A "class A" vent pipe is a two wall metal pipe with insulation between the
metal walls. They are used with solid fuel equipment such as wood burning
stoves and oil burning furnaces. They are made for higher temperatures.

"kevin" wrote in message
ups.com...
I have never heard of A-vent or C-vent. But B-vent is indeed a douible
wall vent pipe. They usually have clever connectors at the ends -- you
don't just cut them and fit them together, you buy the lengths you need
and they couple together. I repaired a B-vent pipe on the side of our
house for a water heater (a "cold water heater", that is ... for
heating my very cold well water!) a few years ago. I was told by a
plumber and a house inspector both to ask for B-vent. At the plumbing
supply, I asked for B-vent, and they immediately knew what I wanted,
although we had to sort out exactly which connector I had and needed.

It may be that different manufacturers have different incompatible
couplings -- I don't know much about that.

-Kevin