Sticky Motorised Valve

[caveman2]

I have a CH system with 2 X 2 port Drayton Motorised Valves. The same problem has developed which came and went last year. The CH valve appears to allow water through when not required. When watching the valve it seems to operate correctly when programmer says on or off. But the valve remains open when CH is off resulting in hot radiators when only the hot water is required.
Is there a way to prove if an Actuator Head replacement would clear fault or is it obvious that the whole thing needs replacing. Thanks.

 

[stem]

When watching the valve it seems to operate correctly when programmer says on or off.

That seems to be contradicted by

The valve remains open when CH is off

Can you clarify what you mean please.

Meanwhile, is there a frost thermostat somewhere that is overriding the heating controls?

 

[caveman2]

The motor seem to drive on and off and operate the micro-switch. But when programmer turned off and the motor has driven back to the rest position, the valve remains open. It's as though the motor is not in fact driving the valve because it's permanently open.

I obviously cannot see what the valve is doing only the motor (actuator)

 

[oldbuffer]

Provided its the removable head type, take head off and try operating valve manually (only about 20 degrees turn between off and on).

If it turns reasonable freely and shuts the hot to the CH when off, then the problem is in the motor or micro switches, otherwise could be sticky valve.

 

[MANDATE]

The motor only drives the valve to the open position, it does not drive it to closed position.
When power is removed the motor unwinds by a return spring. If it does not unwind the spring may be the problem, but it's more likely to be worn gear teeth stopping it returning.
Or the valve splndle may be too stiff for the return spring
With no power to the valve motor, try moving the lever.
If the motor is in the rest position, there will be some resistance as the motor is wound forward and the spring will unwind the motor when the lever is released
If there is no resistance, then the motor is already forward and stuck.