When using combustion air ducts to supply makeup air, they shall not be screened when terminated in which location?

• A. Basement
• B. Exterior wall
• C. Crawlspace
• D. Attic

Answer: (D)

Makeup air for combustion appliances must be delivered with an unobstructed opening. If the duct ends in an attic, it must not be screened because an attic is not a true outdoor air source, and any screen would quickly become clogged with dust, insulation fibers, and lint, adding a restriction that reduces the airflow the appliance relies on. This can lead to insufficient combustion air and potential venting problems. Terminations in exterior walls or crawlspaces are considered outdoor or near-outdoor air sources where screening is more acceptable to keep pests out while still allowing airflow, but in attic terminations the air supply must remain unimpeded.