Venting, Combustion Air, and Flue Gas Analysis
What the exam tests
Venting categories, draft, combustion-air provisions, flue-gas analysis, and the safety consequences of spillage or blocked vents.
Core rules
- Category I: negative-pressure, non-condensing, B-vent, typically 80% AFUE. Category IV: positive-pressure, condensing, PVC/CPVC, 90%+ AFUE.
- Confined-space rule: 50 ft³ of indoor air per 1,000 Btu/hr input. Two openings: each 1 in² per 1,000 Btu/hr if using indoor air; outdoor air via vertical openings 1 in² per 4,000 Btu/hr, horizontal ducts 1 in² per 2,000 Btu/hr.
- Combustion-air openings: one within 12 in. of the floor, one within 12 in. of the ceiling.
- Analyze O2, CO, CO2, stack temperature, and draft. Air-free CO should stay low; condensing flue gas is about 100–130°F, while 80% flue gas is about 350–500°F.
Worked example
A 100,000 Btu/hr furnace in a sealed closet needs 5,000 ft³ of indoor air (50 × 100). If the closet is 1,000 ft³, it is confined. Using two indoor-air openings, each must be 100 in² (1 in² per 1,000 Btu/hr). A 10 in. × 10 in. grille is only 100 in² nominal, and free area is less, so use larger grilles or direct outdoor air.
Traps
Do not treat a pressure switch as proof of adequate combustion air; it proves only a pressure difference. Do not terminate a Category IV vent where condensate can freeze or recirculate. Never downsize vent connectors. Spillage at a draft hood is a red tag.