banging Potterton

[Durn]

Hello, I have a banging combi (Potterton Lynx 2), it works fine when CH or DHW are operating individually. But if the CH is on, when hot water is drawn it makes a single fairly loud bang.
Suggestions as to the cause greatly appreciated!
p.s. As I'm writing this, I've just heard it bang again as the pump stopped?!

 

[ollski]

It may be that the ball in the diverter valve is swelling and getting stuck in heating as the springs are trying to return it to hot water.It will hold stuck for a second the bang the pipe as it flies across.

 

[Durn]

Thanks for the advice ollski. The ball was indeed swollen - so much so that it was virtually in contact with both seats simultaneously so certainly must have been restricting flow as well as sticking.
I was pleased to discover that the ball and seal were available a service kit, though shocked at the cost... almost £40!! There's probably a cheaper way to obtain a rubber ball but anyway, very pleased that the banging is cured.