In message , GB
writes
In the area where the pump is I've now removed all the housing so I acn see
what's going on.

The pump impeller shaft is definitely turning, but I have no way of knowing
whether the impeller has come off the shaft. So, that's a possibility.

I have a pair of pipes labelled bf and br - boiler feed and return. the bf
pipe has the pump on it.

The boiler feed pipe leads down to a 3 way valve. Honeywell VC4013-02. One
arm leads off to the hot water cylinder. That pipe is hot. The other arm
goes to the central heating flow pipe. That side is all coldish. So, I'm
coming to the conclusion it's actually the motorised valve that's not
working correctly.

I took the actuator off, and there's a spring loaded piston on the hydronic
part which I can't move. I'm not sure how stiff this is meant to be? This
part of the system is all in 28mm pipework, so it's a really chunky valve
and I'm not clear how easy it ought to be to move the piston?

In case it's any help, I found a spec sheet for the valves he
http://www.honeywell.be/DocsAdobePDF...ear/VC4013-601
3_ps.pdf




There's lots of info in the DIY-FAQ


--
geoff