On Mon, 21 Oct 2019 19:53:41 +0100, ARW
wrote:

I have never heard of it before.

Nor had I but it makes sense (or did before GW etc).

I just turn it on when it's cold and the date has nothing to do with it.

Quite, but if you polled the population you might find that most turn
theirs on about now.

During the last hot spell, an elderly neighbour we keep an eye on /
help asked me to look at her heating system as it seemed to be heating
the rads when it was set for HW only.

Removing the diverter valve head and manually setting the valve to HW
only didn't stop the CH cct from getting hot and that suggested that
the valve itself was leaking / bypassing (and I got her a new valve
plate ready for when it cooled down in the loft).

During this recent cold snap she asked if she could have the heating
on and I arranged that for her but it seemed there were additional
(/new?) issues.

1) The valve wasn't returning to the default (HW) position, even when
disconnected from the valve itself and completely powered off, unless
you gave the motor gear a little nudge. I replaced the motor (cheap)
and whilst it's better, it's not 100% (suggesting the return spring
might be getting weak or something else is seizing up).

2) The system wouldn't run on CH only, but would run alongside HW.
Taking the PCB out of the head so I could get directly onto the micro
switches showed that both switches had an issue (O/C), one on the NO
connection, the other on the NC.

So I've ordered a complete new head and can test that on the old valve
body, before draining down, flushing then draining again and replacing
the valve itself (n refilling / inhibitor etc), when the weather is
right (mild).

It's a Y plan system but with no room stat, relying on thermostatic
rad valves and the boiler stat for CH. Simple and seems to work fairly
well, all be it not very 'adjustable' (but she rarely has the need to
adjust it it seems).

One way to stop the Mrs fiddling with the room stat is to not have
one. ;-)

We don't have one either, but then we don't have CH ...

Cheers, T i m