Most importantly, which way should the valve be fitted, and how can I
solve this problem?

It sounds like it's the *pump* that you need to turn round, rather than
the
valve!

Been thinking about this untill I got a reply, and came to the conclusion
that the pump should be reversed to that all the water goes in the same
direction. Was a bit worried about reversing the pumped from in case it
affected any of the TRV.


You say that you have ony 2 pipes going to the boiler - 28mm - and that
the
22mm heating circuit branches off these.

Yep, thats right.

In HW-only mode, gravity flow will be from the top of the boiler and
return
into the bottom. When you turn the pump on, the direction of flow through
the pump will determine which way the water goes round the CH circuit -
all
the way back to the boiler - including through the bits of 28mm pipe which
are shared with the HW circuit. Here lies the rub! If your CH flow uses
the
*bottom* connection for flow (which isn't a good idea anyway, because the
boiler thermostat will be at the top!) and the *top* connection for the
return, this will interfere with the gravity flow - and either stop it
altogether or make it go round the "wrong" way.

Think you are right. When the HW is on (i.e gravity) water heats ok. When CH
is on heating is OK. When HW and CH are together we get no ho****er.

So make sure the pump is the right way round. Was your system ok before
you
fitted the valve? Have you touched the pump during your upgrade operation?

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.

Incidentally, I doubt whether the valve really *cares* which way the water
goes through it - but you might as well line up the flow with the arrow
while you're at it. [Unless anyone knows different?]