Blasps,
You're right.
However, in three separate instances when local plumbers were asked about a problem on three different systems, only one of which was my own, the answer has been the same: new boiler. In each case it was fixed by replacing relatively inexpensive parts, namely a fan, a PCB and diaphragm.
Anyway,
At the moment he pump runs but when the CH is on. I assumed it was fully pumped because I can hear the pump running when the boiler is on. How could I tell a gravity fed system? Don't they have larger pipes so the water circulates via convection?
I'm not at home so I can't check the tanks but there was only cold coming from the hot tap and I could hear the usual noises as the tank fills after say, running a bath.
Someone told me to check the cylinder stat and the motorised valve.
I don't know where these are. I couldn't see a stat on the tank nor a valve near the boiler but I assume they exist.
In my old place, the stat was on the HW cylinder and the valve adjacent to it.
It could be coincidence that the HW problem happened after I partially drained the rads, but I'm not convinced.
Thanks again,
Paul.