Combustion Air for a Confined Mechanical Room
A contractor installs a 120,000 Btu/h natural gas furnace and a 40,000 Btu/h natural gas water heater in a dedicated mechanical closet measuring 6 ft by 8 ft by 8 ft with a tight-fitting, weatherstripped door and no louvers. The closet is on an interior wall of a conditioned basement. The contractor installs two 6-inch round ducts from the closet to the basement space, one terminating 6 inches above the floor and one 6 inches below the ceiling, and considers the combustion air requirement satisfied. The building official questions whether the closet is 'confined' and whether the two openings are adequate.
1. Determine whether the closet is a confined space and whether the two 6-inch openings are adequate. Show your reasoning.
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Step 1 - Volume and classification. Closet volume = 6 ft × 8 ft × 8 ft = 384 ft³. Total input = 120,000 + 40,000 = 160,000 Btu/h. The threshold for a confined space is typically based on 50 ft³ per 1,000 Btu/h of total input. Required volume for 'unconfined' = 160,000 ÷ 1,000 × 50 = 8,000 ft³. 384 ft³ is far below 8,000 ft³, so the closet is a confined space. The tight door means infiltration cannot be counted.
Step 2 - Opening sizing. For combustion air from an adjacent indoor space, each opening is typically sized at 1 in² of free area per 1,000 Btu/h, with two openings (one high, one low). Required free area per opening = 160,000 ÷ 1,000 = 160 in². A 6-inch round duct has a gross area of π × 3² ≈ 28.3 in². Even before accounting for louvers or grilles (which reduce free area), 28.3 in² is far below 160 in².
Conclusion: The closet is confined, and the two 6-inch openings are grossly undersized. The installation does not satisfy the combustion air requirement.
2. What corrections should the inspector require, and what else should be checked about the openings?
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Corrections:
- Resize the openings: Provide two openings, one near the floor and one near the ceiling, each with free area of at least 160 in² (based on 1 in² per 1,000 Btu/h for indoor air). If the openings use louvers or grilles, the free area must be calculated after the louver deduction, so the physical opening may need to be larger.
- Alternative: If the adjacent space cannot reliably supply the air, provide outdoor combustion air using the applicable sizing method, which typically requires larger openings and may require ducts sized accordingly.
Additional checks:
- Opening location: The high opening should be within the upper portion of the space and the low opening within the lower portion, as required, so the space can ventilate.
- Duct terminations: Ducts conveying combustion air must terminate in the same pressure zone and not be obstructed; verify they do not draw from a space that can become negatively pressurized.
- Appliance clearances: Verify clearances to combustibles and that the appliances' own combustion air inlets are not blocked.
- Verification: Require the corrected design to be documented and re-inspected, and confirm the appliances operate properly (draft and spillage check) after correction.