Replacing an Atmospheric Water Heater With a Condensing Tankless Unit
A property owner is replacing a 50-gallon atmospheric gas water heater located in a 6-foot by 8-foot closet with a 199,000 BTU/hr condensing tankless unit. The existing vent is 4-inch single-wall galvanized pipe with a 6-foot horizontal run and a 10-foot vertical rise through the roof. The closet has two 12-inch by 12-inch louvers for combustion air. The gas line is 3/4-inch black iron, 20 feet from the meter with four elbows, and also serves a 60,000 BTU/hr furnace. The meter outlet is 7 inches water column. The new tankless unit has a factory maximum vent length of 35 feet and requires Category IV venting. The owner wants to reuse the existing vent and gas line to keep the project under budget. The local inspector requires a permit, gas pressure test, and vent inspection. The owner asks whether the existing vent and gas line can be reused.
1. Can the existing vent be reused for the condensing tankless unit?
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No. A condensing tankless is Category IV, positive pressure, and typically requires sealed PVC, CPVC, or polypropylene venting listed by the manufacturer. Single-wall galvanized is not acceptable and will leak flue gases. The existing 4-inch size may not match the manufacturer allowed diameter; follow the installation manual vent table, including equivalent length for the 6-foot horizontal and 10-foot vertical run, elbows, and termination. The 35-foot maximum must include fittings. The vent must slope back to the heater for condensate, and condensate must go through a neutralizer or approved drain. The roof termination must meet clearance requirements. Reusing the vent is not an option.
2. Evaluate the combustion air provisions for the closet.
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The closet is only 6 x 8 x 8 = 384 cubic feet, far below the volume needed for 199,000 BTU/hr if using indoor air, typically 50 cubic feet per 1,000 BTU/hr or 9,950 cubic feet. The two 12 x 12 louvers may provide enough free area only if sized by the code and if free area is calculated after louvers; do not count gross dimensions. Use the adopted code method: two permanent openings, one near the top and one near the bottom, sized for the appliance input and free area, or install a direct-vent sealed-combustion unit. Verify clearances and that the closet door does not block air.
3. Evaluate the gas line and identify the other required components.
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Total load with the furnace is 259,000 BTU/hr. A 3/4-inch line at 20 feet with four elbows is unlikely to carry 199,000 BTU/hr plus the furnace at 7 inches w.c.; use the longest-length table and likely upsize to 1 inch or larger, or run a dedicated line. Check meter capacity and regulator. Pressure-test the gas piping before inspection. Also install the required T&P relief valve, expansion tank, and freeze protection; a condensing unit produces condensate that must not be trapped or allowed to freeze.