potterton suprima 30 hot water

Hello, I am experiencing problems with the above unit, lately no hot water even though the pipes are red hot nothing hot is coming out of the taps. The status light is flashing green.
It will fire up for a few seconds and then stop with the green light flashing again.
The central heating is not on just the hot water. There is another problem when the CH is on namely loud banging, gushing of water through the pipes, then quiet and normal function returns, this can happen a couple of days in a row or even go a week or so before the banging starts again.
The PCB was replaced by a Potterton engineer about 3 years ago.
Appreciate any help.
Thanks.

First thing to check would be that the pump is working correctly.
See FAQ's for how to do this.

flashing green light means the boiler is up to temperature. from the noises you describe the pcb is probably ok so faulty pump,blockage, airlock are all likely culprits.

Thank you both for the quick replies and advice, I will check what you say, very much appreciated.

the pcb is probably ok as it's been changed so check faulty pump,blockages, airlocks e.t.c