5.2 Combustion Air: Outdoor Air Methods and Sizing

Sourcing Outdoor Air

When a dwelling is too tightly sealed or the mechanical room is too small, combustion air must be drawn directly from the outdoors. This air can be sourced through permanent openings in exterior walls or by using ducts that connect to naturally ventilated spaces such as attics or crawl spaces. Under IRC G2407.6, inspectors evaluate outdoor air installations using either the two-permanent-openings method or the one-permanent-opening method.

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The Two-Permanent-Openings Method (G2407.6.1)

The two-permanent-openings method uses natural convection to circulate air. Cool outdoor air enters the room through a low opening, and warm air exits through a high opening.

Location of Openings

• Upper Opening: Must commence within 12 inches of the top of the enclosure.
• Lower Opening: Must commence within 12 inches of the bottom of the enclosure.

Duct Orientation and Sizing Rates

Ducts create friction and restrict airflow. Horizontal runs suffer from higher pressure drops than vertical runs. Therefore, the required opening sizes depend on the duct configuration:

1. Direct Openings or Vertical Ducts: When the openings communicate directly with the outdoors through an exterior wall, or connect via vertical ducts extending into a ventilated attic or crawl space, each opening must have a minimum net free area of: $\text{Free Area} \ge 1 \text{ in}^2 \text{ per } 4,000 \text{ Btu/h of total input}$
2. Horizontal Ducts: When combustion air is brought in through horizontal ducts, each opening must have a minimum net free area of: $\text{Free Area} \ge 1 \text{ in}^2 \text{ per } 2,000 \text{ Btu/h of total input}$

Worked Example

A mechanical room contains a 110,000 Btu/h furnace and a 50,000 Btu/h water heater (total 160,000 Btu/h).

• With Vertical Ducts: Each opening requires $160,000 / 4,000 = 40 \text{ in}^2$ of net free area.
• With Horizontal Ducts: Each opening requires $160,000 / 2,000 = 80 \text{ in}^2$ of net free area.

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The One-Permanent-Opening Method (G2407.6.2)

To simplify installations, the code allows a single permanent opening to supply all outdoor combustion air. However, because there is no convection loop, the single opening must be larger and the appliances must have specific clearances around them to ensure airflow.

Sizing and Location

The single opening must commence within 12 inches of the top of the enclosure. The net free area of this opening must satisfy the larger of the following two calculations:

1. Input-Based Sizing: Minimum net free area of 1 square inch per 3,000 Btu/h of aggregate appliance input.
2. Vent Connector Sizing: Minimum net free area not less than the sum of the cross-sectional areas of all vent connectors within the space.

Clearance Requirements

To ensure air can circulate around the appliances to the high opening, the following clearances must be maintained:

• Minimum 1 inch from the sides and back of all appliances.
• Minimum 6 inches from the front of all appliances.

Worked Example

A closet contains a gas furnace rated at 90,000 Btu/h. The furnace has a 4-inch round vent connector (cross-sectional area of approximately 12.6 sq in).

• Input-Based Sizing: $90,000 / 3,000 = 30 \text{ in}^2$.
• Vent Connector Sizing: $12.6 \text{ in}^2$.
• Result: The required opening size is the larger of the two, which is 30 sq in.

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Louver and Grille Free Area Calculations (G2407.10)

The sizing calculations determine the required net free area—the actual open space through which air can pass. Since grilles and louvers block a portion of the opening, the gross physical opening must be larger than the calculated free area.

Prescriptive Default Values

If the manufacturer does not specify the net free area of a louver or grille, the code requires using the following defaults:

• Metal Louvers/Grilles: Assume 75% free area (0.75).
• Wood Louvers/Grilles: Assume 25% free area (0.25).
• Motorized Louvers: Must be electrically interlocked with the appliance burners. The burners must not ignite unless the louvers are proven to be in the fully open position.

Sizing Adjustment Formulas

• Metal: $\text{Gross Area} = \text{Required Net Free Area} / 0.75$
• Wood: $\text{Gross Area} = \text{Required Net Free Area} / 0.25$

Worked Example

If a mechanical closet requires 45 square inches of net free area:

• A metal grille requires a gross opening of: $45 / 0.75 = 60 \text{ in}^2$.
• A wood grille requires a gross opening of: $45 / 0.25 = 180 \text{ in}^2$.

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Opening Construction and Protection

• Minimum Dimension: No combustion air opening or duct can have a dimension of less than 3 inches (G2407.6). This prevents narrow slits that create high airflow resistance.
• Screens: Openings must be protected with corrosion-resistant wire mesh. The mesh openings must be:
  • Not less than 1/4 inch (to prevent clogging by dust, lint, or ice).
  • Not more than 1/2 inch (to prevent pest entry).
• Attic Terminations: If a duct terminates in an attic, the duct must extend at least 6 inches above the ceiling joists and insulation to prevent loose-fill insulation from blocking the duct inlet.

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Summary of Outdoor Sizing Methods

graph TD
    A[Outdoor Combustion Air Sizing] --> B[Two-Opening Method G2407.6.1]
    A --> C[One-Opening Method G2407.6.2]
    
    B --> B1[Location: 1 High & 1 Low within 12 inches of top/bottom]
    B --> B2[Sizing: Direct or Vertical Ducts]
    B2 --> B2a[1 sq in per 4,000 Btu/h each]
    B --> B3[Sizing: Horizontal Ducts]
    B3 --> B3a[1 sq in per 2,000 Btu/h each]
    
    C --> C1[Location: 1 High within 12 inches of top]
    C --> C2[Sizing: 1 sq in per 3,000 Btu/h]
    C --> C3[Limit: Not less than sum of vent connector areas]
    C --> C4[Clearances: 1 inch sides/rear, 6 inches front]