Venting and Combustion Air
Domain 2, Venting and Combustion Air, tests vent categories, draft, termination, and the air supply needed for safe combustion and dilution.
Vent categories
Category I appliances operate under negative pressure and are non-condensing; they commonly use Type B vent. Category II is negative pressure and condensing. Category III is positive pressure and non-condensing. Category IV is positive pressure and condensing, typical of high-efficiency furnaces with PVC/CPVC vent. Draft hoods dilute flue gases and stabilize draft on Category I equipment.
Draft and termination
Vent connectors slope upward about 1/4 in. per foot toward the chimney. Type B vent needs 1 in. clearance to combustibles; single-wall vent needs 6 in. Follow the 2-10 rule: the vent should terminate at least 2 ft above any roof surface within 10 ft and at least 3 ft above the roof penetration. Keep terminations away from windows, doors, and fresh-air intakes. Spillage at a draft hood is a red flag for backdrafting.
Combustion air
For a confined space using indoor air, allow 50 ft3 per 1,000 BTU/hr input and provide two permanent openings, one near the floor and one near the ceiling. With outdoor air through vertical ducts, each opening needs 1 in2 free area per 4,000 BTU/hr. Horizontal ducts need 1 in2 per 2,000 BTU/hr. Use free area, not the gross louver size.
Worked example
A 100,000 BTU/hr furnace uses outdoor air through vertical ducts. Each opening: 100,000 ÷ 4,000 = 25 in2 free area. If the ducts are horizontal: 100,000 ÷ 2,000 = 50 in2 per opening. A 25 in2 louver with 50% free area provides only 12.5 in2 and is undersized.
Traps
Do not count a bedroom door or a ceiling joist space as a permanent opening. Do not ignore exhaust fans, dryers, and tight construction, which can depressurize the space. A blocked-vent switch is a backup, not a substitute for correct vent sizing.