Tankless Retrofit with Undersized Gas Supply
A homeowner is replacing a 50-gallon atmospheric water heater with a 199,000 Btu/h condensing tankless unit in a 2,400-square-foot, three-bath house. The existing natural-gas meter is rated 250 cubic feet per hour at 7 inches water column. The gas piping is 3/4-inch black iron, 45 feet from the meter to the heater location. Other gas loads are a 100,000 Btu/h furnace, a 65,000 Btu/h range, and a 35,000 Btu/h dryer. The tankless unit will be mounted on an exterior wall; the manufacturer permits PVC venting and requires condensate drainage. The incoming water is 18 grains per gallon hard, and the homeowner has complained about scale in the old tank. The local utility requires the meter and service to be sized for the connected load, not just the largest appliance.
1. Calculate the connected gas load and determine whether the existing meter and 3/4-inch line can serve the tankless unit. What size line is needed?
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Add the connected loads: 199,000 + 100,000 + 65,000 + 35,000 = 399,000 Btu/h. At about 1,000 Btu per cubic foot of natural gas, that is roughly 399 cubic feet per hour. The existing meter is rated 250 cfh, so it is undersized before any pipe sizing is done. The 3/4-in black iron line is also too small: at 45 ft equivalent length and a typical 0.5-in w.c. drop, 3/4-in pipe carries only about 110 to 130 cfh, far below the tankless unit alone. Use the longest-length method with the actual equivalent length, including fittings, and size for the total load. Expect a meter and service upgrade and a 1-1/4-in or larger gas line, depending on the table and available pressure. Do not size only to the tankless nameplate.
2. What venting and condensate details matter for this condensing tankless installation?
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The tankless unit is a Category IV condensing appliance. Use only the vent material listed in the manufacturer’s instructions-often PVC, CPVC, or polypropylene-and do not common-vent it with the atmospheric furnace or water heater. Maintain the listed termination clearances, slope the vent per the manufacturer, often back to the heater, and protect the termination from snow and recirculation. Condensate is acidic; install a neutralizer and drain it through a trap per the instructions, with freeze protection. Provide a condensate drain that does not create a slip hazard. Check that the exterior wall location meets clearance to windows, doors, and intakes.
3. How would you address the water hardness and the unit’s operating limits?
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At 18 grains per gallon, the water is hard enough to scale the heat exchanger, especially in a condensing tankless unit. Scale reduces flow and efficiency and can void the warranty. Recommend a softener or a listed scale-control device, and install isolation valves and flush ports so the heat exchanger can be descaled. Set the temperature and flow so the unit is not short-cycling. Confirm the manufacturer’s hardness limit and maintenance interval. Also check the minimum activation flow: a tankless unit may not fire for a small draw, so the homeowner should know which fixtures trigger it.