According to KLS :
On Mon, 23 Jan 2006 23:53:08 GMT, "Joseph Meehan"
wrote:

There are two typical smells from a gas furnace. One is the actual gas.
It does not smell like "burn" but very distinctive.

Joe is right, but the smell he's talking about is not the actual gas
Rather, it is the aromatic additive put into the gas by the suppliers
to make the gas actually detectable by people. That's a safety
feature, and the smell is unpleasant and totally different from
anything else most of us would smell in a routine life.

"Totally different from anything else most of us would smell in a routine
life"?

Depends. Does your "routine life" include rotten eggs or skunks?

The additive is methyl mercaptan, aka methanethiol - CH3SH, or
ethyl mercaptan, aka ethanethiol - C2H5SH.

["mercaptan" and "thiol" are terms for the "SH" bond]

_Intensely_ aromatic. H2S (Hydrogen Sulfide - most people only come
into contact with this from rotten eggs, but it's the "sour" in "sour
gas", and is highly poisonous and corrosive) and skunk aroma
are mercaptans too and smell quite similar.

A drop of this stuff will stink up a building with poor ventilation something
awful. You can smell this stuff as low as 50PPB (parts per _billion_).

It's potent stuff.

If you smell it, call the gas company ASAP.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.