On Wed, 10 Sep 2008 12:47:45 +0100, The Natural Philosopher wrote:

You probably need some sort of secondary pump and seection of morosed
valves that will circulate that hot water thruogh all primaries instead
of te boiler. I.e. some form of changeover switch so that when there is
enough heat being produced by the fire, it takes over as a 'boiler'

No a solid fuel boiler must be able to dump it's heat without any
power(*). ie an open (as in no valves, other than isolating ones for
maintenance) gravity loop. You interface this to the existing system with
a "dunsley neutraliser".

It would be wise to have a radiator (more than a towel radiator as well)
on the wood burners gravity loop as a place to dump heat when the HW
cylinder is boiling (literally). This can be controlled with a thermosatic
valve, that is one with a wax sensor on the pipework which opens the valve
when it gets to hot.

(*) Partly so you don't run the risk of having a valve fail closed or a
power cut when you are not around with the fire burning and the thing
exploding or generating rather more hot water, steam and pressure than one
would like. And, probably more important so that you don't have to shut
down your independant source of heat when there is a power cut.

--
Cheers
Dave.