On my friends central heating I can't get HW on its own. It is basically a Y
plan system. CH on its own is OK; the valve goes to position B, CH and HW OK
valve in position A + B, but HW on its own the valve stays in the mid
position. For HW only I should have both white and grey wires to valve
inactive, but I am getting white staying live.
If I disconnect Orange at this point the valve moves correctly to pos A.
I am looking at this because of an original fault where the boiler case
shorted through the supply wire insulation. This caused the HW contact on
the programmer to stick in the ON position. Is it likely that the same fault
damaged the contacts in the three pos valve?
Can I replace the actuator without draining the system?