Combustion Air and Draft on a Sealed Mechanical Room
A 100,000 Btu/h natural-draft gas furnace and a 40,000 Btu/h natural-draft water heater share a 10 ft x 12 ft x 8 ft mechanical room in an Anchorage basement. The room was recently tightened: weatherstripping on the door, sealed rim joist, and a new insulated door. The furnace now soots, the burners lift and flutter, and the water heater's draft hood spills on cold startups. A CO test in the room reads 35 ppm ambient after 10 minutes of operation. The room has one 3 in. round duct to the exterior, installed years ago.
1. Evaluate whether the existing combustion air provision is adequate and identify the governing calculation.
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Total input is 140,000 Btu/h. Standard practice for confined spaces uses the '50 cubic feet per 1,000 Btu/h' rule: 140,000/1,000 x 50 = 7,000 ft³ required. The room volume is 10 x 12 x 8 = 960 ft³ - far below 7,000, so this is a confined space and must draw combustion air from outdoors or from a large unconfined volume. The single 3 in. duct is grossly undersized; two permanent openings (high and low) sized per the applicable code (roughly 1 in² free area per 4,000 Btu/h for vertical ducts, or 1 in² per 2,000 Btu/h for horizontal ducts, with free-area correction for louvers) are needed. For 140,000 Btu/h with horizontal ducts that is about 70 in² each, corrected upward for any grille free area - the 3 in. duct (~7 in²) is nowhere close.
2. The CO reading is 35 ppm in the room. State the immediate action and the diagnostic sequence.
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35 ppm ambient CO in an occupied-adjacent space is an emergency: shut down both appliances, ventilate, evacuate/verify occupants, and do not restart until corrected. Diagnostically, the sequence is: (1) confirm adequate combustion air is restored and the room is no longer depressurized; (2) check the flue/vent for blockage, correct sizing, and proper rise; (3) measure draft at the hood and in the flue to confirm negative pressure at the appliance connector; (4) verify the burners are clean and adjusted and the heat exchanger is intact; (5) re-test CO in the flue and ambient after fixes. The sooting and flame lifting point to starved air and/or backdraft, consistent with the sealed room and undersized opening.