I have a gravity fed CH and HW system, approx 12 years old. We are now not
using the CH part (summer??) but notice when the timer comes on to heat the
water (twice a day) all the radiators get hot aswell just as though the CH
was also selected. The system is run by a single timer unit with seperate
programming for water and heating. The CH programme is definately set to
off. Does anybody have a clue where I should start my investigations?
It may have a little lever, on one side.
You may be able to hear it move (putting a long screwdriver against the
case and the other end on the bone just uder your ear may help - by
conducting the sound)
If you /very carefully/ remove the cover - bearing in mind that there is
mains voltage in there - you may be able to see it move, or try to move.
If there is no apparent movement, no matter what - then you really need
a test meter to see if its electrical input is changing. If that is, but
the valve isn't moving, then it's stuck. If the electrical input isn't
changing, then there is a wiring or controller problem.
It is fairly easy to replace a valve. The problems come with draining
and refilling and with undoing nuts that may have seized in place.
Some makes and models allow the "silver box" to be replaced, leaving the
It may just have got 'confused'. You can reset it by removing *all* power
from the system - by which I mean by turning the mains off at the FCU, not
just at the programmer. When you do this, you may hear the valve move back
to its rest (HW-only) position under the action of its spring return. It may
then work ok when you switch back on again.
If that doesn't work, you need to investigate further by removing the
actuator (the electrical part) from the 'wet' part of the valve. If, when
removed, the actuator socket (the bit which fits over the spindle of the
valve) immediately rotates to a diffeent position, it indicates that the
valve is seized - or, at any rate, too stiff to be moved by the actuator's
spring return. You should be able to rotate the spindle to and fro by
gripping it with finger and thumb. If you can't, you may be able to free it
by turning it to and fro with a pair of pliers. If that doesn't work, you'll
need to replace the valve - which involves a partial drain-down of the
You may *think* that - in which case your 'experience' is rather limited!
The actuator in a 3-port valve is actually quite complicated - with diodes
etc., which cause the motor to stall in the mid position under appropriate
circumstances. Mine has certainly got confused  on occasions, and has
been perfectly ok again as soon as it has been reset by removing *all* power
I have suggested the same solution in the past to a number of previous
posters with a similar problem - and many have come back and said that it
fixed the problem.
 where 'confused' is defined as a state in which the valve stays in one
position and fails to respond to changes in demand even though the wet part
of the valve isn't seized. This may well be due to a micro-switch failing to
open or close at the right time, but is often fixed by doing a reset.
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