On Jul 28, 9:06*am, "Dave Liquorice"
wrote:

I can't get me 'ed around the requirement for four and then dragging
the HW circuit into the equation. I guess there could be a second
loop from the wood burner to a coil in the HW cylinder...

In both boilers the CH and HW circuits are connected - i.e. each have
two loops from the heating chamber - thus four pipes. My concern was
that it might be possible for the return flow from the radiators,
which is connected by a T-junction to the return flow of the
woodburner, to not just go back to the oil boiler but also to go
through the woodburner CH return, through the woodburner boiler and
then through the woodburner HW return and generally lose heat where it
shouldn't before finding its way back to the oil boiler return.

I imagine one could argue that the suction effect of the CH pump is
likely to draw the water flow back towards the oil boiler and so it's
not likely to flow towards the woodburner CH return, but a little
experiment I have just done - switching on the oil boiler and closing
the feeds but not the returns to the woodburner for a short while -
soon resulted in a small, but significant increase in the temperature
of the woodburner CH return. So as far as I can see, 4 normally open
valves would be needed to completely stop any possible flow throuhg
the woodburner boiler.

I assume standard normally open motorised valves cannot stick closed
and are normally used in solid fuel systems and fail safe in the event
of a power failure.

And to reassure the guy concerned about a steam explosion, although
the plumbing of the woodburner has generally been bodged, they did at
least fit a steam release valve!!!

Keith