I think that when it rains hard in a certain direction the flue on my
condenser boiler lets in rain water. This then runs through the boiler and
drips out from the bottom front corner. Does this sound possible? It is
definitely rain water as it never happens when it is not raining.
Any ideas on how I can prevent this?
When you consider that the boiler itself should be completely sealed, then
the water dripping from the corner of the casing is most likely coming
through a gap between the flue pipe and the brickwork of the wall. Is there
a collar around the outside of the flue? If not, then some sealant, in the
form of some sand and cement mortar, might be a good idea.
If this is happening you have a serious problem which should be
investigated immediately. The flue is connected (obviously) to the
combustion chamber and that combination should be totally sealed to the
room. It could be water is leaking round the outside of the flue and
finding its way into the room from there. But that too should be sealed.
Or, of course, the condensate drain if it's a condensing boiler.
*Why is the man who invests all your money called a broker? *
Dave Plowman email@example.com London SW
Yes I've experienced similar myself, where I've had a leak at the bottom
of the boiler which I've traced back to condensate running back along
the (near-horizontal) flue and emerging from the outer sheath of the
flue, above the boiler due to a failed 0-ring there. No big safety
issue there. I imagine that incoming rain would/could follow the same
Also, I've been whinged at by CORGISs about my boiler flue not being far
enough away from a facing wall (it's about 0.6 m away) - I've been told
that this is close enough for steam/condensate to be pushed back down
the flue. If that's plausible, then I'm sure incoming rain is too!
Thanks for all your replies. Luckily the boiler is in the garage and not in
the house, but I will put a CO detector in there. The outside of the flue
does have a collar and is also sealed with brown silicon. No rain today and
no leaks, so definitely rain water.
The water is coming from the front right corner of what the installation
manual calls the air box, where the fan and condensate trap are located.
Does this provide any clues?
Also on a side note, why do boilers and fires require flues but not gas
ovens? I have always wondered about this.
They are regarded as small intermittent use devices and they rely on
There is nothing dangerous given off when you burn gas properly with plenty
of fresh air.
It does give off a huge amount of water vapour which will make everything
mould if you use it for heating without throwing most of the heat away with
Boilers are usually sealed units. Where the air is drawn in from outside
and expelled back outside through the flue. Not all boilers are like that,
but most today are. This reduces the need to have permanently open vents in
the same room as the boiler, so keep draughts out when you want it warm
Gas fires and stoves need a permanently open vent from outside, into the
room where they are installed. If you haven't go one on an outside facing
wall, then provision has to be made for a forced air intake and outlet
system into the room where the gas appliance is installed. A gas fire must
also have a flue to the outside, with other safety regulations that go along
Did you think it was as simple as fitting a stove in a cupboard under the
stairs, and that's it? There is a lot more to gas safety regulations than
I'm sure more knowledgeable others will go into more details for you.
Telling of how the extraction does this, while the Bunsen effect makes it do
that. But I'll leave it there. :-)
Is the condensate not removed with pipework to the outside? If not, then
get a pipe installed to remove it safely to a drain point.
If you have pipework for this purpose, then make sure rain is not
overflowing into it and back into the boiler. That may be why you are
noticing this problem more when it rains.
Could be condensate or water from the heating system.
Condensate is relatively corrosive, so steel metalwork it
comes in contact with rusts quickly. In time, it can corrode
through sealed casings, rendering them unservicable.
With my Keston, I check for leaking condensate by leaving the
hosepipe trickling slowly into the end of the flue outlet. The
water should pass through and away down the condensate drain.
Make sure it is (don't let it fill up the heat exchanger or it
will likely damage the ignitor and gas parts). Make sure you don't
run water into the boiler air intake instead of the flue outlet.
I don't know how suitable this test is with other condensing
boilers. Boilers which pump out the condensate might not do so
when the burner isn't firing or the power is off for servicing.
At your own risk, as they say.
[email address is not usable -- followup in the newsgroup]
I asked recently about rain ingress as my new boiler has been fitted
with a flue that points upwards, about 5 degress from horizontal,
which to my mind is bound to see some rain come in.
It was pointed out (helpfully by most!) that that is the right way to
fit the flue for the boiler that I have (of course, others may be
different). And the same point about condensate was made.
Hope that helps!
Today I tried to find the source of the leak, I stripped the boiler down as
far as I dared. It seems to be coming down the righthand side of the inside
of the air box, between the air box and the combustion box.
So I reckon it is either from the flow or return into the heat exchanger or
something to do with condensate from the flue at the top. The condensate
trap seems to be filling and emptying correctly and is clear of debris.
The next stage I reckon would be to remove the heat exchanger assembly, but
for this I would need to drain down. The wife wants to call someone in, I am
near to agreeing with her.
One question, as the water in the system is under pressure then wouldn't the
leak drip all the time rather than just when the boiler is operating? If so
then it must be condensate, mustn't it?
Or rain running along the inside of the outer pipe on the flue. Is there
any water mark along the inside bottom of the pipe? Or any water marking at
all along the flue pipes? If it's condensate, you should see where it has
been dripping or running along the metal, as it leaves a bit of a mess
behind because of what contains.
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