Common-Vented Natural-Draft Appliances Spilling CO
A 35,000 Btu/h natural-draft water heater and an 80,000 Btu/h natural-draft furnace are connected to a common 6 in. masonry chimney in a 1,200 ft² bungalow. The chimney has an unlined 8 in. x 8 in. clay tile flue and a rain cap. The basement is tight: a 300 cfm kitchen exhaust fan and a 100 cfm bathroom fan are both running during a test. The water heater draft hood spills flue gases within 90 seconds of furnace startup; a CO alarm in the basement reads 70 ppm. The technician measures -5 Pa in the basement relative to outdoors. The furnace vent connector rises 2 ft to the chimney; the water heater connector rises 1 ft. The chimney is on an exterior wall and is cold. There is no combustion-air duct. Both appliances have draft hoods.
1. Explain the likely chain of events causing the water heater draft hood to spill, using the pressure and appliance details.
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The basement is depressurized to -5 Pa relative to outdoors because the 300 cfm kitchen fan and 100 cfm bathroom fan exhaust more air than the tight house can replace. Natural-draft appliances need a supply of combustion air and a chimney that is warm enough to produce draft. This chimney is on an exterior wall and is cold, so draft is weak at startup. When the 80,000 Btu/h furnace starts, it competes with the 35,000 Btu/h water heater for the limited air and for space in the common vent. The two draft hoods interact: the furnace can pressurize the common vent or reduce the water heater's draft, and the water heater draft hood spills. The 70 ppm CO reading confirms that flue products are entering the basement. This is a pressure and venting interaction, not simply a blocked chimney.
2. A helper suggests removing the rain cap or increasing the water heater vent connector size. Evaluate those proposals.
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Neither proposal is an acceptable fix. Removing the rain cap might reduce resistance slightly, but it does not correct the -5 Pa depressurization, the missing combustion air, or the cold exterior flue. A larger vent connector is likely worse: it slows flue-gas velocity, allows the gases to cool, and can weaken draft further. The common vent must be sized and installed according to B149.1, with proper connectors, rise, and chimney condition. The root causes here are inadequate combustion air and a venting system that cannot handle the combined appliances under negative pressure. Altering the cap or connector does not address either cause and may create a new hazard.
3. What immediate safety actions and permanent remedies should be taken?
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Immediately shut off both appliances, ventilate the basement, and keep occupants out if CO is present. Tag the appliances out of service and do not relight until repairs are complete. Permanent remedies include providing code-compliant combustion air to the basement, such as permanent openings or a properly sized duct connected to outdoors. The venting must also be corrected: options include relining the chimney to the correct size, separating the appliances so each has its own vent, replacing the water heater with a power-vented or direct-vent unit, or installing a listed common-vent arrangement designed for the combined input. If large exhaust fans remain, interlocking them with the appliances or providing makeup air may be necessary. After repairs, verify draft at each draft hood, measure CO in the flue and occupied space, and confirm the appliances do not spill during fan operation.