I have Potterton Netaheat 16-22 which has banging pipes when the water is heating. This is caused by hot water returning to the feed/expansion tank to such an extent that the tank overflows.
The heating had worked OK up until today when it stopped completely. The timer and room thermostat seems to be working OK. The water still heats and the banging is still there. The pump is working OK (though a little noisy). It has been suggested it could be boiler thermostat?
Any solution are more than welcome.

Trevor Gamesby, April 2008

could well be thermostat. if hot water is blowing over f/e tank you need to get it looked at soon as possible. i have seen plenty of tanks melt due to overheating. also constant pumping over will add oxygen to heating water which in turn will cause system to sludge up.

spook15, April 2008