For safety reasons, all gas fireplaces should be installed with the proper venting by a professional HVAC contractor. How a fireplace is vented depends on where it is being installed inside your home and if you’re converting a pre-existing wood-burning fireplace to gas. 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.