Running other naturally vented appliances, such as a clothes dryer, while the fireplace is in operation can potentially cause backdrafting, a condition where air comes back down the flue, bringing combustion gases into the living space.
Backdrafting occurs when exhaust air is drawn back into the home, typically carrying dangerous combustion gases with it. The air is drawn back into the home because of a negative pressure situation indoors. Any naturally vented appliance, such as a clothes dryer or exhaust fan, can create a negative pressure situation. The appliance attempts to draw air from the point of least resistance, such as from the B-vent flue of the fireplace. If the draw is strong enough, it can pull exhaust air down the flue and into the home, potentially causing serious health risks for the home's occupants.
Cause 1:
Not installing a combustion air duct or triple-wall flue
A traditional B-vent gas fireplace takes combustion air from inside the home and vents the exhaust air outside the home. Because B-vent fireplaces draw combustion air from inside the home, they're at the greatest risk of backdrafting. Backdrafting occurs if another appliance is pulling air from the home at the same time as the fireplace, creating a negative pressure indoors. This negative pressure makes it difficult for the B-vent flue to work properly. Instead of exhaust air leaving the home, it's drawn back down the flue and into the living space.
ProTip!Make sure only those duct materials specified by the manufacturer are installed.
Solution
The best way to prevent backdrafting is to install a direct vent fireplace instead of a B-vent fireplace. If a B-vent fireplace is installed, however, include a combustion air duct to minimize the risk of backdrafting. A combustion air duct brings outdoor air directly to the firebox to satisfy the need for combustion air. Properly attach the combustion air duct to the fireplace unit, and route it through the exterior wall. An option is to replace the B-vent flue with a triple-wall flue. A triple-wall flue has three concentric circles—the center circle is for exhaust air, the middle circle is an insulation layer that keeps the center of the flue as warm as possible, and the outside circle provides a cooler layer. A triple-wall flue improves the drafting capabilities of the flue.
