Vent Connector Routing Through a Chase
A natural-draft, Category I gas-fired water heater (40,000 Btu/h) and a Category I furnace (100,000 Btu/h) share a single-story house with a 6-inch diameter Type B vent serving both through a common vent. The water heater connector runs 12 feet horizontally with two 90-degree elbows before entering the common vent; the furnace connector runs 4 feet with one elbow. The common vent terminates 2 feet above the roof surface, and the roof pitch is 6:12. The vent passes through an interior chase that also contains a 2-inch PVC drain. The house is tightly constructed with a measured air infiltration rate of 0.30 air changes per hour, and the water heater is in a closet with a solid door and no dedicated combustion air opening. There is no draft hood spillage observed on a mild day.
1. Evaluate the venting arrangement and identify every condition that is likely noncompliant, with reasoning.
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Several problems stand out:
Termination height. For a 6:12 roof pitch, the vent must extend well above the roof - the common rule is at least 3 feet above the highest point where it passes through the roof and at least 2 feet higher than any portion of the building within 10 feet horizontally. A 2-foot termination above the roof surface does not satisfy this.
Connector length and elbows. The water heater connector at 12 feet horizontal with two 90-degree elbows is long and tortuous. Category I connectors have limits on horizontal run and total equivalent length, and each 90-degree elbow adds equivalent length. This arrangement likely exceeds the allowed developed length and creates a strong spillage risk.
Common venting. Combining a 40,000 Btu/h natural-draft water heater with a 100,000 Btu/h furnace in a single common vent requires the vent and connector sizing to be checked against the venting tables for the combined input, height, and connector rise; a 6-inch common vent may be oversized for the combined load and short height, which promotes condensation and poor draft.
Combustion air. A tightly constructed house at 0.30 ACH with a solid-door closet and no dedicated combustion air opening cannot be assumed to provide adequate combustion and dilution air for 140,000 Btu/h of natural-draft appliances. This is a serious safety issue.
Chase clearance. The vent passing through a chase shared with PVC drain piping raises clearance-to-combustibles questions and must be checked against the listed vent's clearance requirements.
2. The installer says 'there's no spillage on a mild day, so it's fine.' Explain why this test is inadequate and what testing is actually needed.
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A no-spillage observation on a mild day is the least demanding condition and proves very little. Natural-draft appliance draft is weakest when the temperature difference between flue gases and outdoor air is smallest and when the house is under negative pressure - cold, windy weather, exhaust fans running, and closed-up tight construction all worsen draft. A mild-day check can pass while the appliance spills under real winter conditions.
Proper verification includes: (1) combustion air adequacy calculation for the combined input in a tight house, with a dedicated opening or ducted outdoor air if required; (2) worst-case depressurization testing - operate all exhaust fans and the clothes dryer, then check for spillage at the draft hood of each appliance; (3) draft measurement in the vent connector and at the appliance outlet; (4) verification of vent termination height and connector length/elbow limits against the venting tables; (5) confirmation that the common vent is correctly sized for the combined input and height. Until these are addressed, the installation should not be accepted.