In an earlier contribution to this discussion,
Chris wrote:

Thanks for all the replies. You won't believe it but it has started
working properly again as mysteriously as it stopped! However I
suspect it will stop again one of these cold frosty mornings!
From reading the posts I gather the problem is almost certainly with
the 2-port valve. I did try manually opening the valve but the boiler
still would not fire up until I turned on the HW circuit. The valve
always closes quickly with a 'whizz' sound but is quite slow opening
with more of a quiet creak than a whizz. Not sure if this is normal,
sounds like during opening it is tightening a spring loaded coil or
something like that.

Is it tricky to replace the motorised valve? I know how to drain the
system and basic plumbing. Might be a job for my plumber though.
Thanks for all the help which has been very informative.

Cheers,
Chris.

It's not difficult - especially if you've got any manual gate or lever
valves which enable you to isolate the motorised valve. Otherwise, it's at
least a partial drain down job.

From what you now say, it sounds as if the valve is just a bit stiff rather
than seized. You may be able to free it up by removing the actuator and
turning the exposed spindle (of the valve itself) back and forth a few
times. You should be able to rotate it through 90 degrees with finger and
thumb - but may initially need to use pliers if it is stiff. You don't need
to drain anything to do that. Don't disturb the electrical connections to
the actuator.

To get an idea of what it's supposed to sound like, open the valve manually
and operate the actuator while it is off the valve. If you call for heating,
the motor will turn in order to think it's opening the valve - and will wind
up the return spring. When it gets to the end of its travel, the motor will
simply stall and - hopefully - the contacts will close and turn the boiler
on (which is why you need to make sure that the valve is physically open
before doing this). When you turn the heating off, power will be removed
from the motor, and the return spring will move the actuator shaft back to
the valve closed position.

When the valve is moving freely, the opening and closing sounds will be the
*same* as the sounds made by the detached actuator.
--
Cheers,
Roger
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