In an earlier contribution to this discussion,
Steve wrote:

here are some photos that might make things clearer

pipework
http://i86.photobucket.com/albums/k8...pipelayout.jpg

one end of electrical bit
http://i86.photobucket.com/albums/k8...ann/oneend.jpg

other end of electrical bit
http://i86.photobucket.com/albums/k8...n/otherend.jpg

Having now had a closer look, its a Honeywell 9123. Info on the
"other end" gives a part number for head replacement - is that likely
to mean that the electrical bit is removable?

I have tried "rebooting" but nothing moved. On closer inspection, the
lever slot is marked Auto on the left and Manual On on the right.

I have now found one combination of lever position and on/off settings
where the lever does move under its own steam - If I push the lever to
the right, then put Water ON while heating is OFF, the lever moves
back to the centre position


Thanks for taking the time to help me to understand this

Steve

Nice photos! It's a fairly conventional fully pumped Y-Plan system, but I
think you mis-described it a bit in an earlier post.

Water comes from the boiler, through the pump, and up to the central inlet
of the 3-port valve. The LH outlet goes to the radiators. The RH outlet goes
to the cylinder coil. The bottom water pipe (not the gas pipe) comes back
from the coil and returns to the boiler. It is probably joined by the CH
return under the floorboards. The small pipe with the gate valve on it is a
by-pass so that if the heating is on but all the radiator TRVs are closed,
the water still has somewhere to go.

The fact that a part number is quoted for replacing the head almost
certainly means that the actuator can be detached. Maybe there are screws
going up into the bottom of it, through the brass flange on the top of the
wet bit of the valve?

Are you absolutely sure that the valve isn't returning to the HW position?
When it does, it requires pressure on the lever to move it elsewhere. when
it is in the mid or CH position, the lever flaps about without doing
anything. Note that in the manual position, the lever only moves the valve
to the mid position. There is no way of moving it manually to the CH
position.
--
Cheers,
Roger
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