stevep wrote:
I have a gravity fed HW and pumped CH system with a programable
thermostat. If I have the thermostat programmed to come on all day
inc. overnight then the boiler keeps tripping in to heat the water
despite heat not being required for the CH.
Can I fit a thermostat to the airing cupboard tank so that when the
room temp is OK the boiler will not light up or is there some other
easy solution?


Yes - I did that in two previous houses and I can recommend doing it.
The problem is that the system tries to keep the water inside the boiler
jacket hot all the time that the heating is on and it just pumps water
to feed the radiators when the room thermostat is cold. You don't need
to keep that boiler hot and a cylinder thermostat is the answer.
Previously the boiler rarely stopped, afterwards it rarely came on - I
think it made a big difference to bills but didn't measure the savings.
Let's have a go at explaining it:

I had a clock timer with two outputs - 'need room heating' and 'need hot
water'. As it was a gravity fed system you have to have hot water when
you have room heating. There was a little cam or peg I recall on the
one which was a mechanical clock, a switch or wiring setting on the
electronic one, to set that rather than having them independent as you
can have with solenoid valve systems. Your timer will be set like that
already I expect.

I fed the circulating pump from the output of the room thermostat.

The hot water cylinder thermostat was a two way changeover switch - the
common went to the gas solenoid (via the overheat thermostat inside the
boiler). The contact which is made when the tank is cold went to the
timer contact 'need hot water'. The other one to the output of the
heating thermostat.


An ascii sketch:

BEFORE
-------
+ HW-timed--------------*-------------------- to gas solenoid
!
Clock !
! room stat
+ heating-timed----/ ---*--------------------- to pump
-------


AFTER
-------
+ HW-timed------------*---! cylinder stat
! -------------------- to gas solenoid
Clock ! !
! room stat !
+ heating-timed----/ ---*-+--------------------- to pump
-------

The room stat is 'down' or closed in the sketch when the room is cold.
The cylinder stat is 'down' in the sketch when the cylinder is hot so
gas is only supplied if
- timer set for hot water AND the tank is cold OR
- timer set for (heating plus hot water) AND the room is cold.

Neutral connections are not shown, but it's worth checking whether your
room thermostat needs and has one - cowboys often omit it as it needs
another cable but omitting it causes sluggish switching with the rooms
hot for a bit then freezing cold an hour or so later.

If you have a combined thermostat and clock then you might have
commections at the points marked --*-- . You'll certainly have two outputs.

Hope this helps, but email if you have any questions.
good luck!

Phil