On Sun, 01 May 2011 13:55:39 +0000, YAPH wrote:

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.



Oops! Yeah - sorry. After T/sw + room stat.


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).



Personally, I wouldn't be happy with just an external stat for lag boiler
control but YMMV. IMHO the vagaries of the English climate would leave
you twiddling that stat until doomsday trying to get it "just right".


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!

Wouldn't be the first time that's been done either! (by me ...)


The pumps still have me confused. The 15/60 can handle a greater head
than the 15/50 so why were the external 15/50 pumps fitted? Is the head
still too much for the internal 15/60s alone, and they were added to give
them a bit of help I wonder? I could understand that if it was
anticipated that only one boiler would run at once. I'm not a heating
engineer, perhaps someone else can help? I suspect that the 15/50s will
be set for full flow and the 15/60s throttled back to give about 1/2 the
15/50s flow each. Only a guess.

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
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