Vaillant VCW GB 242, poor CH??

[garethr83]

Just fitted a new servo valve and diaphragm due to hot water being poor. Central heating was fine.

I then got it all running again, bled the system etc, but I have a problem with the CH.

It will spark and fire up but only stay on for around 10-30secs then knock off. It will keep repeating this every few minutes.

The rads aren't getting hot.

What do you suggest?

 

[garethr83]

Forgot to add that I have removed the big screw in the centre of the pump and not sure what I should have expected but only a few dribbles came out. I could see it spinning however, but not sure how fast or powerful it should be?

I also removed the two connections to the overheat switch on the underside and this kept tripping the red light and black reset switch to pop out. I then put them back on and it seems to be running better with the rads now getting hot to touch.

How long on average should the burners light for and knock off and then back on again when CH is called for?