Combustion-Air Openings for a Boiler Room Addition
A 30 ft by 18 ft by 12 ft boiler room houses a 1.5 MMBtu/h natural-draft boiler and a 200,000 Btu/h natural-draft water heater. The room has two permanent openings to outdoors, each 18 in. by 24 in. with louvers having 70% free area. One opening is 12 in. below the ceiling and the other 12 in. above the floor. The owner wants to add a 500,000 Btu/h boiler in the same room. The installer says the existing openings are adequate because the room has 'plenty of volume.' The local code adopts the IFGC combustion-air provisions: 50 ft³ per 1,000 Btu/h for indoor air, or two permanent openings sized at 1 in² per 4,000 Btu/h each when using outdoor air. The boiler room door is tight-fitting and there is no mechanical supply.
1. Calculate the room volume and compare it with the combustion-air volume required for indoor air, before and after the addition.
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Room volume = 30 × 18 × 12 = 6,480 ft³. For indoor air, required volume = 50 ft³ per 1,000 Btu/h. Existing total input = 1,500,000 + 200,000 = 1,700,000 Btu/h → 1,700 × 50 = 85,000 ft³. Proposed total = 2,200,000 Btu/h → 2,200 × 50 = 110,000 ft³. The room volume is only about 7.6% of the existing requirement and 5.9% of the proposed requirement, so the 'plenty of volume' argument fails; the room must use outdoor combustion air.
2. Calculate the required free area per opening and compare it with the existing openings, both for the current appliances and for the proposed total input.
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Each existing opening: 18 × 24 = 432 in² gross; at 70% free area = 302 in² free. Required per opening with two outdoor openings = 1 in² per 4,000 Btu/h of total input. Existing 1.7 MMBtu/h → 1,700,000/4,000 = 425 in² each. Proposed 2.2 MMBtu/h → 550 in² each. Existing openings are already undersized by 123 in² each before the new boiler; after the addition they are short by 248 in² each. The high/low placement is correct in concept, but free area and total input govern.
3. State your recommendation on the proposed boiler and the combustion-air upgrade.
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Do not approve the 500,000 Btu/h boiler on the existing openings. Options: enlarge both openings to at least 550 in² free each (for example, about 786 in² gross each at 70% free, such as 30 in. × 27 in. louvers), or provide a mechanical supply interlocked with the appliances and sized by an engineer. Verify that the openings are unobstructed, that louvers are fixed or interlocked, and that the room has no tight-fitting door that prevents relief air. Also check the boiler venting and draft hood dilution air; adding input may require a venting upgrade. After modification, commission with draft and combustion tests at all firing rates.