Sizing a Garage Heater Branch on an Existing 1/2-Inch Line
A homeowner wants a 30,000 Btu/h natural-gas unit heater added in a detached garage. The existing 1/2-in. Schedule 40 black-steel line leaves the meter, runs 35 ft, and feeds a 40,000 Btu/h water heater and an 80,000 Btu/h furnace. The proposed branch adds 25 ft of pipe, three 90° elbows, a union, and a shutoff valve. The meter outlet is 7 in. w.c.; the utility says the meter can deliver 250 ft³/h. The longest run from meter to the new heater is 60 ft. The contractor proposes teeing off the existing 1/2-in. line near the furnace and using the same size to the garage. The customer does not want to repaint or remove drywall, and the garage wall is finished. The technician must decide whether to proceed, upsize, or test first.
1. Determine whether the existing 1/2-in. line can supply the added load. Show your sizing logic and state what you would do if it is undersized.
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Total connected load is 150,000 Btu/h (40,000 + 80,000 + 30,000), not 120,000. The longest length is 60 ft, but fittings add equivalent length; using the longest-length method, 1/2-in. Schedule 40 at about 60 ft has a capacity well below 150 ft³/h (roughly 70–90 ft³/h at 0.5 in. w.c. drop), so the proposed tee is undersized. Do not rely on diversity to justify a smaller pipe. A 3/4-in. run at 60 ft is closer to the required capacity but must be checked with the actual sizing table, fitting allowance, gas specific gravity, and available pressure drop; if the table result is marginal, use 1-in. Run a dedicated line from the meter rather than extending the existing 1/2-in. branch. Before cutting, confirm meter capacity: 250 ft³/h is adequate for 150 ft³/h plus other loads, but notify the utility if the meter set must be changed.
2. What pressure test would you perform on the new piping before putting it into service, and what are the key steps?
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Isolate the new piping from existing appliances and the meter as required. Test with air, nitrogen, or carbon dioxide-never oxygen or fuel gas-at 1.5 times the maximum working pressure but not less than 5 psi. With 7 in. w.c. (about 0.25 psi) working pressure, the 5 psi minimum governs. Use a gauge whose range puts the test pressure near mid-scale, such as 0–15 psi, hold for the code-required period (commonly 15 minutes), and check for zero pressure drop. Soap-test all joints at operating pressure after purging. Do not use a low-pressure manometer for the 5 psi test, and do not leave the test gas connected to appliances.
3. Where must the shutoff valve, union, and sediment trap be located at the new heater, and why?
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Install a manual shutoff valve in the same room as the heater, accessible and ahead of the union and appliance controls. Place the union downstream of the shutoff valve so the appliance or its controls can be removed for service. Install a sediment trap at the appliance inlet at the lowest point, before the controls, with a capped nipple long enough to collect debris-typically at least 3 in., subject to the current code. The trap protects the gas valve and orifice from pipe scale and dirt. The shutoff must be clearly identified, and the appliance connector or rigid piping must not be concealed or stressed.