Problems with a Potterton Netaheat 10-16 Boiler?

i presume you have a open vent sysyem (header/expansiontank ect) almost certainly your motorised valve which controls the route of flow to c/h , h'w or both is not operating properly and there fore allowing flow around all pipework, rather than shutting the entry to the c/h when in hot water mode. you can check by alternating h/w -c/h call up and seeing if the valve moves to its appropiate position.

Desman
October 2006