On 26/11/2011 21:41, Simon T wrote:

A quick follow up for those still reading this thread.

Went back round the other day and checked the continuity of the grey &
orange wires on both valve actuators and both were fine.

Got continuity when the valve was open and no continuity when the valve
was shut.

Switched the power back on, system seems to be working again. System
shuts down when the valves are closed.

I'm wondering if my mother buggering about with the programmer might
have had something to do with it not working (though the timer was off
when I checked it out last time when the pump was still running).

Well you could get pump overrun for a short time but that would be a few
minutes rather than hours so if it was well after the programmer
switched off then it will have to be an intermittent fault. Intermittent
faults are a pain to trace as they tend to go away just when you think
you have got to the root of the problem.

I have had such a fault for years which causes the overheat stat to fire
as the bypass circuit can't cope if the boiler keeps firing when all the
valves are closed. But it only occurs very occasionally and usually late
at night. I think I have finally cured it having found it in action when
I was forced out of bed in the early hours by bladder pressure but I am
not at my best at 4 in the morning. It was the newest of my 3 two port
valves which I have discounted before because it was so new. Since
changing the head there has not been a recurrence but it is early days yet.

--
Roger Chapman