests to check the nest relays are switching properly. They can be damaged and the contacts weld together if they wired wrong. It sounds like one of the relays is stuck on if the old controller works, If there was a valve problem I would expect it to be a problem with the old controller too.

Tests to make at nest receiver:
With the heating set to on terminal: 3 should become live, heating off it should be dead (and terminal 1 should then become live),

with hot water set to on: terminal 6 should become live, hot water off it should be dead (and terminal 4 should then become live).

Take care when testing, ensure you are able to use the tester safely on mains voltages before proceeding.

Hey, you may be right, I only see one valve at the hot water tank, perhaps the other is above the boiler itself, one of the engineers muttered something like this the other day as they came back to clear the condenser pipe which was blocked, they failed to check when installing. So year apologies about that!

Thanks for the info, I'll have a look at connecting it all again this weekend and seeing what happens.

 

[stem]

Sometimes during installation, if a short circuit occurs then the switching contacts inside the Heat link can get welded together so the output remains electrically live even when Nest has switched off.

The tests recommended by @Echo the husky will confirm if this is the case.

 

[madnessx9]

Okay, this is a fun one.

Tested terminals, as soon as it was powered on hot water relay was on, sounds like stuck relay like you said tried to turn it off and on no change, always powered on both hot water and heating set to off on the unit. I was having issues connecting to the nest via the app on my phone today oddly so I factory reset the unit, as the heat link was waiting for sync I decided to test the terminals, both the relays were off!? since the factory reset it has turned on and off without issue. Now the only problem I have is being unable to access the hot water schedule.

This seems to be solved now.

Appreciate your help.