potterton profile 50l boiler bangs when h/water only is on

[slackjawgaze]

Hi
I have a potterton profile 50l boiler.

When the boiler comes on for the hot water only - after 5 minutes the pipes start banging really loudly increasing in volume !!! (this only happens if we dont give the boiler at least a 4hr period of non-use).

If i then turn on the central heating the noise stops straight way !

Can anyone explain 1. why it stops the noise ?
2. What i can do to fix this problem ?

thanks

Carl.

 

[picasso]

it sounds like the cold feed is blocked or the small tank in the loft is empty, there is probably enough water in the system for the pump to circulate but you need a complete circuit for the gravity hot water to work properly.