Central heating strange behaviour

[JonMad]

Our boiler was as old as the house, 37 years, noisy and probably very inefficient. We replaced it with a new one - a Vaillant, and took the opportunity to also get a new Honeywell 3 port (Y) valve, a new central heating pump and a Nest thermostat (shiny, but not really that 'smart'). It's doing heating and hot water from a tank in the loft and a cylinder in the airing cupboard.

We have some odd behaviour where the boiler switches the heating on without any apparent requesting of heating from the thermostat. It doesn't happen all the time, but when it does happens it's
- usually late at night, typically about 1 1/2 hours after the heating had been on, and with the [heating only] having been the last mode it was in, not [water only], or [heating + hot water].
- when the thermostat is down at 16/17 degrees and the ambient temperature is way above that, say 20/21, from the heating previously being on.

It's not the Nest being in Learning Mode as I've switched that off.
When this happens the Nest is still showing that the heating is off, but it's most definitely on and the radiators get hot.
Each time I've been able to turn the heating off again by flicking the water on then off.
It doesn't do it consistently.

First thought was that it was a Nest issue, e.g. dodgy wifi (see my comments on the wi-fi extenders thread). But I have wifi extenders near each device and there are no errors on the Nest or the app to indicate a connection issue.
Then I was reading about some boilers being sensitive to stray voltages within the valve circuitry, when the last mode was [heating only], causing them to turn back on. Sounded like just the issue we're seeing, so we got a voltage suppressor fitted to stop this. I was confident that would fix it but the problem happened again last night.

We have the firm who installed our new boiler happy to continue looking at this issue with us until its resolved, but I'm just throwing it out there for any other thoughts or experience.
Next step might be to put a simple programmer back in rather than the Nest. Or replace the valve for another new one as maybe one from a dodgy batch.

[scotta]

We had similar. It was one of the motorised valves that was sticking. as a result the boiler was running when the central heating was switched off.

Where is the boiler located? If its somewhere un-heated it could also potentially be the frost protection kicking in.

[jamcg]

We had similar. It was one of the motorised valves that was sticking. as a result the boiler was running when the central heating was switched off.

Where is the boiler located? If its somewhere un-heated it could also potentially be the frost protection kicking in.

All of this. Did you have the 3 way valve fully replaced or just a new head?

[Jobbo]

It was one of the motorised valves that was sticking.

That is such a common issue that it ought to be the first thing to check. Even if they're not particularly old, the microswitch inside the motorised valve can stick on, meaning the boiler has a signal to fire despite the thermostat not calling for heat.

I've replaced three Drayton motorised valves since moving to this house 5 years ago. It only has two, so you can see how long even a new one lasts

[integrale_evo]

Got similar issues with ours again at the moment, the valve is either sticking or not giving the right signals as when it’s supposed to be hearing the hot water the radiators are getting hot.

Pretty sure I only changed it just over a year ago, which is pretty shit.

[drcarlos]

We've got an issue that makes our boiler trip out at least every couple of weeks.

Boiler is on a service plan and all seems OK as they've been out several times and can't find anything wrong (and it's not like they have fault logging and diagnostics), valve and tank stat were replaced last month with decent Honeywell stuff, the old cruddy wiring centre that seemed to confuse the old valve (hot water was never off) was removed and then I found the switch in the switched spur was dodgy and kept making the programmer flicker, so I am thinking that it's damaged the programmer or our friend (gas safe engineer and plumber) said we might need to check the gas pressure is steady.

With your behaviour the Programmer is the master that will switch circuits live and react to valves and thermostats being open/closed, then liven the output to the boiler so if the boiler is coming on at odd moments it could be a faulty programmer.

[jamcg]

We've got an issue that makes our boiler trip out at least every couple of weeks.

Boiler is on a service plan and all seems OK as they've been out several times and can't find anything wrong (and it's not like they have fault logging and diagnostics), valve and tank stat were replaced last month with decent Honeywell stuff, the old cruddy wiring centre that seemed to confuse the old valve (hot water was never off) was removed and then I found the switch in the switched spur was dodgy and kept making the programmer flicker, so I am thinking that it's damaged the programmer or our friend (gas safe engineer and plumber) said we might need to check the gas pressure is steady.

With your behaviour the Programmer is the master that will switch circuits live and react to valves and thermostats being open/closed, then liven the output to the boiler so if the boiler is coming on at odd moments it could be a faulty programmer.

Programmer activates valves, valves activate the boiler. There’s no direct wiring between a programmer and boiler in a y or s plan system

[drcarlos]

We've got an issue that makes our boiler trip out at least every couple of weeks.

Boiler is on a service plan and all seems OK as they've been out several times and can't find anything wrong (and it's not like they have fault logging and diagnostics), valve and tank stat were replaced last month with decent Honeywell stuff, the old cruddy wiring centre that seemed to confuse the old valve (hot water was never off) was removed and then I found the switch in the switched spur was dodgy and kept making the programmer flicker, so I am thinking that it's damaged the programmer or our friend (gas safe engineer and plumber) said we might need to check the gas pressure is steady.

