Spillage in a Tight Basement with a Common Chimney
A 1978 two-storey house has a 100,000 BTU/h natural-draft furnace and a 40,000 BTU/h power-vented water heater in a 12 ft × 14 ft basement room with a 7 ft 6 in ceiling. The room has one louvered door (24 in × 12 in, metal louvers) to the finished basement; there is no outside combustion-air duct. The furnace vents through an 8 ft horizontal 6 in single-wall connector into an exterior 6 in × 6 in clay-tile masonry chimney. The water heater connector enters the chimney above the furnace. During a cold snap, the CO alarm on the floor above sounds. The technician finds frost at the chimney top, spillage at the furnace draft hood, and a room pressure of −12 Pa with the dryer and two bath fans running. The furnace nameplate input is 100,000 BTU/h; the water heater is 40,000 BTU/h. The homeowner reports the problem began after new windows and a sealed attic hatch were installed.
1. Determine whether the existing combustion-air opening is adequate for both appliances. Show the calculation and state what additional information you need.
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Combined input = 100,000 + 40,000 = 140,000 BTU/h. The room volume is 12 × 14 × 7.5 = 1,260 ft³. Using the common confined-space rule of 50 ft³ per 1,000 BTU/h, 140,000 BTU/h would require 7,000 ft³, so the room is confined. Two permanent openings are required, each with 1 in² of free area per 1,000 BTU/h, i.e. 140 in² free area each, one near the ceiling and one near the floor. The single 24 × 12 in metal-louvered door gives 288 in² gross; if the louvers have 75% free area, that is about 216 in², but it is one opening and it connects to the conditioned basement, not outdoors. It cannot be counted as two openings, and the adjacent space may also be depressurized. I would need the louver free-area rating, whether the basement has outside air, the exhaust-fan CFM, and the chimney liner condition before approving. The −12 Pa reading is a red flag: it exceeds normal limits for naturally aspirated appliances and can overcome draft.
2. Explain why the power-vented water heater can worsen spillage at the furnace draft hood, and outline the tests you would perform before recommending a repair.
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A power-vented water heater fan can pressurize a common chimney, especially a clay-tile chimney with no liner. The furnace draft hood is a neutral-pressure device; if the chimney is positive or the room is negative, flue gases spill at the hood instead of going up. The water heater may also cycle and add a burst of pressure each time it runs. Tests: measure draft in the furnace vent connector and at the chimney base with only the furnace, only the water heater, and both running; perform a spillage test at the draft hood after 5 minutes of operation; measure CO in the flue and in the room; record room pressure with the dryer and bath fans on and off; inspect the chimney top, cap, and liner. Do not leave the appliances operating if spillage or CO is present. If the chimney is unlined or oversized, a liner or separate venting may be required.
3. List the corrective actions in priority order and explain why simply adding a CO alarm is not acceptable.
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Priority: shut off gas and tag both appliances; ventilate and confirm CO is clear. Then provide outside combustion air: two openings sized at 1 in² free area per 1,000 BTU/h each, or a direct duct sized to the appliance inputs, terminating outdoors. Next correct the venting: line the chimney with an approved liner sized for the appliances, separate the water heater and furnace vents, or power-vent the furnace with a listed kit. Repair the chimney cap and any liner defects. After reassembly, verify draft, spillage, manifold pressure, and combustion; install CO alarms as a backup. Adding a CO alarm alone is not acceptable because it does not remove the source of CO, does not restore draft, and does not correct the depressurization. The appliance can produce CO continuously while the alarm is only a warning device.