Venting, Flueing, and Combustion Air
Domain 3 covers venting, flueing, and combustion air: how products of combustion leave the appliance and how replacement air enters. The exam tests vent category, termination clearances, draft, common venting, and confined-space calculations.
Venting fundamentals
Appliances are categorized by flue pressure and temperature. Natural-draft appliances rely on a draft hood and a chimney; fan-assisted appliances use a draft fan but may still be Category I; direct-vent appliances take air from outdoors and discharge outdoors. Vent connectors must slope, be supported, and be sized to the appliance. Terminations must avoid recirculation, snow, windows, doors, and mechanical intakes per code clearances.
Combustion air
A confined space is generally one with less than 4.8 m3 per kW (1 ft3 per 20 Btu/h) of total input. Two permanent openings are commonly required, one near the top and one near the bottom, each with free area of 1 in2 per 1000 Btu/h (about 2200 mm2/kW) of input when using interior air.
Worked example
A room measures 8 m x 6 m x 2.4 m = 115.2 m3. A 30 kW appliance needs 30 x 4.8 = 144 m3 to be unconfined. Since 115.2 is less than 144, the room is confined. Each opening needs 30 kW x 2200 mm2/kW = 66,000 mm2 (about 102 in2) free area, unless the code permits a different method.
Traps
- Sizing a vent by appliance input only, ignoring connector rise and category.
- Terminating under a deck, near a window, or beside a mechanical air intake.
- Using interior air in a tightly built or depressurized house.
- Common-venting a fan-assisted appliance with a natural-draft appliance without code approval.
- Blocking the draft hood relief opening.