The Furnace Closet That Cannot Breathe
A 4 ft by 4 ft by 8 ft closet contains an 80,000 Btu/h induced-draft Category I furnace and a 40,000 Btu/h natural-draft water heater with a draft hood. The closet door is a louvered bifold with 60 square inches of free area, opening into a central hallway. The installer says the louver is the combustion air opening and plans to leave the closet under negative pressure by using it as the return-air path for the hallway. The closet has a drywall ceiling and no other openings. The owner had the house blower-door tested at 2.5 ACH50, and the contractor says that proves the house is tight enough that indoor air cannot be used anyway.
1. Does the closet volume satisfy the indoor-air method? Show the calculation.
Read the worked answer
No. The indoor-air volume method requires not less than 50 cubic feet of volume per 1,000 Btu/h of total appliance input. Total input = 80,000 + 40,000 = 120,000 Btu/h, so the required volume is 50 x 120 = 6,000 cubic feet. The closet volume is 4 x 4 x 8 = 128 cubic feet. Even if the closet were permanently opened to the hallway, the combined volume of all connected spaces would have to be computed and the openings sized to that combined volume, with no doors that can close off the spaces. A 60-square-inch louvered door does not create the required volume, and the blower-door result does not change the arithmetic-the volume method fails on its face.
2. If outdoor combustion air is used, size the openings for the two-opening and one-opening methods, accounting for louver free area.
Read the worked answer
Two permanent openings, one within 12 inches of the top and one within 12 inches of the bottom, each at 1 square inch free area per 4,000 Btu/h: 120,000 / 4,000 = 30 square inches net each. Single-opening method: 1 square inch per 3,000 Btu/h = 120,000 / 3,000 = 40 square inches net, located within 12 inches of the top. Free area is adjusted for the louver: a metal louver at about 75 percent free area needs roughly 40 square inches gross to deliver 30 net; a wood louver at about 25 percent would need 120 square inches gross. If ducts are used, they are sized from the same free-area basis and must terminate outdoors with approved openings.
3. May the closet be used as the return-air path? What code provision controls?
Read the worked answer
No. The duct-system chapter prohibits taking return air from a closet or space containing a fuel-burning appliance, and the prohibition is not cured by a louvered door. Using the closet as a return path would also depressurize the space relative to the hallway, degrading draft and increasing the chance of spillage from the water heater draft hood-a safety issue even where the appliance fires properly. The louver may serve as a combustion-air opening only if it delivers the required free area. The correction is a dedicated return outside the closet and properly sized combustion-air openings to outdoors or to a qualifying volume.