Actuator or not?

[petit_pablo]

Might sound silly but do you have an immersion too? Check this hasnt been accidently turned on.

Otherwise id say the mid position valve(3way) is passing

 

[davecash]

Thanks. Yes I have an immersion heater. I have checked and it is switched off.

 

[D_Hailsham]

I have overheated water all the time, either when the CH and HW are on, or even when just the CH OR HW is on.

If set to HW only on the timer and the boiler goes on and off when the tank stat is turned up or down, then there should not be any overheating. In this condition the valve is not in circuit, so the HW (B) port is permanently open.

Check that the valve is the correct way round: Port A to Radiators, Port B to HW cylinder.