Furnace and Water Heater in a Confined Closet
A 100,000 BTU/h, 80% AFUE Category I gas furnace is replacing an old unit in a closet measuring 40 in. wide by 30 in. deep by 8 ft high. The closet has a solid, weather-stripped door and no combustion-air openings. The furnace is common-vented with a 40,000 BTU/h natural-draft water heater into a single masonry chimney. The furnace vent connector is 6 in. diameter with a 2 ft rise and an 8 ft horizontal run containing two 90° elbows. The water heater connector is 3 in. and enters the chimney above the furnace connector. The closet also stores paint cans and a vacuum cleaner. A CO alarm is installed in the hallway. The furnace input is based on a manifold pressure of 3.5 in. w.c. and a gas heating value of 1,000 BTU/ft³.
1. Calculate the closet volume and the combustion-air opening area required if the closet is connected to the adjacent room by two permanent openings. Show the code basis.
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Closet volume:
- 40 in. × 30 in. × 96 in. = 115,200 in³
- 115,200 ÷ 1,728 in³/ft³ = 66.7 ft³
Total input = 100,000 + 40,000 = 140,000 BTU/h.
Using the standard indoor-air method of 50 ft³ per 1,000 BTU/h, the required volume would be 140 × 50 = 7,000 ft³. The closet is only 66.7 ft³, so it is a confined space and cannot rely on its own volume.
For two permanent openings connecting directly to an adjacent room, NFPA 54/IFGC requires each opening to have at least 1 in² of free area per 1,000 BTU/h of total input, with a minimum of 100 in². Therefore:
- 140,000 ÷ 1,000 = 140 in² free area for each opening
- One opening near the top and one near the bottom of the closet
If vertical ducts are used, the area can be smaller per the code table, but the direct-opening method gives 140 in² each. Verify the local code and the furnace manufacturer's instructions, and use louvered openings with free area, not gross opening dimensions.
2. Evaluate the common venting arrangement. What code and safety problems do you see, and what would you verify?
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Common venting a Category I furnace with a natural-draft water heater into a masonry chimney is allowed only under specific conditions:
- The chimney must be lined, clean, and properly sized for the combined appliances. An unlined or oversized masonry chimney can run cold, condense flue gases, and deteriorate.
- The vent connectors must slope upward toward the chimney at the required rate (typically 1/4 in. per ft) and be supported.
- The 8 ft horizontal furnace connector with two elbows may exceed the maximum run or require a larger diameter; verify against the vent tables and the manufacturer's instructions.
- The water heater connector entering above the furnace connector must be arranged so one appliance does not spill flue gases into the space. The smaller appliance is commonly connected above the larger one, but the chimney and connector sizing still must be checked.
- The closet has no combustion air and stores paint cans and a vacuum cleaner. Negative pressure from a clothes dryer or exhaust fan can reverse draft and spill CO into the home.
Verify with a combustion analyzer and draft gauge: check CO air-free, oxygen, stack temperature, and draft in the connector and chimney. Perform a spillage test at the draft hood after several minutes of operation.
3. What immediate actions and final verifications would you require before leaving the installation?
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Immediate actions:
- Remove all paint cans, solvents, and other combustible or flammable storage from the closet.
- Install the required combustion-air openings, or convert the installation to a direct-vent/sealed-combustion arrangement if the closet cannot be opened.
- Verify clearances to combustibles from the furnace and vent connector labels.
- Confirm the chimney is lined, clean, and properly sized; repair or reline as required.
Final verifications:
- Gas leak test at all connections and verify manifold pressure at 3.5 in. w.c.
- Combustion analysis: CO air-free, O2, and stack temperature within the manufacturer's range.
- Draft test and spillage test at the draft hood; confirm no flame rollout or spillage.
- Verify the vent connector slope, support, and termination.
- Check thermostat operation, ignition, flame sensor, limit, and blower operation.
- Confirm the CO alarm is functional and explain to the homeowner that it is not a substitute for proper venting.
Document all readings and corrections; do not leave the appliance operating if spillage or unsafe combustion is present.