HI Martin

On Thu, 23 Mar 2006 23:26:22 +0000, HandyMart
wrote:

The plan now is to disconnect the oil fired aga and fit a higher output
wood (multi fuel) burner as a replacement to heat water and rads. There
is already a heat dump rad permanently across flow and return with three
zone values controlling circuits to DHW cylinder coil, bathroom towel
rails, all other rads.
The programmer would be redundant as the firing up of the burner is
entirely manual but should the zone valves be retained with some simple
manual switching maybe using the cylinder stat to give hot water
priority or would it be simpler to remove those also.

Martin

Are you fitting a woodburning 'stove' (ie. something that sits inside
the house and looks reasonably decorative whilst directly heating the
room it's in, as well as having a back boiler) - or what you might
call a 'furnace' (something whose sole purpose is to put heat into the
circulating water circuit) ?

The only reason I ask, it that we've found that with our multifuel
stove it's a fine balance between 'direct heat to the (big) room it's
in' and 'heat to the water circuit'. You can easily have 'too much'
direct heat - while trying to get the radiators heated up. Depends on
the layout of your home - but the lesson seems to be that the
woodburner needs to be in a big room !

Anyway - you've got the heat dump rad - so that's good.
It needs to be on a gravity flow circuit - so's it will still get
water if the mains / pump fails. In our case this meant siting it
where I wouldn't choose to put a radiator (in a narrow hallway that
backs onto the main fireplace) - but needs must !

As to controls - we neded up fitting all remaining rads (except the
bathroom one) with TRVs - and this works fine. They recommend that you
have a thermostatic switch on the woodburner boiler that kills the
pump if the temperature drops below about 40c - something to do with
corrosion of the boiler, ISTR ??

Bear in mind that the circulating water may well be cooler than you
are used to (ours is generally between 50 and 60c, measured at the
flow from the boiler) - this may affect the settings on your
thermostats / controllers.

After much agonising g, we ended up with a very simple control
system - two-way valve that routes the water through the DHW cylinder
if the stat on the cylinder demands it, and though the general
radiator circuit at other times. So far, so good !

No doubt there are more complicated ways of doing this - but be
careful of the 'sledgehammer and walnut' situation - a woodburner is a
'lo-tech' thing, very different from a oil / gas boiler , and (IMHO)
doesn't require a high-tech control system.....

....but then - I'm not an expert g

Good luck
Adrian
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