I have acl drayton 2 port valves - on the side of each is a switch
> marked A and B - what is this for?
>
> cheers
I assume that it's an independent switch (i.e. not electrically connected to the motor wiring) which closes when the valve is fully open, and can be used to switch your boiler and pump on. This is how 2-port zone valves are used in S-Plan heating systems.