I had an interesting problem with my Keston Celcius 25 condensing boiler
today. Switched boiler on to warm up a cold house. Arrive home a couple
of hours later to find radiators only luke warm and a wonderful gurgling
noise coming from the boiler. Actually, it was really quite loud if you
went outside to the flue terminal. Someone reported similar symptoms
here some months back and I speculated the combustion chamber was half
full of water.

I was a little surprised the condensate trap could have blocked up in
the six months since I cleaned it out, but anyway off with the front to
look at the U-trap (you can see half of it as it's made of transparent
tubing). Well, it looks remarkably clean, but maybe the muck is in the
half you can't see as it's black plastic. Take the trap out and clean
it through in the sink. One thing I didn't consider when installing the
boiler was how easy it would be to empty the condensate U-trap, with
the result that pretty much the only way to empty it results in
chucking the water down the wall and over the floor -- there's nowhere
to hold a bucket to catch it before it's been deflected in all directions
by the pipework under the boiler. Oh well, something to bear in mind if
I install another one. Still, remarkably little muck came out, but at
least testing it under the tap shows water runs through it. Reassemble
U-trap, prime it by pouring a pint of water in the flue terminal (my
neighbours must think I'm bonkers;-), and fire up the boiler.

Next snag, it won't light -- oh bugger. It's going through the sequence,
but claiming the gas won't ignite. I'm wondering if the condensate
water got into somewhere it shouldn't have when draining it, e.g. maybe
no spark. Extract the spark electrode (as the EHT cable absolutely
refuses to unplug from it -- I suspect the heat has soldered the
connector on over time). Anyway, a nice healthy spark, so that's not
it. Check voltage on gas valve -- that's OK, although I can't see if
it actually operates. However, I can smell gas at the flue terminal
after it fails to light, so at least some gas must be getting through.
The fan is clearly running. There's no air flow pressure switch in the
Keston. Puzzled. Time for a cup of tea...

Have another try after the tea -- no joy, but I wander round outside
whilst it's trying to light -- gurgling noise loud and clear from the
flue. Where's the water come from -- the boiler hasn't actually lit
since I cleaned out the U-trap? The sealed system pressure hasn't
gone down at all since I last filled it 2.5 years ago. Oh well,
out the condensate trap comes again, more water all down the wall and
over the floor, although actually rather little. I'm now beginning to
get suspicious that the combustion chamber is half full of water which
won't drain out. Back outside, up the ladder, another pint of water
in the flue (neighbours now sure I've lost the plot), but nothing comes
out of the condensate drain (and it's not a problem in the trap as I
haven't refitted it yet).

Next, remove the burner assembly from the heat exchanger chamber so I
can look inside. What I can see is remarkably clean, but you can't see
much due to the heat exchanger construction, and certainly not down to
the bottom of the heat exchanger chamber. Pour a jug of water into the
chamber, but nothing comes out. Try another jug incase the pressure of
more water will dislodge whatever is stopping it draining, but this just
fills the chamber up above the top of the heat exchanger. I'm trying to
think how to access the bottom of the chamber. I could disconnect the
flexible flue pipe from the heat exchanger, but that connection has to
be very waterproof or condensate running back down the flue will leak
into boiler casing and corrode it (as happened to someone else in this
newsgroup), so I'm not keen. Instead, I disconnect the top of the pipe
(where the direction it is socketed into the flue spiggot is not so
critical for being watertight). Poke a stick down and give it a good
wiggle around, but it doesn't clear whatever is blocking the condensate
drain. The only other access is through the condensate drain. Find a
short length of mains flex to poke up there and wiggle around. Success,
and the two pints of water I had poured in plus the condensate which
was stuck in the come out, gushing all over the wall and floor of course.
Actually, I caught a good deal of it in a bucket this time, in order to
find what was blocking the drain. Nothing very conclusive -- water is
quite clean, although there is the remains of something which looks like
a small jellyfish in there. Pour several more jugfulls of water through
the heat exchanger to flush out anything else.

Reassemble the boiler, back round outside to prime the U-trap with yet
another jug of water down the flue (neighbours probably got the camcorder
on a tripod by now in preparation for a "You've been Framed" contribution).
Start the boiler -- SUCCESS!

Maybe this storey will be useful to some of you with Kestons.
I could also suggest to Keston that although they have a detector for
condensate pipe blocked after the boiler, they have no detection for
condensate U-trap blocked or the heat exchanger chamber filling with
condensate for some reason (as in this case). This might be worth
detecting, and would be easy to do using the existing detector which
just relies on water contacting an exposed terminal. OTOH, it would
seem that the boiler actually won't light in this state, although if
already lit, it manages to carry on going for a while at least.

Back to a nice warm house at least...
--
Andrew Gabriel