To seal a furnace with trapped air, first inspect the furnace door and flue for gaps using a flashlight and felt pen. Apply high-temperature caulk or mastic to seal joints and insulate the flue with ceramic fiber blanket. Ensure the blower fan and intake vents are tightly closed. Test for air leaks by holding a smoke pencil near seams; visible smoke indicates failure. Finally, install a carbon monoxide (CO) detector and schedule annual professional inspections.

Sealing a furnace with trapped air prevents combustion byproducts like carbon monoxide (CO) from escaping into living spaces. The U.S. Environmental Protection Agency (EPA) reports that improper furnace venting causes 1 in 4 CO poisoning deaths annually, often linked to blocked or漏气 flues. Trapped air also reduces efficiency, as the Department of Energy notes that 20% of heat loss in homes stems from air leaks. Insulating the flue minimizes condensation, which can corrode metal components and reduce lifespan. A 2021 study in Energy Efficiency Journal found that sealing leaks saves 8–15% on heating bills. However, improper sealing risks CO buildup, as noted in a 2019 CPSC report where 60% of CO incidents involved poorly sealed heating systems. Always hire certified technicians to avoid over-sealing, which can trap dangerous gases.