Common-Vent Spillage in a Tight Basement
A 1,250 ft² basement with an 8 ft ceiling contains an 80% AFUE, 100,000 BTU/h natural-draft furnace and a 40,000 BTU/h natural-draft water heater. Both appliances connect to a common 6 in. B-vent with a 12 ft vertical rise and an 8 ft connector that has two 90° elbows. The basement is tightly constructed and has a 110 CFM bath fan and a 200 CFM clothes dryer. There is one 4 in. outdoor combustion-air duct, but it terminates 18 in. above the floor and is blocked by insulation. During a worst-case depressurization test with both exhaust fans running, the water heater draft hood spills flue gas and the CO reading in the spillage is 65 ppm. The basement pressure is -4 Pa. The furnace runs without spillage when the water heater is off.
1. Calculate the combustion-air volume required by the 50 ft³ per 1,000 BTU/h indoor-air method and compare it with the basement volume. Does the basement qualify as an acceptable source?
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Total appliance input is 100,000 + 40,000 = 140,000 BTU/h. At 50 ft³ per 1,000 BTU/h, required volume is 140 x 50 = 7,000 ft³. The basement volume is 1,250 x 8 = 10,000 ft³, which is larger than 7,000 ft³. Volume alone therefore looks adequate, but the basement is described as tightly constructed. Under the indoor-air method, tight construction requires outdoor air or a tested equivalent; the volume comparison does not make the space acceptable. The blocked 4 in. duct also means the intended outdoor-air source is not working. The correct conclusion is that the basement does not qualify as a reliable combustion-air source until the outdoor-air path is restored and the worst-case condition is re-tested.
2. Why does the common vent spill only when the water heater is on, and what test sequence confirms the cause?
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A natural-draft water heater has a draft hood that is easily upset by negative pressure in the room. The bath fan and dryer exhaust air from a tight basement, driving the space to -4 Pa and starving the water heater of combustion air. When the furnace runs, its own draft may mask the problem; when the water heater fires alone, the basement negative pressure and the blocked outdoor-air duct allow the draft hood to spill. Confirm with a worst-case depressurization test: close windows and doors, operate all exhaust fans and the dryer, fire the water heater alone, and measure draft in the vent connector, spillage at the draft hood, room pressure, and CO in the spillage. Then restore outdoor air and repeat. Also inspect the vent connector rise, common-vent termination, and any blockage.
3. List corrective actions and explain why a draft inducer or jumper on the water heater is not a complete fix.
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Correct the air supply first: unblock and properly terminate the outdoor combustion-air duct, size it to the appliance input, and remove restrictions. Seal or isolate the basement from exhaust fans where possible, add make-up air, and verify worst-case draft after correction. If common venting remains unsafe, separate the venting or install a listed power-vented water heater. A draft inducer may force flue gas out of the water heater, but it does not solve air starvation or negative pressure and can create unsafe operation if the appliance is not listed for it. Jumpering a safety device is never acceptable. Re-test draft, spillage, and CO with all exhaust devices operating before leaving the job.