Draft Hood Spillage After a Basement Renovation
A two-storey house has an 80,000 Btu/h Category I natural-draft furnace and a 40,000 Btu/h natural-draft water heater sharing a 5 in. common vent with a 6 ft rise and a 20 ft lateral. The basement was recently renovated: sealed doors, a 110 cfm bathroom exhaust fan and a 300 cfm range hood were added. The homeowner reports a faint exhaust smell near the furnace and occasional draft-hood spillage when the bathroom fan runs. A draft gauge at the furnace flue reads -2.5 Pa in the basement relative to outdoors; the water heater draft hood spills when tested with a lighter flame. Outdoor design temperature is -25°C. The furnace has a 4 in. combustion-air collar capped off, and the basement door has a 1 in. undercut. The vent connector has two 90° elbows and a 12 in. rise before the lateral. A CO monitor near the furnace reads 12 ppm in the basement air after 20 minutes.
1. What is the most likely cause of the spillage, and what immediate safety actions are required?
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This is a combustion-appliance depressurization problem, not simply a dirty flue. The house was tightened and 410 cfm of exhaust was added; the only makeup path is a 1 in. undercut, far too small to supply both appliances and the exhaust fans. With the bathroom fan running, the basement becomes the combustion-air zone and pressure falls. A -2.5 Pa reading at the flue is enough to overcome the weak draft produced by a 6 ft rise and a 20 ft lateral with two elbows; the water heater draft hood is the most sensitive point and spills first. The 12 ppm CO rise confirms that products are entering the living space. Immediate action is to shut off gas to both appliances and red-tag them, ventilate the basement, keep the exhaust fans off, and advise the owner not to operate either appliance until corrected. Test CO in the flue and ambient air, check the vent for blockage, and repeat spillage testing at the draft hood. Do not treat an open window as a permanent repair.
2. Evaluate the combustion-air provisions and describe acceptable correction options under B149.1.
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Total connected input is 120,000 Btu/h. The 1 in. door undercut cannot be counted as a designed permanent opening, and a louvered door with 1 in. clearance is not a controlled combustion-air supply. Use the confined-space method in B149.1. For indoor air from the basement, two permanent openings are required, each sized at 1 in² per 1,000 Btu/h of total input, so each opening needs at least 120 in² free area and must be located high and low; confirm the current code minimum. Because the basement is routinely depressurized by exhaust fans, indoor air is unreliable. A stronger option is two openings to outdoors sized by the outdoor-air ratio, or ducted outdoor air to the appliance area. The best solution is a sealed combustion-air zone using direct-vent appliances. Whichever method is chosen, the openings must not be blocked by the door or stored goods, and the exhaust fans must not create a negative pressure that exceeds the appliances' draft capability.
3. What test would you perform to prove the venting is safe, and what remediation is likely?
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Perform a worst-case depressurization test: close all doors and windows, operate every exhaust fan and the clothes dryer, and measure draft at the furnace and water heater, spillage at the draft hood, and CO in the flue and ambient air. Also verify common-vent sizing for 120,000 Btu/h and the vent connector rise. If spillage occurs under worst-case conditions, the common vent cannot remain in service. Likely remediation is to convert the water heater to power-vented or direct-vent and the furnace to direct vent, or to provide a dedicated combustion-air supply and interlock the exhaust fans, or to reline and resize the common vent per the appliance manufacturers' tables. The capped 4 in. combustion-air collar must be connected to outdoor air if the furnace is listed for direct vent. After repair, repeat the worst-case test and record draft, CO, and spillage results.