In article ,
says...

We want to remodel our kitchen, but a large 9" round chimney vent to
our roof connected to our furnace and hot water heater runs through
one of our cabinets, taking up a fair amount of space. In the
remodel, we are putting in a new wall to separate the kitchen area
from the dining area that is next to the existing chimney.

A handy friend of mine said that I could convert the round pipe to a
rectangle (3 x 10) that could run through the drywall.


Your 'handy' friend is completely, and dangerously, wrong. But no one is
explaining to you what he had in mind.

IF, and I mean IF, this was a hot air duct, the ones that distribute the
furnace's warm air to the registers in the rooms, you could indeed convert a
round duct into a rectangular wall stack (but not a small 3 x 10). But that is
NOT what you are dealing with.

You are proposing to move the main vent of the chimney and the hot water
heater. This contains very hot combustion products loaded with carbon monoxide.
This MUST be ducted through a special, double walled vent pipe called a "B"
vent. That is what the pipe in question is.

B vents have many requirements and restrictions to insure a safe installation.
A heating contractor is needed for this job, and it may prove very difficult to
relocate. You will more than likely NOT be able to reduce the size of the vent,
so the alternatives are to relocate it, build around it again, or get a new
furnace and water heater with direct, sidewall venting.

Dennis