To seal a flue opening effectively, first ensure safety by turning off the appliance and allowing it to cool. Use high-temperature caulk or fireproof sealant specifically designed for flue openings. Apply the sealant uniformly around the edges, pressing firmly to create an airtight and fire-resistant barrier. For metal flues, consider using metal flashing or a pre-made flue cap. Regularly inspect the sealant for cracks or wear and reapply as needed.

Sealing a flue is critical to preventing dangerous gas leaks and maintaining combustion efficiency. Unsealed flues allow carbon monoxide (CO) to escape into living spaces, a leading cause of poisoning-related deaths in the United States. According to the CDC, CO poisoning accounts for over 400 annual deaths and 20,000 emergency room visits, often linked to faulty heating systems. A properly sealed flue ensures gases vent safely to the outdoors, reducing health risks. Additionally, sealing gaps improves appliance efficiency by 5–15%, as noted by the U.S. Environmental Protection Agency (EPA), which estimates that 30% of residential energy loss stems from drafts and leaks. The National Fire Protection Association (NFPA) standard 211 mandates proper flue maintenance to prevent fire spread from chimneys. Neglecting this step can lead to costly repairs, legal penalties, or even catastrophic fires. Regular inspections by certified professionals are recommended to address wear and ensure compliance with safety codes.