"Danny" wrote in message
om...
Again Roger, thanks for all of your help.

Thats right, it only trips when the CH motorised valve is closed. I
fully opened the by-pass valve, with the CH circuit off, I noticed
that the water wasn't passing through the by-pass valve, massive
temperature difference on either side of the valve. I'm thinking that
this valve may be faulty and is stuck closed (it's not motorised),
should I replace this or is there another method of fix?

Regards

Sounds like you need to replace the valve. You might consider using an
automatic by-pass valve - which only opens when a certain (adjustable)
pressure is reached, so that nothing goes through it when either or both CH
and HW circuits are operating. Screwfix do a suitable valve for about £25.

Roger