In an earlier contribution to this discussion,
wrote:

Hi there. I have a Baxi Solo boiler connected to a Honeywell timer
control system. The control system consists of a ST699 time
controller, a T6360B Room Thermostat, a L641A Cylinder Thermostat and
a V4073A Motorised Mid-position valve. The entire system is installed
about 7-8 years I believe.

Right. The problem I have is that the boiler is firing up randomly
when it shouldn't be on. The timer says no, but the boiler fires up,
stays on sometimes a few seconds and sometimes an hour or two, then
shuts down again. Sometimes it will then come on again within seconds
or a few minutes. I've replaced the room thermostat and the time
controller to no avail. At one point the heating portion wouldn't work
at all but setting the lever on the mid-position valve to manual
enabled the heating when the hot water was due to be on. Now the lever
on that keeps shifting itself back towards the auto position, not sure
if this is anything to do with it.

This is becoming a real pain as the boiler is in a cupboard in the
bedroom and is very annoying if it comes on at 3 in the morning. It
generally is fine when the timer says it should be on, and I can get
it to work on demand, but it's the randomly coming on at other times
bit that is the issue.

Any thoughts on this? Please? I'll be your bestest buddy if someone
can help me with this. I don't want to call a heating engineer yet as
the expense with not knowing quite what is wrong is worrying, when I
mentioned this at the service a few weeks ago they didn't know what it
could be and I generally trust that company.

Has it ever worked properly while you've owned the house, or has it always
done this?

It's very difficult to envisage a fault which could develop on a correctly
installed Y-Plan system to make it come on when the programmer is off.
[S-Plan systems, on the other hand, *do* have possible failure modes which
can make this happen].

With Y-Plan (which is what you've got) the only thing which should be live
[1] when the programmer is off is the HW-OFF terminal on the programmer -
which is connected to terminal 2 of the cylinder stat and to the grey wire
of the mid position valve. See Y-Plan in
http://content.honeywell.com/uk/homes/systems.htm for a wiring diagram. Your
system should be wired exactly like this except that the pump is almost
certainly controlled by the boiler - so the boiler will have a permanent
live feed in addition to the switched live shown in the diagram, and the
pump will be connected to the boiler's pump terminals rather than just wired
in parallel with it.

If your system has *never* worked properly, I suspect that the cylinder stat
may be wired incorrectly. It's vital that it's wired exactly as per the
diagram. If any of the wires are crossed, all sorts of funny things will
happen - possibly explaining your symptoms, although I can't quite get my
head round the exact mechanism.

[1] except for the permanent live to the boiler, if it controls the pump -
but this won't make it fire.

--
Cheers,
Roger
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