12.4 Combustion Air & Basic Venting Requirements (Clause 8)

Every gas appliance needs a continuous supply of air for combustion, and appliances with draft hoods also need dilution air. Modern airtight houses no longer leak enough air on their own, so Clause 8 of CSA B149.1 (Venting systems and air supply for appliances) tells you how to bring air in and how to get flue gases out safely. The Canadian method differs from the U.S. National Fuel Gas Code: B149.1 sizes air supply from tables, not from the U.S. "50 cubic feet per 1,000 Btu/h" confined-space rule.

1. What the Air Is For

1. Combustion air: about 10 cu ft of air per cu ft of natural gas burned (about 25 cu ft for propane).
2. Excess air: extra air (typically 20% to 50%) that ensures complete combustion.
3. Dilution air: air drawn into a draft hood or barometric damper to cool the flue gases and stabilize draft—only appliances with draft control devices need it.
4. Ventilation air: keeps the appliance room from overheating.

Starve a burner of air and it produces carbon monoxide and soot, and flames roll out toward the burner vestibule.

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2. Appliances Totalling Up to 400,000 Btu/h (Clause 8.2)

Clause 8.2 covers the air supply for central furnaces, boilers and water heaters with a combined input up to and including 400,000 Btu/h (120 kW). Work through three questions:

1. Is the building tight? Clause 8.2.1 defines tight construction. Most new homes qualify; if in doubt, treat the building as tight.
2. Do the appliances have draft control devices (draft hood, draft diverter or barometric damper)?
3. Which table?

Enter the table at the total input, rounding up to the next listed input. The middle column gives the minimum free area of the air supply; the last column gives the minimum round duct size. In the loose-construction tables, a "0" means the structure's volume is large enough that no outdoor air supply is needed (round the volume down to the next listed value).

Mixed appliances (Clause 8.2.2): size from Table 8.1 (or 8.3) using only the appliances with draft control devices, size from Table 8.2 (or 8.4) using the total of all appliances, and provide the larger of the two.

Worked examples (tight construction)

• 80,000 Btu/h boiler + 50,000 Btu/h water heater, both with draft hoods: total 130,000 Btu/h → Table 8.1, 150,000 Btu/h row → 22 sq in. of free area, minimum 5 in. round duct.
• Two 100,000 Btu/h fan-assisted boilers, no draft control: total 200,000 Btu/h → Table 8.2 → 14 sq in.
• 75,000 Btu/h draft-hood appliance + 50,000 Btu/h appliance without draft control: Table 8.1 at 75,000 Btu/h gives 11 sq in.; Table 8.2 at 125,000 Btu/h gives 9 sq in. → provide 11 sq in.

Other Clause 8.2–8.3 rules

• Single water heater (Clause 8.2.3): a single water heater of 50,000 Btu/h or less needs no outdoor air supply, provided no other appliance in the space requires one.
• Enclosure inside a loose structure (Clause 8.2.6): where the building volume is large enough, a room can draw its air from the rest of the building through a permanent opening of 1 sq in. per 1,000 Btu/h, located 6 to 18 in. above the floor, plus an equal ventilation opening placed high if any appliance has a draft control device.
• Ducts (Clause 8.3): a square or rectangular air duct may have no dimension smaller than 3 in. (so never less than 9 sq in.).
• Ontario CAD clause 7.1.12: a new or replacement residential boiler under 300,000 Btu/h that is not direct-vent must have its combustion air supplied from outdoors under Clauses 8.2.4 and 8.3 regardless of building construction, and the home needs CSA 6.19 carbon monoxide alarms at the sleeping areas.

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3. Appliances Totalling Over 400,000 Btu/h (Clause 8.4)

Example: 1,400,000 Btu/h of appliances, all with draft control:

$$\frac{1{,}000{,}000}{7{,}000} + \frac{400{,}000}{14{,}000} = 142.9 + 28.6 = 171.4\text{ sq in.}$$

The ventilation opening must be at least $0.10 \times 171.4 \approx 17.1\text{ sq in.}$ (above the 10 sq in. minimum).

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4. Louvres, Grilles, Screens and Dampers

• Free area: a grille's bars reduce the open area. Use the manufacturer's free-area rating (commonly 35% to 60% of the gross opening). Required gross area = required free area ÷ grille efficiency; for 36 sq in. through a 60% grille, $36 \div 0.60 = 60\text{ sq in.}$
• Opening size: openings in a fixed louvre, grille or screen may be no smaller than 1/4 in. (6 mm), so lint and frost cannot block them.
• Dampers: manually operated dampers and manually adjustable louvres are not permitted; automatic dampers are allowed only when interlocked so the burner cannot fire unless they are fully open.

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5. Venting Basics, Draft Hoods and Spill Switches

Natural draft

Category I appliances rely on natural draft: hot flue gas is lighter than the outdoor air, so it rises. Draft increases with vent height and with the temperature difference between the flue gas and outdoor air.

The draft hood (draft diverter)

A draft hood on a Category I appliance:

1. isolates the burner from changes in chimney draft by providing a neutral pressure point at the flue outlet;
2. admits dilution air, cooling the flue gas and stabilizing draft;
3. lets a downdraft escape at the relief opening instead of blowing into the combustion chamber; and
4. relieves a blocked vent by spilling flue gas at the hood rather than smothering the flame.

Spill switches

Because spillage carries carbon monoxide into the house, many draft-hood appliances have a spill switch: a normally closed, usually manual-reset thermal switch near the draft hood relief opening. Hot flue gas spilling over it opens the switch and shuts off the gas valve. A tripped spill switch means a venting problem to find, never a switch to bypass.

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6. Vent Termination Clearances (Clause 8.14.8)

Clause 8.14.8 lists places where a vent may not terminate. They include:

• directly above a paved sidewalk or paved driveway located between two single-family dwellings and serving both;
• less than 7 ft (2.1 m) above a paved sidewalk or driveway on public property;
• within 6 ft (1.8 m) of a mechanical air supply inlet to any building;
• above a meter/regulator assembly within 3 ft (900 mm) horizontally of the vertical centreline of the regulator vent outlet, up to a vertical distance of 15 ft (4.5 m);
• less than 1 ft (300 mm) above grade;
• within these distances of a window or door that can be opened, a non-mechanical air supply inlet, or another appliance's combustion air inlet: 6 in. (150 mm) for inputs up to 10,000 Btu/h, 12 in. (300 mm) over 10,000 and up to 100,000 Btu/h, and 3 ft (900 mm) over 100,000 Btu/h; and
• under a veranda, porch or deck, unless it is fully open on at least two sides beneath the floor and the vent termination is at least 1 ft (300 mm) below the underside of the floor.

Manufacturers add their own clearances (inside corners, soffits, expected snow level), and the more stringent of code and instructions applies.