In an earlier contribution to this discussion,
Geronimo W. Christ Esq wrote:

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.
When installed properly, a Baxi Solo PF *definitely* needs to control the
pump in order to provide pump over-run. Imagine the scenario where the
boiler is going flat out, and is suddenly stopped by the room stat. The
metal parts of the boiler are *very* hot, and the water inside the boiler
will get dangerously hot if it doesn't continue to circulate for a while to
carry the residual heat away. If the pump is simply wired in parallel with
the boiler, it will stop when the boiler stops - and the boiler will
overheat. It may not be a problem with your existing installation because
the pump is always on. But when you put a 'proper' control system on it, it
will be!

So, as I suggested earlier, you actually need 5 wires beween the boiler and
airing cupboard *unless* you cheat! The boiler controls the pump via a
thermostat - and keeps it running until the water has cooled sufficiently.
But this over-run period is typically a minute or less. What you *could* do
is to use a timer - like the ones used with bathroom fans - to keep the pump
going for (say) 2 minutes after the boiler demand has been cut. The timer
could be installed in the airing cupboard.


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.

The immersion heater is probably on a 15A radial circuit. You can spur an
FCU for the heating system - fused at 3A - off that with no problem.

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.
As others have said, for energy conservation you need to turn the boiler
right off once both the CH and HW demands are satisfied - and by far the
best way of doing this is by using a room stat and a cylinder stat.

One or two people have mentioned by-pass circuits. You may or may not need
one. A by-pass provides somewhere for the water the go - principally during
the pump over-run period, when the zone valves are closed. If you go for an
S-Plan system with 2 x 2-port valves, you will definitely need a by-pass. If
you go for a Y-Plan system, with a single 3-port valve, you may not -
because there is always at least one port open. Provided you follow the
recommendation of having one radiator with two lockshield valves and no
TRV - so that it can't be turned off - you're probably ok with a Y-Plan. If
you *do* need a by-pass, the easiest way would be to install an automatic
by-pass valve between the flow and return, just before the zone valve(s).
--
Cheers,
Set Square
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