Combi boiler firing of it's own accord, at random

[Masood]

A little bit of a puzzler here - Vaillant combi boiler with a Honeywell RF programmable room stat. Customer says she comes home from work to find the rads warm, even though the programmer is set to "off". No symbols on the boiler display at the time.

I had a look at the programmer, it is synched to the receiver, and works as it should. There are no DIP switches on it to alter RF frequency, so I can't fully rule out the possibility of a neighbour's programmer switching it on, but customer says there's no pattern to it - it fires up at random. Also the flats are not new-build, so not very likely that they all have identical units.

The only discrepancy I have found is that the bathroom thermostatic mixer valve is letting by - cold water comes out on full hot setting, even after hot water has been isolated from the boiler. Do you think this could be back-feeding into the boiler and triggering the flow switch, and in combination with a faulty DV, is warming the rads? Seems a bit unlikely to me, but I can't find anything else obvious. British Gas changed the DV a few months ago - she has the receipt - so that is pretty new. Any ideas, folks? Ta!

 

[diamondgas]

I'd be checking the Room stat settings first, if it's programmable? Then suggest the custard leaves the boiler on water only (if possible) when she's off to work? Only other possible is frost protection kicking in?? Depends on where the boiler is fitted etc.

 

[Masood]

I'd be checking the Room stat settings first, if it's programmable? Then suggest the custard leaves the boiler on water only (if possible) when she's off to work? Only other possible is frost protection kicking in?? Depends on where the boiler is fitted etc.

I thought of frost protection, but this has been ongoing for a couple of months, and it's only just turned cold down here in Londonshire. Should have mentioned, she doesn't use the programmer as a timer - just turns it on and off manually, so when she's out, the programmer is set to "off"...

 

[diamondgas]

What's the make and model of the boiler masood? Sounds like a possible hw pre-heat issue? Maybe combined with a clogged p-hex...??? Just a guess without testing TBH

 

[AWheating]

check that the honeywell stat is set up correctly. some honeywell programable stats have a setting that when the heating is set to off it has a back ground temp that triggers the heating to come on between time settings/daytime to make sure the house does not drop below 16 degrees(hypothermia trigger temp). this temp can be adjusted or turn on and off in the installer settings.

 

[SteveTheGas]

Pre-heat, diverted valve letting by?

 

[Masood]

check that the honeywell stat is set up correctly. some honeywell programable stats have a setting that when the heating is set to off it has a back ground temp that triggers the heating to come on between time settings/daytime to make sure the house does not drop below 16 degrees(hypothermia trigger temp). this temp can be adjusted or turn on and off in the installer settings.

Cheers AW - it's a very old model, I don't think it's got that function, but I'll have a closer look when I go back.

 

[JJ The Plumber]

Switch off the preheat and see if the problem still occurs.
It will be one less thing to cause you a problem.
I had something similar recently and I found that the prog stat seemed quite unusual at times. I replaced the batteries as they said it was about 2 years old and now all is well.
It seemed to be sending unusual signals as the batteries failed.
Might be worth a shot.

Cheers JJ