I'm reading another thread about furnaces, the long thread where the
house won't heat up "furnace blowing all the heat up the chimney" and
someone wrote that flues are double-walled. Is that always the case?

Maybe his furnace is gas and mine is oil? Because my flue is single-
walled. Is that bad? As long as it doesn't cool off so much that the
chimney doesn't draw well, any heat lost from the non-insulated flue
would heat the basement.

When I get a new oil furnace (no gas supply) should the new one have a
double-walled flue?