In article ,
"John" writes:

I don't think you have answered what the OP asked Andrew - It seems to me he

I was following up Set Square's point, not the OP's.

was asking if the valve would (should) stay in the fully powered up i.e.
open to heating position when the clock, thermostat, etc turns off. The
answer is yes it does stay in that position unless a water "call" is applied
or as you suggested ALL power is turned off at the isolator. It is a defect
in the design of the Y plan as it can lead to the motor spending very long
periods energised although it has been a known problem for many years and it
is unlikely to be modified now. I prefer to install S plan for just this
reason.

I'm not actually familiar with the common 'plans'. My mid position
valve and boiler demand for heat are driven by a computer program
of my own design. I did however have to sit down and carefully
work out how to drive a mid position valve in order to do this,
and it wasn't as obvious as it seemed at first. I talked with one
of the manufacturers who were extremely helpful and put me in
direct contact with one of their valve designers, who explained
the residual magnetism issue (I kept having the valve stick in
the middle position until I discovered I had to degauss the yoke).

--
Andrew Gabriel