[See thread " Thunderstorm in Dorset "]
" However, have just had call from son over at Canford Heath (north
suburban Poole), and they had intense rain, vivid lightning and
substantial hail - the wind-driven rain/hail entered the gas boiler
flue and entered said boiler, putting it out! "

This has obvious meteorological interest, but the second part of this
post is really so that anyone with the same boiler type/configuration
can pick it up on a web search.

Apparently, many gas boilers of this type are fitted as described
below, and could experience the same problems.
From the thread on the 21st January, thunderstorms in the east
Dorset (Poole/Bournemouth/Wimborne/West Moors area) ...
Had a look at the radar imagery for the times in question (between
~1915Z & 2015Z)
( http://www.meteox.com/ ... use 'archive' link )

and based on this (presumably) course resolution (~5km) the intensity
of rainfall in the area was at least 50 mm/hr, and possibly around 100
mm/hr. Based on the description from this ng, and posts from other
contributors, I would (grossly) estimate that the rainfall total
within ~6 mins was about 15 mm - I don't think it was any more, as I
don't think we'd be talking about a boiler failure, but a complete
flooding of the lower floor of the house & there was never any such
threat.
This would, however, put it firmly into the 'noteworthy'
classification using the old British Rainfall criteria.

[ this is where it drifts OT ... ]

Was puzzled as to how the rainwater managed to penetrate the flue,
given that it terminates horizontally and is clear of obvious
obstructions. It turns out that the rain water entered via the vented
system boiler (for a diagram see for example ...

http://www.diydoctor.org.uk/projects/boilers.htm )

which has an external safety outflow fitted. When the boiler had been
installed (last year), the outflow had been connected into the house
guttering downflow *without* a non-return valve.

Now, the manufacturers (as we've just found out after a representative
engineer called), don't recommend letting this outflow enter a
drainage downpipe of this type, for the very reason that the storm of
the 21st Jan demonstrated; a rapid back-build of water *up* the
downpipe will contaminate the boiler! There should have been a
non-return valve fitted and the installers (British Gas), admit that
this was a mistake, but the manufacturers say that even this is
fraught with problems: the NRV can itself become contaminated after a
few heavy rain events and fail, meaning that it might not allow water
to pass *out* from the boiler safety valve - again causing failure.

We're going to alter the configuration of the downpipe so that there
can be no back-pressure of water given the same situation.

Martin.

--
Martin Rowley
E:
W: booty.org.uk