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.

--
Cheers
Dave.