On Fri, 29 Apr 2011 03:52:16 -0700 (PDT) Cynic wrote :
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.

That was the Keston suggested method.

--
Tony Bryer, Greentram: 'Software to build on' Melbourne, Australia
www.superbeam.co.uk www.eurobeam.co.uk www.greentram.com