About 18 months ago we had a new central heating system installed
useing a Potterton Performa 24 Combi Boiler. This worked very well for
the heating and ho****er. Quite recently a very odd fault developed.
When the Boiler was switched to hot water only the central heating
continued to operate. I am not sure if a valve operates to divert the
system to hot water only and this is stuck, or it is something
electrical. I have tried turning the boiler off and letting it cool
for a couple of hours, but when it is switched on the same thing
happens.

john wrote:
About 18 months ago we had a new central heating system installed
useing a Potterton Performa 24 Combi Boiler. This worked very well for
the heating and ho****er. Quite recently a very odd fault developed.
When the Boiler was switched to hot water only the central heating
continued to operate. I am not sure if a valve operates to divert the
system to hot water only and this is stuck, or it is something
electrical. I have tried turning the boiler off and letting it cool
for a couple of hours, but when it is switched on the same thing
happens.

Yup, sounds like the divertor valve...