In message m, mick
writes
On Fri, 29 Apr 2011 13:45:43 +0100, Dave Liquorice wrote:

On 28 Apr 2011 22:51:03 GMT, YAPH wrote:

I'm inclined to just have one boiler on load all the time and an
outside thermostat to bring the second in during cold weather (sort of
weather compensation scheme)

Think you would be better off monitoring the flow and return temps and
fireing both boilers until the difference got less than say 10C then
drop one of the boilers off. The logic can probably be done with simple
series connection of flow/return pipe stats, they might need a change
over switch but fairly sure they all have that.

Back feeding one boiler to the other can be controlled with a 2 port
valve wired in the normal way, call valve valve switch boiler.

I like the system Mr Bryer posted, particulary the switch to set which
boiler is "lead". When one goes phut it's simple to use the other albeit
with reduced capacity on the system.

Finally it might be worth looking at a PLC (Process Logic Controller) to
do the logic and switching rather than hard wired stats and relays. The
heating/HW system here is effecively four stat controlled zones, but
with two pumps (2 zones on each pump) from a single boiler with pump
overrun. It works but the logica isn't perfect, if the pump overun is
active for one zone and another calls for heat the previously active
pump overun doesn't drop out. I could probably add more relays, it
already uses six, but if I'd gone for a PLC it would have just been a
tweak of the porogramming.


You could use an "intelligent relay" here. You can get them with 4 n.o.
mains-rated contacts and 6 or 8 digital inputs. They include built-in
time switching, timers, logic etc. unfortunately the programming software
and lead is usually charged for, but not as much as for a proper PLC!
Most manufacturers do their own flavour of these. Have a look at the
Crouzet Millenium 3 range. This is a complete kit:
http://uk.rs-online.com/web/search/s...seAction.html?
method=getProduct&R=5368979

Of course, a real tech-head would build a board with a PIC and relays on
it and program it using LDmicro ladder logic for free! http://cq.cx/
ladder.pl


He couldn't really do "any of the above" could he

AIUI, its in a customer's house and he would be responsible for whatever
he installed

Lets get back to reality


--
geoff