Strong Smell Of Gas from Flue

[eevoob]

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!!

[BigWallop]

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.

[David Hearn]

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 iron ones).

D

[BigWallop]

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).

[Andy Hall]

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 .....

[James]

If the smelly stuff (added to the gas) is not rendered odourless as the gas is burnt, why doesn't a flueless appliance (like a gas cooker) smell of gas when it is working?

James

[Stefek Zaba]

So far so plausible...

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.

logic. Ya gotta love it...

Stefek

[BigWallop]

But it does.

[BigWallop]

> 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, didn't I.

[Nick Atty]

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.