C/H system shut down issue

8 Feb 2011
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United Kingdom
Problem: whole system stays on until I turn down room thermostat until it clicks (i.e. it isn't up to temp yet). This switches the boiler off, but the system still stays on pumping for about 15 minutes until the boiler (?) has cooled down. Changed the programmer/timer and the overheat thermostat in the boiler, and the room thermostat to no effect.

Currently have thenew timer set so the water goes off (and therefore so does the boiler) 30 minutes before the CH and then it shuts down fine. If I use the room thermostat to try and shut the system down it doesn't do it.

Faulty electronic thermostat circuit board maybe, or faulty overheat cutoff?

Boiler's a Vaillant ThermoCompact VC GB 142 EH.

Thanks in advance for anyone's help.

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That is probably the pump overrun, nothing to do with overheat.

This is allow any residual heat in the heat exchanger to be dissipated through the radiators. Its not really a problem if its a bit longer than needed, as it can also help to distribute the heat evenly throughout the system

Not sure about your boiler, but on mine it is altered by parameter D2, which can be set from 2-60 minutes.

I expect there is a setting on yours somewhere.

Thanks Keith and I think you're in the right direction. But the system used to work fine and I'm puzzled as to why the room thermostat doesn't shut the system off when it gets to temp. It used to. So because it used to work OK I've assumed somethings busted, that's why I thought of thermostats.

Hopefully there is someone here who is familiar with this boiler?

If you have the installation manual, there may be some pointers there.

There could be another stat controlling pump overrun by measuring temperature, which is worn out.
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Only new things fitted have been timer/programmer and the overheat thermostat in the boiler but still the same. Interestingly, having read Keith's post I was prompted to try the room thermostat again and it DID shut down the system when I lowered it. Yesterday when I did it later in the evening when everything was warmer, it didn't. In the past, when the system shut down according to the timer, we always heard a click from the room thermostat whatever temp it was set at. Now we don't. A puzzling one eh?

Thanks for your input.


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