Set Square wrote:

Depending to some extent on how many conductors there are in the cable
running between the boiler and airing cupboard, and on where the whole lot
is powered from, what you want to do is probably possible.

I would have to take a look, but the only wire I know about running up
to the cylinder is to control the pump. This is via a fused switch, so I
suspect it is just a mains wire coming up from the timeswitch. I am not
100% sure but I do not believe there is any pump control coming from the
boiler (a Baxi Solo 3/50 PF); I think the switch just turns the both the
boiler and the pump on or off (the pump runs constantly when the system
is on). I wouldn't be surprised to crack it open and find both the
boiler and the pump appliances wired to the same terminals.

So my idea is, instead of sending the mains signal up to the pump, I can
just send the mains down to the boiler. That may involve ensuring that
an appropriate mains ring is available. Would it be really stupid or
non-compliant to try to get away with hanging it off the same ring that
the immersion heater is on ? Having an electrician in to run another
ring up to the cylinder would override my objective of avoiding having
to rip out walls or tiling to do wiring.

It makes sense to concentrate as much of the electrics as you can in the
airing cupboard. The pump is already there. The cylinder stat needs to go
there. It's the best [only viable?] place for the motorised valves. The only
items which need to be somewhere else are the boiler and the room stat. You
already have a cable going to the boiler. You can use a wireless room stat,
with the actual switching bit in or near the airing cupboard, and with the
wireless sensor/programming unit anywhere you like.

This is probably a stupid question, but is a room stat worth bothering
with in the first place, when TRVs are in use and installed ? I see them
in new houses around here but most people I know don't have a stat.

You will, of course, need a source of mains power in the airing cupboard. If
the cable from the boiler has enough conductors you may be able to use
that - otherwise you'll need to spur an FCU off a nearby power point.
Whatever happens, put a proper 10-way junction box in the airing cupboard
and wire it all up as per standard S-Plan stuff.

Good advice, thank you.

Your main constraint - if there is one - is going to be the existing cable
between airing cupboard and boiler - particularly if the boiler needs to
control the pump in order to provide pump over-run. You could potentially
need five conductors in this cable - Live, Neutral, Earth, Switched Live,
and Pump Control. If the boiler doesn't need to control the pump, and
doesn't need a permanent live, you can get away with three conductors.

I think I may just about get away with this. The timer seems to be very
simple, when it is on it seems to just switch mains into the boiler and
pump, when it is off both are stone-dead. I am assuming both the boiler
and the pump are not major power users so it should be simple enough to
wire them off the main that I have.

Thanks for the advice, greatly appreciated. I think I will see about
putting this plan into action.