System worked ok before, either HW only or HW & CH. Pump not touched
only the valve put in place which seems to have affected flow.

Spent more time trying to work out what is happening, then fitting the
control in the first place! - never mind good excercise up the ladder.

The only effect of the valve (provided it is open!) should be to put a
little bit of extra restriction into the HW circuit - but not enough to
matter. [It *is* a 28mm valve, isn't it?]

Yep, definatly a 28mm fitting - the bore through the valve is a bit smaller
but shouldnt be a problem as the system gives HW only which is just gravity
fed.

Have you checked that, when you are not getting hot water, the valve is
actually *open* and that the boiler is still running for the CH? I'm trying
to think of how ?you could have mis-wired it to make the valve close when
the CH is on, but I can't instantly see how to do this. Anyway, it worth
double-checking the wiring - and maybe getting someone else to check it
too.

Wiring OK. If HW only, the valve opens, boiler fires, pump off. If CH only,
valve closed boiler fires, pump runs, if HW&CH then Pump runs, valve open
boiler fires. - If the valve is open and boiler on there should be HW.

Seems very odd. Maybe the HW circuit has *always* flowed the other way
when the pump is on, and maybe it doesn't matter.

I think you may have got its- its always flowed the other way round and no
one noticed, even though it is a bit odd. The gravity circuit runs the
oposite way round depending if pump is on or gravity only.

From what I can make out (and I think I understand it now) through the maze
of plumbing, what happens is a follows.

When the pump is running, the CH return pumps from its 22mm into a T to the
28mm which then goes (Boiler Return ^ HW). This is resulting in 1/2 the CH
return being pumped to the boiler, and 1/2 to the HW (which then becomes the
Flow). When the HW pipes return it arrives at the 28mm T with the 22mm CH
Flow comming off. As the HW return cant move the against the pumped CH flow,
what happens is that the HW return water get "sucked off" the T piece by the
pump (i.e CH FLOW ^ HW RETURN) so CH flow is a combination of Boiler Flow
and the HW return. (or to make it simple, CH returned water is pumped as HW
flow, and HW return if then pump round the CH circuit as CH flow)

This then results in HW being passed through the cylinder from the bottom up
but is obviously pumped when pump is on.

And you wonder why I am confused about all this. Anyway this now seems to
have resulted in that we do get HW when CH on (and it must have always been
like this), but that I have just been confused over the last 3 days! I
suspect that there was a load of air in the system which caused the initial
problem. (you could hear it in the pump, and boiler cut out a couple of
times), which now seems to have cleared.

Time to put the tools away (I hope) - thanks for all your help.