Added Boiler Gas Branch Size and a Questionable Pressure Test
A gas fitter is adding a 140,000 Btu/h input natural gas boiler to a house. The meter outlet pressure is 7 in w.c. The house has a 3/4-inch Schedule 40 black iron trunk that runs 30 ft from the meter to a tee. From the tee, a new 1/2-inch Schedule 40 black iron branch runs 25 ft with four 90° elbows to the boiler. The existing trunk continues 15 ft to an 80,000 Btu/h furnace and a 40,000 Btu/h water heater. The gas fitter isolates the new boiler, caps the branch, and pressure-tests the new piping at 10 psig for 30 minutes. The gauge starts at 10.0 psig at 72°F and ends at 9.6 psig at 87°F. The fitter says the 0.4 psi drop is only thermal expansion and wants to set the boiler to fire. The boiler manufacturer requires inlet pressure between 4 and 10 in w.c. and a sediment trap at the appliance.
1. Evaluate the new branch size and the existing trunk using the longest-length method. State what must be changed before firing.
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Natural gas at about 1,000 Btu per cubic foot means the new boiler needs about 140 cfh; the total connected load is 140 + 80 + 40 = 260 cfh. The longest run to the boiler is 30 ft of trunk plus 25 ft of branch plus fitting allowance. Four 1/2-inch 90s add roughly 10 to 12 equivalent feet, so the branch is about 65 to 67 equivalent feet. A 1/2-inch Schedule 40 black iron line at that length and a 0.5 in w.c. drop is only in the 60 to 80 cfh range, far below 140 cfh. The branch is undersized and must be increased, normally to 3/4 inch or larger after a full sizing calculation. The 3/4-inch trunk is also suspect: it carries 260 cfh after the tee, and 3/4-inch pipe at 55 to 70 equivalent feet is generally below that. A proper longest-length calculation may require a larger trunk or a separate line from the meter. Do not fire the boiler until the piping is resized and the leak test passes.
2. The gauge fell 0.4 psi while the temperature rose 15°F. Is that a leak? Show the reasoning and the next action.
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Use absolute pressure and absolute temperature. At the start, 10.0 psig + 14.7 = 24.7 psia; 72°F = 532 R. At the end, 87°F = 547 R. If volume is constant and there is no leak, P2 = 24.7 × 547/532 = 25.4 psia, or about 10.7 psig. The gauge should have risen about 0.7 psi, not fallen 0.4 psi. The observed end pressure of 9.6 psig is 24.3 psia, about 1.1 psi below the temperature-corrected expectation. That is a leak, not thermal expansion. Isolate the appliance, confirm the test pressure is acceptable for the piping and any connected components, and locate the leak with soap solution or an electronic detector. Repair, then repeat the test at a stabilized temperature. Also verify gauge resolution and that no regulator or appliance was exposed to 10 psig.
3. List the appliance connection and pressure checks required before placing the boiler in service.
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Install a sediment trap at the boiler, a full-port manual shutoff valve, a union, and a drip leg arranged so debris does not enter the gas valve. Use a listed appliance connector only where allowed, protect it from contact with the flue or sharp edges, and do not route it through the cabinet. Verify the regulator vent is open and outdoors, the manifold pressure matches the rating plate, and the inlet pressure stays within the manufacturer's 4 to 10 in w.c. range with all appliances firing. Purge the line outdoors, check for leaks at every joint with a bubble solution or detector, and confirm bonding and grounding. Start the boiler and check for a stable flame, correct gas input rate by meter timing, and no pressure drop that indicates an undersized line.