Whilst it was warm I decided to add some controls (i.e none before) to the
central heating system.

Exiting system is gravity (pumped heating). Two 28mm pipes out of the
boiler, then a 22mm pumped circuit is taken of the 28mm pipes.

To keep things simple I went with what I think is called "honeywell C Plan".
Wiring OK (after a google of this group) but I have a problem.

Before starting I put heating ok HW only, and checked which was flow &
return. I then fitted the valve on the return pipe (with the correct flow
direction A-B) was recommended doing a search and as access was easier.

After refilling I put HW and CH on, and then noticed that the water was
trying to flow through the valve (i.e B-A) and that the pump was therefore
in the return. (i.e when the pump was on, circulation was in the opposite
direction to what gravity was running before fitting the valve)

With the HW & CH on the house got warm and all the flow/return pipes by the
boiler but the HW coil did not heat. Putting the system to HW only also did
not work. I suspected an airlock somewhere and tried to clear without
sucess.

In the end I guessed that I must have been mistaken which pipe was flow &
return and though that I have fitted the valve in the flow. I then drained
the system again, and swapped the valve around so that the valve was in the
flow with direction (A-B). Upon refilling the system, we now had
HW/HW&CH/CH. When HW only was on, the only the HW feed got hot, then the HW
return. CH was cold. All seemed ok.

This morning, put HW only on, and I notice that the water is now flowing in
the original direction (i.e through the reversed valve B-A) and then up the
HW feed. There is also now some gravity circulation up the CH feed (but no
heat in any rad). HW is heating.

How can the water travel in opposite directions between gravity & pumped. Is
it possible that gravity flow runs either direction for some reason?

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