On Fri, 06 Feb 2009 10:50:32 -0800, heeley wrote:

Hello people,

I've recently encountered a puzzling problem with a Vaillant GB 242 .
Hot water works fine, but on calling for central heating the boiler
appears to be dead. There is no roomstat and on on operating the
external programmer I can hear a relay click inside the boiler control
box. On switching the boiler off at the spur and on again the boiler
will fire up and I then get central heating for about a minute, the
boiler then shuts down again.

I'm inclined to think its an electrical fault maybe the heating
thermistor being out of range, but since I don't know what the correct
resistance of it should be I can't draw any conclusions. I assume the
overheat stat will prevent the boiler firing up in both hot water and
heating modes? so I can't imagine it is that. Any experts around ? I
deserve an easy boiler after my last (successful ) escapade with an
Isar.


I presume this is the Vaillant VCW 242 GB which is a combi with two white
metal doors and a grey metal fascia?

This problem could stem from a number of sources. The fact that the
boiler works for DHW means a lot of stuff must but OK.
So we know the pump, fan, gas valve, ignition, DHW flow detector, DHW
microswitch, diverter servo valve are good. Also the servo valve must be
able to go to the right from neutral to DHW mode.

No CH?
Could be servo valve not returning under spring pressure to the CH
position. That's the triangular valve front mid bottom. Or the diverter
valve (back right bottom) can't go right. Or there is some problem with
the control boards, or maybe even external controls. Possible with the
temperature sensor. Servo pipe blocked.

Good chance of it being the servo valve not being returned to CH, because
of general crud/corrosion in the DHW flow detector.

HTH if the above leaves you bewildered get in a pro who knows VCWs


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html