With your behaviour the Programmer is the master that will switch circuits live and react to valves and thermostats being open/closed, then liven the output to the boiler so if the boiler is coming on at odd moments it could be a faulty programmer.

Programmer activates valves, valves activate the boiler. There’s no direct wiring between a programmer and boiler in a y or s plan system

Ignore that I was trying to think through why mine was tripping out and I was thinking programmer issue, it made me go and actually look at the wiring diagram! Might be onto something with the voltage thing for our problem as a nothing happens we generally find out it's tripped as the house gets cold or you wake up cold in the morning, flick the reset switch on the boiler front and away it goes.

[jamcg]

If you need to press the boiler reset it’s a boiler issue. Nothing external would cause the boiler to go into lockout except for a broken pump making it go into overheat. A stuck valve would stop the boiler firing in the first instance so wouldn’t cause a lockout

[jamcg]

@JonMad just had a thought of something for you to try. If you go into the settings on your nest thermostat and go to reset, there’s an option to restart the thermostat. It might be worth a go, as it might be mistakenly sending a signal for heat without actually showing anything on the display or app

[Shlergen]

Sounds similar to ours, as above valve sticking. If you have a hot water tank there will be a motorised valve - take the motor off the top (should just lift off) and use some pliers to move the valve to maximum open & close positions. This solved ours for a while but kept re-occuring. I then Tim the toolman'd and put a new motor in.

Looks like this

[JonMad]

@JonMad just had a thought of something for you to try. If you go into the settings on your nest thermostat and go to reset, there’s an option to restart the thermostat. It might be worth a go, as it might be mistakenly sending a signal for heat without actually showing anything on the display or app

Interesting idea, turn it off and on again.

We had a whole new valve put in with the boiler, thermostat and pump. Might be we have a dodgy valve. The un-requested heating seems to be every other day. 18th March, 20, 22, 24, 26, 29, 31. I’m expecting it to do it tonight again on that basis.

In the meantime I’ve been having fun doing some Nest integration.



Waiting to see the orange line to go up without having crossed the blue line. I.e. when it starts without apparently being requested.

[JonMad]

And lo and behold, almost immediately after I posted that the heating kicked in and took the temp up as far as 22 degrees without being asked. I woke up at about 3am, flipped the hot water on then off again, and within 15 mins (pump overrun period) both heating and hot water switched off and the house started to cool down again.

143DE4EA-2E36-41A1-9E92-FE28F597D4ED.jpeg (28.8 KiB) Viewed 1001 times

The predictability of this suggests a control rather than a mechanical issue.

[jamcg]

My advise would be to a) double check your schedule to make sure you’ve not accidentally set any weirdly timed switch points and then b) do a full reset of the nest to factory settings and start again

Failing that- as a temporary stopgap try to schedule your hot water to turn on for a minute just before bedtime. See if that prevents it. I know this is just hiding the issue rather than curing it but worth a try

[JonMad]

Next plan - they'll be replacing the Heat Link

[Jobbo]

Our hot water wasn’t hot on Tuesday morning, so I immediately assumed it was the zone valve which was only fitted last May.

Replaced it on Friday (thank god for fitting a decent immersion heater timer - not really used in the last 5 years but invaluable in scenarios like this) and… the hot water is still not heating from the boiler. Works fine for heating but doesn’t even turn on the boiler when you manually switch the hot water on at the Evohome receiver, so something must have gone awry with the wiring. I did fit it all into a choc-block in a standard surface mount box so it was a bit untidy. Ordered a Honeywell sundial wiring centre to rewire much more neatly. Fingers crossed I find the problem!

[jamcg]

@Jobbo Checked the batteries in the hot water sender? Evohome hot water stats are 2xAA powered rather than mains

[Jobbo]

I’ve manually turned on the receiver and the motorised valve doesn’t move, so I’m fairly sure it’s the wiring. But I haven’t replaced the batteries (no error codes in the Evohome controller) so will do so to eliminate that, cheers. Sensible suggestion.

[jamcg]

Do you have a multimeter or a voltage tester? If so then some things you can check to rule out anything with wiring and identify the problem as a valve- all colours here relate to the valve wiring:

You should have 240v permanent at the grey wire

Brown should get 240v when you press the manual override on your receiver unit

No easy way to test neutral unfortunately so thats a power off job, needing a visual check of connections and a little pull on the wire to check it’s inserted correctly

Orange is the switched live back to boiler- if you can get the valve to move, and this isn’t live then there’s a fault inside the head

[Jobbo]

I do have a multimeter, but since the wiring is a bit messy I’m happy to wire it more neatly anyway.

Battery replacement didn’t work but I shall get the multimeter out later and check it.