In article , YAPH
writes
On Fri, 29 Apr 2011 03:52:16 -0700, cynic wrote:

If both boilers are fan flues there isn't going to be a significant heat
loss via the idle unit. I'd be incliined to simply parallel the two
(might need a relay in the call for heat circuitry depending on the
boiler innards) and set one boiler thermostat higher than the other so
it does the majority of the work. Not perfect but simple.

Point. Might just try that first. Though with standard-efficiency boilers
(which is what we have here) where the boiler's thermostatic control
operates on the flow temperature I suspect the one set to the lower
temperature would just cycle on and off, which is not ideal.

I'd like to think that the second boiler would only be required for fast
warm up and during the coldest period of the year so you could extend
the life of the installation by only running 1 except when absolutely
necessary and periodically swapping which one is the primary if you want
to balance the wear.

Say only have boiler 2 on when the return temp is below a certain level
or in combination with an outdoor temp stat? Simple relay logic control
could manage that.

Watch out for over complexity and maintainability, I am reminded
occasionally of this when kind people ask who would maintain x, y & z if
I got run over by a bus tomorrow. The answer is universally, "I wouldn't
give a f'ck" but you may be more concerned :-).
--
fred
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