On Fri, 6 Nov 2009 00:29:52 -0600, (Lp1331 1p1331)
wrote:

There is virtually no oil fired equipment in my area, so I all my
experience has been on gas units. The main thing about double walled "B"
vent is allowable clearance to combustibles. It is generally 1 inch with
double and 6 with single. With single wall, a 4 inch pipe would need to
have a 16 inch diameter hole in a roof. Larry

Yes, after about 8 feet of single wall, winding up and around the
furnace to the the metal pipe chimney, and far from combustibles, the
chimney is much bigger in diameter and I think it is double wall, but
I forget why I think that. Probalby from looking at it from the roof
end.

Thanks to everyone.

There is gas 400 feet from my house, but my n'hood of 100 townhouses
has no gas. At least I know my neighbors won't burn my house down
with a gas explosion, but that's the only benefit.

The electric stove broiler isn't hot enough either afaic.