In article .com,
worriedconsumer .com writes:

I bought a new Keston C25 boilers which was installed and commissioned
in late December 05. The installation was inspected in January 2006 by
a Keston engineer, and the boiler has been running well for the last
four months, but more recently it appeared to be losing efficiency.

Why did Keston inspect it only a month after installation?

I investigated further and found the rubber flue tube to be completely
corroded and brittle - the inner sides of the flutes have split so it
is possible to stretch the pipe out like a slinky spring. I ran the
boiler with the casing open and was shocked to see a huge amount of
flue gas coming back into the boiler itself. The boiler casing is
corroded in parts and very rusty looking, yet it is not even six months
old. It - not surprisingly - runs very hot, too.

There's a non-resettable case over temperature sensor which
people have reported blows if the flue splits and discharges
back into the case. It's mounted on the transformer.

The boiler casing is not properly sealed, either. A quick check with a
smoke can showed that the positive pressure of the flue gas was pushing
the flue gas out of the casing into the room cavity in which the boiler
is installed. This boiler room space is not vented to the outside - my
installer informed me that the boiler is self-venting, allowing it to
be installed in domestic spaces such as kitchen cupboards etc.

It is obviously a known design fault. I note - with a little bit of
investigation on the internet, including posts in this thread
describing a cracked 'gasket pipe' from January '05 - that the latest
version of the C25 boilers no longer contains this rubber pipe and has
been redesigned with a solid one. This can only be because someone
realised there was a serious problem.

My 2002 model has the black consertina pipe. I recall someone
saying earlier ones had a blue flexible flue pipe which Keston
swapped out. The black pipe in mine is still flexible (I had
to disconnect it to clear a blockage in the condensation
outlet). However, I have almost never run my boiler very hot;
the temperature knob pretty much only went past pointing straight
up (about 60C flow) during commissioning. In normal running, it
is set pointing to the left (about 45C flow). I'm curious what
position/temperature yours is normally set to -- maybe this has
a significant effect on the flue pipe life?

--
Andrew Gabriel