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When I select 'ch only' on my control unit the lever on the three port valve moves over to 'H' and the pump switches on/off according to the thermostat telling the pump when the room has reached/not reached temperature. When I select 'hw only' on my control unit (central heating off) the lever on the valve moves to 'W' and heats the water in the tank until it reaches temperature fine.

When I select 'ch and hw' the lever on the valve moves to 'H' and central heating is on fine, when the room thermostat clicks off, the lever stays in the 'H' position and the pump on, the heating stays on and the lever does not move over to heat the water.

I then selected the 'hw' off and put 'ch' on, which by itself works fine on and off according to the thermostat, when I turned the thermostat down the pump clicked off, I then switched 'hw' on via the control unit, the lever on the valve stays on 'h' and the pump starts heating up the rads even though the thermostat is not on.

Any ideas please?

Has this fault suddenly appeared or has it always been this way?

Have you checked the tank thermostat and what it does if you adjust it?

Is there some confusion between a mid position 3 port valve and a diverter 3 port valve.
Can you give make and model number of the valve.
Is this a new fault or has the system always beenlike this?

Checked wiring and there was a loose connection, now working fine, thanks for replies