Old Vaillant VCW GB 242 ignores Thermostat- always on -help?

[Grangey]

Hi all,

Took over a property about a year ago with an old Vaillant VCW GB 242, always had a theory the central heating wasn't playing ball.

Long story short, last week we replaced the old thermostat and separate time clock which we confirmed isn't working (turning down would have no effect, and boiler didn't show obvious signs of following in the timer) in favour of a Nest.

During installation our Electrician couldn't get it to show obvious signs of behaviour it was recognising instant demand from the thermostat, he tried multiple configurations from both a mirror of how the old one was (not that we know this ever actually worked), to a few others possibilities based on the wiring diagram for the boiler.

It's been 48 hours since install, confirmed the thermostat has issued no demand to the boiler, and the house's rads are still warm/hot and the internal temp hasn't moved from 28oc! So the boiler is clearly always powering the central heating.

Can anyone suggest what might cause this? With it being such an old (but seems highly regarded and reliable boiler), it seems like it needs someone that really knows this kind (or at least age) of boiler as it's stumped our electrician.

Thanks in advance!

 

[stem]

I have been to several installations with the same fault you describe. They all had one of two faults, neither of which relate to the boiler itself.

1) The external control wiring was from an older installation and although in one instance 230V was present, the wiring hadn't been re-connected to a replacement boiler which had been installed, so weren't actually used.

2) Incorrect wiring of the controls, so that the switched live was permanently energised. There are quite a few possibilities of incorrect wiring, so an inspection would be required.

It would be simple for someone competent to disconnect the existing control wiring from the boiler (assuming that it is connected of course) and test the boiler stand alone, by adding and removing a 'link' in the external control wiring terminals, to see if the boiler then responds as it should or not.