In an earlier contribution to this discussion,
Steve wrote:

I have a gas boiler that provides the heat for radiators and water. I
have an electronic timer that allows me to set two on and off times
each day. The boiler will run at those times if required and I can
select "off", "auto", "once" or "on" for heating and water separately.

My normal summer setting is "off" for heating and "auto" for hot
water.
In the last week, I have noticed that the radiators are heating up
when the boiler is running, even though the controller is set to
"off" for heating.

I have no room thermostat, but each radiator has a thermostatic valve,
so I can turn them all off, but that isn't really the point. I want
to get things running smoothly before the winter comes.

I had the boiler serviced two weeks ago, but he just hoovered the
boiler out, checked gas pressures etc., so I can't see how he could
have caused this problem.

The only other components of the system that I can see are one
motorised valve (I'm not sure if that controls the water or the
heating), and a pump - presumably for the heating.

Can anybody suggest where the problem might be and whether I need to
call in professional help to fix it. I can do basic electrics and
plumbing, but I've never drained down a central heating system.

TIA

Steve

Did the problem start when the boiler was serviced? Even if no other parts
were touched, the system could have been tested after the service and - if
you have a 'sticky' zone valve - that could have jammed in the mid position.

I am assuming at the moment that you have a Y-Plan system - which is fully
pumped, with a 3-port valve to direct water either to the heating coil in
the HW cylinder or to the radiators or to both at the same time. The valve
has a small motor to move it to the mid (both) and CH positions, and a
spring to return it to the HW position. Sometimes, if the 'wet' part of the
valve gets a bit stiff to turn (particularly in the summer, when it doesn't
get any exercise) the spring is insufficient to move it back - and it sticks
in the mid position so that whenever the HW is on, the radiators get hot
too.

Many - but not all - 3-port valves allow you to remove the electrical bit
(actuator) from the wet bit by undoing a couple of screws. If yours is like
this, remove the actuator and check how easy it is to rotate the shaft of
the valve. You should be able to move it by gripping it with your finger
and thumb - or certainly by using *light* pressure from a pair of pliers.
[It only moves through 90 degrees, with the mid-position being at 45
degrees.] If it is very stiff to turn, you may be able to free it by
gripping with pliers and moving it back and forth. If not, the wet part of
the valve will need to be replaced - which involves a partial drain-down.

While the actuator is off - but still connected electrically - you can test
it by selecting all permutations of HW and CH at the programmer/thermostats
and check that the actuator moves to the expected position in each case.

If your system isn't like this, we'll need to think again. In that case,
answer the questions posed by Christian about the number and type of valves
etc.
--
Cheers,
Roger
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