Air sealing and insulation weatherization work can affect the combustion safety of a home. Appliances with natural draft chimneys, such as gas or wood-burning fireplaces, or gas and oil-burning appliances like boilers, furnaces, or water heaters, may rely on natural air leakage through the enclosure to provide the makeup air for combustion. Some homes may have dedicated combustion air vents (that must not be sealed during weatherization work). If extensive air sealing and weatherization work is performed on a home with a natural draft chimney, this makeup air may be reduced to a point where the equipment or fireplace may backdraft and spill combustion gases into the home. When weatherization work is undertaken, a direct supply of makeup air for these systems may be required. It can be more difficult to provide make-up air in homes with unvented fuel heaters or gas fireplaces, so these may need to be removed prior to weatherization work.

Note
Weatherization work in homes with gas, oil, or wood-burning equipment requires special consideration and combustion safety testing. Guidance for addressing combustion safety, testing, and remedial measures are beyond the scope of this guide. Contractors performing weatherization work should be knowledgeable on these issues prior to performing any work.