I've just had a check done by a CH engineer who thinks (not sure) that my
motorised valve is set up wring, in terms of the ports.
At present :
A goes down through the floor
AB connects to the pump (then goes into the loft, where there is a wierd
t-port device which then goes back down through the floorboards) - assumed
to be source from boiler
B goes to the hot water cylinder
So, what he says is the problem is that when the heating is on so too is the
hot water, which cannot switchitself off (thermo stat bypassed in favour of
central heating thermostat).
I must admit to having installed this MV myself but only after going to a
local plumber merchant and asking for like-to-like mv replacement.