Furnace and Water Heater Sharing a Sealed Closet
A 100,000 BTU/hr, 80% induced-draft furnace and a 40,000 BTU/hr natural-draft water heater with a draft hood share a converted closet measuring 10 ft × 12 ft × 8 ft. The closet is closed with a solid door and has no other openings. The contractor proposes to cut a single 14 in × 6 in return-air grille in the door, use it for combustion air, and also use the closet as part of the return-air path since the platform is open to the hallway ceiling. The furnace flue is single-wall pipe that passes through the closet wall with no thimble and slopes back toward the furnace. The local inspector is due tomorrow, and the homeowner wants the drywall closed today.
1. Is this closet a confined space? Compute the volume and the required combustion air openings for the two-permanent-opening method with metal louvers.
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Volume = 10 × 12 × 8 = 960 ft³. Total appliance input = 100,000 + 40,000 = 140,000 BTU/hr.
The unconfined-space rule of thumb is 50 ft³ of volume per 1,000 BTU/hr of input: 140 × 50 = 7,000 ft³ required. With only 960 ft³, the closet is confined by a wide margin (it would support about 19,000 BTU/hr as an unconfined space) and must be provided with outdoor/communicating air.
Two-permanent-opening method, one opening within 12 inches of the top and one within 12 inches of the bottom:
- Free area = 1 in² per 1,000 BTU/hr of total input for each opening = 140 in² free area each.
- Metal louvers are typically credited at 75% free area: 140 ÷ 0.75 = ≈187 in² gross each. Two 14 in × 14 in metal grilles (196 in² gross) satisfy it; wood louvers at 25% free would need 560 in² gross each, which will not fit this door.
A single 14 × 6 grille (84 in² gross, ~63 in² free) fails on both count and location - it is one opening, in the wrong place, and undersized by more than half.
2. The contractor wants to use the 14 × 6 door grille as the return-air path from the closet. Explain why this is unacceptable and what it can do to the occupants.
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Three separate failures:
- Prohibited return-air source. The mechanical code prohibits taking return air from a room or closet containing fuel-burning appliances (an exception exists only for direct-vent appliances). The closet is not a return plenum.
- Depressurization and backdrafting. A return grille in that closet pulls the space negative. The natural-draft water heater's draft hood has no fan to fight back; the negative pressure spills flue gases into the closet instead of up the chimney. The induced-draft furnace makes this worse when its own combustion blower runs.
- Distribution of CO. Return air from that closet travels through the ductwork and is delivered to every room in the house. A spillage event becomes a whole-house exposure.
It also fails as a combustion air opening: it is a single opening, at one height only, of unknown free area, and it communicates with the conditioned space rather than outdoors.
A 14 × 6 hole is not a compromise or a shortcut - it actively converts a venting defect into a CO hazard. The correct answer is proper high/low combustion air grilles sized to 1 in²/1,000 BTU/hr free area each, or a direct-vent/outdoor combustion air kit.
3. The drywall is going up tomorrow. What do you correct, in what order, and what do you put in writing?
Read the worked answer
Correct the life-safety items before anything is concealed - an inspector who has not seen the inside of the wall cannot approve it, and you should not want him to.
Order of work:
- Combustion air - cut two properly sized openings per the calculation, or install a manufacturer-approved outdoor air kit; do not rely on the door grille.
- Flue connector - replace the single-wall connector where it passes through the wall with listed Type B vent and a proper thimble/clearance. Single-wall connector needs roughly 6 inches to combustibles; Type B needs 1 inch, per listing and NFPA 211.
- Slope - the connector must rise at least 1/4 inch per foot toward the chimney, never back toward the appliance. Correct it, secure each joint with at least three screws, and confirm the connector does not pass through a concealed space.
- Return air - remove the return from the closet entirely; seal the platform opening to the hallway so the closet is not part of the return path.
- Verify - run both appliances and perform a draft test at the water heater draft hood and a CO test in the closet and the supply airstream; confirm no spillage.
In writing: photograph the pre-correction conditions, note the measurements (960 ft³ vs 7,000 ft³ required, opening sizes, flue slope and clearance), and state plainly that the closet may not be closed up until items 1–4 are corrected and re-inspected. Offer the documentation to the inspector and the homeowner.