Adding a 90,000 Btu/h Furnace to an Existing 1/2 in. Gas Line
A remodel adds a 90,000 Btu/h furnace in the attic. The existing 1/2 in. schedule 40 black-iron line from the meter tees off and runs 40 ft equivalent length to the furnace. The same 1/2 in. line continues 15 ft to a 40,000 Btu/h water heater, and a 1/2 in. branch runs 25 ft to a 35,000 Btu/h range. Gas pressure at the meter is 7 in. w.c.; the installed system is being tested at 10 psi. The contractor proposes to keep the 1/2 in. furnace line because the furnace is new and high-efficiency.
1. Using the longest-length method, evaluate whether the 1/2 in. furnace branch is adequate.
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Total connected load is 90,000 + 40,000 + 35,000 = 165,000 Btu/h. The furnace branch is 40 ft equivalent. In typical schedule 40 black-iron tables at 7 in. w.c. and a 0.5 in. w.c. drop, 1/2 in. pipe at 40 ft carries only about 60,000–70,000 Btu/h. The furnace alone at 90,000 Btu/h exceeds that capacity, so the 1/2 in. branch is undersized. A 3/4 in. branch is required, and the main and other branches must be rechecked for the total load and longest length.
2. What gas-piping details should be verified at the furnace and water heater before the inspection?
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Each appliance should have a listed shutoff valve, a sediment trap on the appliance side of the shutoff, and an approved union or connection for service. The sediment trap leg should be at least 3 in. deep and installed as close to the appliance as practicable. The furnace and water heater also need proper venting, combustion air, and a drip leg arranged so debris does not enter the controls. The 10 psi test must be held for the code-required duration without leakage, with the meter and appliance controls isolated.
3. The contractor argues that high-efficiency furnaces use less gas, so the 1/2 in. line should be fine. How do you respond?
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Gas piping is sized on the appliance's rated input, not on average operating consumption. A 90,000 Btu/h furnace has a 90,000 Btu/h input rating and must be supplied at that rate under full demand. High efficiency affects the amount of heat delivered, not the gas input needed for sizing. The line must be sized for the nameplate input plus the other connected loads and the longest-length pressure drop.