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High-efficiency gas furnaces use sealed combustion and direct venting, so they remove exhaust gases through vent pipes to the outdoors rather than relying on a traditional chimney. These systems typically use an inducer fan and two vent pipes (often PVC): one brings in outside air for combustion, the other exhausts the combustion gases directly to the outside. Because the exhaust is cooler and the system is sealed, a masonry chimney isn’t required. You can terminate the venting on the side of the building or through the roof with approved venting, but a chimney isn’t a necessity for these units.

In some installations, an existing chimney might be used, but that’s not required and isn’t the standard setup for high-efficiency models.