"keith" wrote in message
...

........ Possibly the motor has died at
some time and someone has pushed the drive unit override lever across
into the manual position which sets the valve to pass hot boilerwater to
both cylinder and radiators. As the motor does not drive the valve all
the way across the internal limit switch does not operate to call the
pump and boiler into life. Turning up the water thermostat or running off
some hot water thus calling for heat to the cylinder does ask the boiler
and pump to operate and so you get heat to your rads.
............
Thanks for replying, I've spotted there's yet another control which I
think is the one you're refering to as it's on the valve.
It slides from "Auto" at one end to "Mid" at the other. It looks like you
can take a diversion on the way to Mid and go to an alternative position
instead. There's a picture that calls this "Manual Override Only".
The slider is set to "Auto" which I think you're saying is correct. (see
valveswitch picture at www.dulwich.net/ch)
Should it be set to one of the other settings?


If its at auto then the chances are that the valve motor is ok if you are
getting heating sometimes. However there is still a possibility that your
motor has seized up with the valve open. Try switching off all power to the
heating system and then going to the valve to assess if the lever is now
"free" or requires pushing across against the spring to move the valve. Put
the power back on and set the programmer and roomstat to heat required
lightly touching against the lever and listening whilst doing this (you may
need an assistant). If the valve is ok you should hear the motor and feel
the lever follow the drive from hot water only to the heating position.
If the valve is ok then the problem will require further tracing.