Hi, we had a combi boiler (Worcester 28i) installed a few months back.
Everything works fine. However, when I am outside in the garden (near
the flue) whenever the boiler ignites (due to a tap being turned on for
eg) then there is a strong smell of gas from the flue. Is this normal?
It seems to stay strong until it "goes out" . I`m thinking this is
wasted fuel or is it that it is burnt smell anyway. It definately
smells gasy!!- mind you, I don`t want to breathe it in too long!!
There is always a hiss of gas just before it ignites, and the smelly stuff
that is added to the odourless natural gas doesn't burn off to well and
lingers around for a little while, especially in an enclosed space like the
housing (casing) of a boiler.
If you smell gas when the boiler is not burning, then close the door that
connects the room to the rest of the house. Then get the room ventilated to
the outside air by opening windows and doors that lead to the outside
directly from the room.
A little gas escaping from the burner before the igniters sparks is quite
normal though. Just be glad it's escaping properly from the flue.
In this situation though, the OP says the smell stays strong until it
"goes out" - which I take to mean when the boiler stops burning gas.
Our neighbour's condensing boiler is the same, as was our neighbour's
one at our previous house - a very strong gas smell whilst it was
burning. I just took it to be the 'normal' smell for condensing
boilers. Our conventional boiler (cast iron) doesn't make that smell
(nor did the previous one, or the one at our old place - all were cast
Natural Gas is odourless, and the smelly stuff is actually a chemical which
is added to it for safety. You wouldn't know if your gas was leaking if you
couldn't smell it, sort of safety. The chemical itself doesn't burn off
like the gas does, so sometimes it can linger around, if the wind is in
right direction or the area around the flue is somewhat enclosed, and this
causes the smell to be stronger than it normally would be in the close
vicinity of the boiler flue.
The smelly chemical is also being constantly pumped out of the boilers
burners along with the gas, so it does build up and cause the smell to stay
in the area around the flue, but the smelly chemical itself is totally
harmless (so they say).
The odourant normally used is Methyl Mercaptan at a rate of about
0.005 ppm. This is because it is has one of the strongest smells per
quantity used known to man.
The recommended exposure safety limit seems to be 0.5ppm .....
If this were true, (a) we'd all smell the 'gas' smell when using gas
cookers and open-flame fires; (b) the designers of the 'safety' feature
would be singularly incompetent, since there would be no difference in
the signal for the rare dangerous case (unburnt gas) and the common safe
case (burnt gas). Either of (a) and (b) alone would make me dubious;
since both are true, I need a large dose of convincing that the smelly
stuff doesn't burn.
In fact, I've just remembered what it's called: mercaptan. Googling on
that word, and finding the natural gas tracer appears to be specifically
methyl mercaptan, it's both a nasty toxin - but NOT in the
concentrations used for gas safety - and highly flammable (e.g.
Ah, logic. Ya gotta love it...
And what happens with the un-burned stuff on the initial hiss of gas and
smelly stuff. The smelly stuff, as I think I've sort of explained, lingers
around for a good while if it isn't burnt off immediately. The smelly
chemical is actually heavier than air as well, and this also helps it stick
to things like the walls of the flue and the interior of the boiler casing.
I did say that it lingered more in an tighter space than the wide open air,
I used to use Ethyl Mercaptan (or beta-mercapto-ethanol as we called it
to be pretentious) in my days as a geneticist.
Wonderfully smelly stuff (not to mention toxic), and one of those things
that has the nasty side effect of temporarily knocking out your sense of
smell when you've been exposed to much of it.
Rather nastier in smell than the gas additive.
After a day of splashing it around, no-one sat next to you on the bus.
In passing, we had a discussion on uk.rec.waterways a while back about
the smelly stuff that's added to bottled gas. We discovered that this
is less volatile than the gas, so you can tell when your bottle is
running out, as the smell gets stronger.
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