Drolet Heat Commander Installation And Operating Instructions Manual page 16

Good system design
Inside chimneys are preferred because
even when no fire is burning, there is
normally upward flow in the system.
4.1.2 Masonry chimneys
The furnace may also be connected to a masonry chimney, provided the chimney complies
with the construction rules found in the building code enforced locally.
- The chimney must have a clay
liner.
- If the masonry chimney has a
square or rectangular liner that is
larger in cross sectional area than
a round 6" (152 mm) flue, it should
be relined with a suitably listed 6"
(152 mm) stainless steel liner.
When passing
-
combustible wall, the use of an
insulated listed thimble is required.
- If a stainless steel liner is installed,
it is recommended to extend the
liner 12" (305 mm) to 18" (457 mm)
above the top of the masonry
chimney to favor the draw.
through a
Inferior system design
Outside chimney may be a problem if there
are long periods of inactivity or low fire
temperatures. A cold chimney may down
draft making start up difficult.