On 04/10/2020 12:00, The Natural Philosopher wrote:
On 04/10/2020 11:45, F wrote:
When the room stat is calling for heat I assume the valve is driven
open and held open but is it driven closed once the stat is satisfied
or the hold released and springs are left to close it?

springs close it. suppsodely 'failsafe' on power loss

Thanks, if I turn the power off it doesn't close.

Though the fault has rectified itself a couple or three times, am I
right to assume I'm looking for a new actuator head?

No. Often the problem is scale/corrosion inside the valve body.
take theÂ* head off and wiggle the valve shaft. If it doesn't move freely
you needÂ* a whole new valve, and unless you use freeze spray or have
isolation valves a drain down of the whole CH or HW primary circuit.

The body was installed in December 2006 so it wouldn't be surprising if
it's in need of replacement.

What is odd is that with the actuator head off, the lever springs
immediately to the off position but with the head on it either doesn't
move back at all or, as just now, creeps across very slowly and not
quite fully. As though it's having to move the motor's splined spindle
as well as the valve?

--
Frank