Vaillant VCW - random shut-off + internal flue question

[drb]

We have a 22yr old Vaillant VCW Gas open flued boiler, doing CH & HW

We've recently had it serviced, including having the heat exchanger fins removed & washed clean of debris. A new thermocoupler was installed too. It then worked fine for approx 3 weeks.

Now it just shuts itself off, including the pilot. Sometimes this takes an hour to happen after firing up, sometimes within seconds of firing up.

This problem happpens 70% of the time. If there is any pattern, the morning cycle (HW only for an hour) runs just fine but when the CH comes on, clunk, off it all goes.

You can relight the pilot just fine but, if either CH or HW is on, clunk off it goes within 30 seconds of it firing up.

I noted a couple of sites on the web suggesting pushing the reset button (makes no difference) OR a microswitch behind the red button (not willing to take it apart) OR a blockage or misfunctioning valve slowing the flow, thus overheating the water (wouldn't know where to start, though I do note the rads get very hot when working so I don't think sludge is the answer)

Any wisdom you could impart would be welcome, if only a pointer to something else I hadn't thought of

Thanking you in anticipation

- David

[Ed Sirett]

This sounds like a non-electronic version with a t/couple. Given that you seem to be able to relight the pilot OK I guess this would be the Overheat cut out coming into force.

You then need to determine if the boiler really is overheating or not.

It could be the pump?

[Psst]

The Sine VCW has a simple pilot light and thermocouple set up. If the pilot light isnt staying lit then its either the pilot burner thats defective in some way i,e partially blocked,or the t/couple. There is an interlink inside the control box but all this does is operate a microswitch when the pilot has been successfully established,so providing power to the next control stage. I have some of my own notes for the VCW including functional flow diagrans, If you send me an email I'll pdf them for you..

email to [e-mail] but dont forget to change the 1999 to 2007 before you click send!