Backdrafting Commercial Water Heater in a Depressurized Closet
A 199,000 Btu/h commercial gas water heater in a 3-foot by 4-foot closet with an 8-foot ceiling serves a 60-seat restaurant. The heater has a draft hood and a 4-inch single-wall flue with 6 feet of horizontal run, two 90-degree elbows, and 12 feet of vertical rise. The closet has two 18-inch by 12-inch louvered doors opening into the kitchen. The kitchen has a 600-cfm exhaust hood and a 200-cfm make-up air unit that runs only when the hood is on. The water heater’s CO alarm sounds at 35 ppm; soot is visible at the draft hood, and the flame rolls out when the kitchen exhaust starts. The owner wants to add a 50,000 Btu/h fryer and a 30,000 Btu/h griddle on the same gas line. A technician proposes increasing the flue to 5 inches and opening a window.
1. What is causing the spillage and soot, and why is increasing the flue size unlikely to fix it?
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The heater is depressurizing its own closet and spilling flue gases at the draft hood. The 600-cfm kitchen exhaust is larger than the 200-cfm make-up air unit, so the kitchen goes negative whenever the hood runs. The louvered doors open into that negative kitchen, not outdoors, so they cannot reliably supply combustion air. A 199,000 Btu/h appliance also needs substantial air: at 50 cubic feet per 1,000 Btu/h for indoor air, about 9,950 cubic feet, while the closet is only about 96 cubic feet. The 4-inch single-wall flue with two elbows and a horizontal run is also a draft liability, but increasing it to 5 inches does not solve negative pressure and may reduce flue-gas velocity and worsen condensation. Opening a window is not a controlled, code-compliant combustion-air solution. The soot and flame rollout confirm incomplete combustion and spillage, not a flue that is merely too small. The correct approach is to eliminate the negative pressure, provide dedicated combustion air or convert to sealed combustion, and verify draft under worst-case exhaust conditions.
2. Calculate the combustion-air requirement and evaluate the two louvered doors as the air source.
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Using the indoor-air method, 199,000 Btu/h ÷ 1,000 = 199, and 199 × 50 cubic feet = 9,950 cubic feet of volume required. The closet is 3 × 4 × 8 = 96 cubic feet, far short. For direct outdoor air through two openings, the typical requirement is 1 square inch of free area per 4,000 Btu/h, or about 50 square inches total for 199,000 Btu/h, divided between high and low openings. Each door is 18 × 12 = 216 square inches gross; at 75% free area for a louver, that is about 162 square inches each, which would be more than enough free area if the openings truly connected to outdoors. But they open into the kitchen, which is depressurized by the exhaust hood, so they are not outdoor-air openings. The appliance is therefore starved for combustion air and backdrafts. Provide two permanent openings to outdoors, sized and located per code, or use a sealed-combustion/direct-vent appliance. Do not count the kitchen as an outdoor air source.
3. What corrective scope would you require before the owner adds the fryer and griddle?
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First, stop using the water heater until it passes a draft test. Interlock the make-up air with the exhaust hood so the kitchen cannot be run at a negative pressure, and verify worst-case draft with the hood, bathroom fans, and dryer running. Provide dedicated outdoor combustion air to the water heater closet, or replace the natural-draft heater with a sealed-combustion or power-vented unit. Repair or replace the single-wall flue with the proper listed vent material, support it, and maintain clearances; the two elbows and horizontal run should be minimized. Install and test CO alarms. Only after the existing heater is stable should the added load be evaluated: 50,000 + 30,000 = 80,000 Btu/h more gas and more exhaust and heat gain, which will worsen the negative-pressure problem. The gas line must be sized for the total connected load, and the kitchen ventilation must be rebalanced. Increasing the flue to 5 inches and opening a window are not acceptable fixes.