Good system design Inside chimneys are preferred because even when no fire is burning, there is normally upward flow in the system.
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.
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 through a 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.
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