On 21/11/2011 16:11, Simon T wrote:

I suspected it might have been a stuck 2-port valve, but having checked
both of these found that neither one (CH or DHW) were stuck and appeared
to be operating fine.

The micro-switch on a 2 port valve could be stuck closed even though the
valve itself had closed which action should have opened the micro-switch
contacts.

--
Roger Chapman