Will I Need a Chimney to Vent the Smoke and Combustion Gases?
Venting with a pre-existing chimney – Most masonry chimneys are eight or nine square inches wide – far too large for a gas fireplace. With a space that big, the exhaust would condense and fall back down into the fireplace before it had a chance to be vented outdoors. The old chimney would have to be fitted with an insulated four or five-inch aluminum pipe.
Direct venting (built-in or stand-alone fireplaces) – Built-in and stand-alone gas fireplaces do not require chimneys, but they do have to be vented directly outside. This is usually done through a wall, but when a wall is not available, the fireplace may be vented through the ceiling.