On Sun, 01 May 2011 12:58:03 +0000, mick wrote:

Hokay...

(Assuming that system has a timeswitch and room stat and that no further
frost protection is needed)

Actually 3 zones of CM907/927 progstats and 2-port motorised valves (plus
HW zone) but yeah ...

Existing: Live to t/sw + room stat instead of feeding directly into SL
on boiler feeds lead boiler signal.

Live from t/sw, before room stat, goes via new return main stat to lag
boiler signal.

Why? That would bring the lag boiler on when there's no call for heat.

Now either specify boiler1 or boiler2 as lead and connect up or arrange
switching (these boilers seem to be operated by a simple switched-live
SL input). If not switched then pump is from whichever is lead boiler.

If switched, a 3-pole 2-way switch would be needed. 2 poles wired as a
reversing switch from lead/lag to boiler1/boiler2 and the 3rd pole
switching the pump live from one boiler to the other to keep it in step
with the lead boiler.

Since the boiler each has its own pump only 2g2w switch required -
standard-ish from elec wholesalers.

With external thermostat it would be simple enough: call-for heat (output
of zone valves combined) goes direct to lead boiler's SL, and via
external 'stat to lag boiler's SL (with the changeover switch swapping
over the connections).

Minimum cost, 1 return main stat. Optional 16C external hold-off stat in
series with it. Optional 3-pole 2-way mains-rated switch for duty select
(must be able to handle the pump load).

Each boiler should have it's own means of isolation, breaking both the
continuous live and the SL connection.

Yup, fan isolation switch to each.


That's a good reason to make the system easy to fix. As I've shown
above, though, in this case you could make this work pretty well without
adding more than a couple of stats and a switch. It's a pretty standard
sort of circuit for dual-boiler heating systems and shouldn't flummox
any heating engineer worth his salt too much. The important thing, above
all else, is to document your work and leave a copy with the customer
(who *will* lose it, but it makes them feel better) and another inside
your control box/ panel/whatever.

Heh, yes - preferable glued in!


--
John Stumbles -- http://yaph.co.uk

Teenagers: tired of being harassed by your stupid parents? Act now! -
Move out, get a job and pay your own bills, while you still know
everything!