Adding a Tankless Heater to an Old Common Vent and Gas Manifold
A three-family dwelling is being converted to a four-family. The existing 1/2-psi natural gas system has a 1 1/4-inch service and a manifold with four 1/2-inch black-iron branches: one 40-foot run to a 75,000-Btu/h range, one 25-foot run to a 35,000-Btu/h dryer, one 30-foot run to a 40,000-Btu/h water heater, and one 50-foot run to a 100,000-Btu/h boiler. The contractor wants to add a fifth 1/2-inch branch, 60 feet long, to a 120,000-Btu/h tankless water heater. The gas company will set a new meter at the exterior. In the basement, the existing atmospheric water heater shares a 5-inch B-vent with a draft-hood boiler; the water heater connector rises 12 inches and slopes back toward the appliance, and the common vent has two 90-degree elbows. The owner wants you to approve the piping and venting so the meter can be set.
1. Using the longest-length method, evaluate the proposed 1/2-inch branch to the 120,000-Btu/h tankless water heater. What load and length must be used, and what would you require?
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For a 1/2-psi natural gas system, use the longest run from the meter to the most remote appliance, not just the new branch. The new branch is 60 feet, but the total developed length includes the manifold, the service, and fittings. The load on the common upstream piping includes all appliances: 75,000 + 35,000 + 40,000 + 100,000 + 120,000 = 370,000 Btu/h. At 1/2 psi and the total developed length, a 1/2-inch black-iron branch cannot carry 120,000 Btu/h; compare the adopted gas table and you will find it is undersized. The tankless heater also has a minimum inlet pressure and a pressure drop that must be checked. Require the branch to be upsized, or convert to a 2-psi system with a properly sized line regulator at each appliance, and verify the meter and service capacity for the total load. Do not approve the new branch just because the existing four branches worked; adding load changes the common manifold and service demand.
2. Evaluate the common venting of the atmospheric water heater and draft-hood boiler. What is wrong with the connector slope and the two 90-degree elbows?
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Both appliances are Category I, draft-hood appliances, so common venting is possible only if both are listed for common venting and the common vent is sized per the adopted gas code table for the combined input and height. The connector must slope upward toward the common vent, not back toward the appliance. As described, the water heater connector rises 12 inches and then slopes back toward the water heater, creating a low point where flue gases can collect and spill. The two 90-degree elbows add resistance; the code limits the number of elbows and the connector length, and the vent capacity table must be adjusted. Also check the common vent height, the chimney liner, and the combustion air openings. If the basement is tight, the appliances may backdraft. Require the connector to be re-pitched toward the vent, the elbow count reduced if possible, and the vent capacity verified; if the common vent is undersized or the chimney is unlined, separate venting or a properly sized liner may be required.
3. What combustion-air, clearance, and testing requirements would you check before approving the installation?
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Check that the basement has permanent combustion-air openings sized for the combined input of all appliances, or that the appliances are direct-vent. Verify clearances to combustibles for the water heater, boiler, and vent connector, and check that the vent passes through the roof or chimney with correct clearances and supports. The tankless water heater’s venting is different from the atmospheric appliances; do not connect it to the same B-vent. Check the gas piping leak test at the required pressure for the allowed duration, the sediment trap at each appliance, and the grounding/bonding of the gas piping. Confirm the regulator vent is outdoors and protected, and that the meter set is properly sized. Before approval, require a draft test and a combustion-air verification; a flame rollout or spillage at the draft hood is a red flag. If the venting cannot be corrected, do not approve the meter set.