In article ,
"Set Square" writes:
In an earlier contribution to this discussion,
wrote:

My mid-position valve stays powered up in the Heating position even if
nothing is demanding. Is this correct, or should it return to the
Water position using the spring return (like it does if you completly
power down the whole central heating system)?

Cheers

Paul

It seems normal for it to stay put. If you look at the Y-Plan wiring diagram
in http://content.honeywell.com/uk/homes/systems.htm you'll see that one or
other wire to the valve is receiving power under just about every condition.

The acid test is to remove all power from the heating system by turning it
off at the FCU (not the programmer). *Then* you should hear the spring
return taking it back to the HW position.

If it was in mid-postion when you remove power, then it is OK for it
to either stay there or return to the hot water position. Mid position
is normally held by feeding the synchronous motor with a DC supply
which locks its rotor. When power is removed, you can be left with
residual magnetism in the field yoke caused by the DC supply having
slightly magnetised it, which can continue to lock it into mid-position.
Often a slight tap in this state or touching the indicator lever is
enough for the spring to overcome this temporary mangetism. When the
unit is powered and needs to return to the hot water position, a tiny
AC current is leaked through the field winding via a 270k resistor which
is just enough to kill this temporary residual magnetism in the field
yoke and allow the spring return to operate.

--
Andrew Gabriel