Vaillant VCW GB 242E Heating failing after long operting period 'intermittently'

The Boiler is some 10 years old, and has developed a intermittent fault ..

Functions normally most of the time, then started to fail to supply hot water for heating system.....This seems to happen if its been on for a long period of time .. ie over night with the house stat turned down (normal would be shut down 1200 till 0730)

When heating has failed -over night- Symptom's are :-

House thermostat demanding heat , (Radiators fully cold) circulation pump running, Boiler 'gas' fan running No flame No green light No orange light.

This situation will remain so indefinitely

Found to cure problem.

Turning on house hot water tap

Fan remains running Can here light 'click' from control box (relay operating) Can hear (primary water circuit) diverter valve operate Ignition system fires Gas valve activates Boiler lights Green lamp lights Orange lights

Hot water will run until tap closed .. normal operation .

Close hot tap Diverter valve operates (can hear) Boiler remains lit / re-lights flame height rises to full then backs off - normal- Heating functions normally

Till at some point the house thermostat shuts boiler down, then randomly the heating fails.to re-start when demanded by thermostat.

I have changed the micro-switch on the hot water demand 'detector' this at first seemed to of cleared the problem .. however this morning .. heating had failed .. open hot tap and close .. heating operated as normal and has remained on all day ..

I don't have a circuit for the flame control module, I only have the user documentation that came with the appliance..The schematic, only shows the inter connections. I can only assume its the part that the water demand sensor feeds that's developed the fault .. may be a 'dry joint' on the board as its seems only to happen after long periods of use ?.... as the hot water function is 100 p/c reliable all of the main parts of the boiler are functioning normally .. its got be a electronic fail ..has anyone got the circuit ?

Sorry for the long post .. but that about describes the problem Any one seen this problem ?

Tnx - Graham.

This seems like it's the diverter valve or the servo valve or diverter valve micro switches. When the CH demand is applied the pump moves the diverter valve from neutral to the left to going to CH mode. When the diverter valve arrives at the full left position one or two micro switches are made to initiate ignition.

IME a good many of the problems with the boiler (which is a generally very reliable unit) are with the water flow detector and/or servo valve assembly. This is the gadget at the low front middle on the boiler.

The piston from the water flow diaphragm get sticky at the beginning or end of its travel. as it works the servo valve and DHW microswitch. However YMMV.

This problem is marginal/intermittent and exercising the valve gear by turning on a tap temporarily clears the problem.