Sizing a Gas Line for Multiple Appliances
A new residential installation includes a 100,000 BTU/h furnace, a 40,000 BTU/h water heater, and a 60,000 BTU/h range. The gas meter is located outside, and the piping will run 30 feet from the meter to the manifold, then branch to each appliance. The available gas pressure is 7 inches water column (WC) at the meter, and the allowable pressure drop is 0.5 inches WC. The piping material will be Schedule 40 black iron pipe. The local code requires the use of the gas sizing tables in the code book, which account for pipe size, length, and pressure drop. The installer plans to use a 1-inch main line and 1/2-inch branches to each appliance. The furnace is located 10 feet from the manifold, the water heater 5 feet, and the range 15 feet. The total connected load is 200,000 BTU/h.
1. Using the longest length method, determine if the proposed pipe sizes are adequate. Assume the longest run from the meter to the farthest appliance is 45 feet (30+15).
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Using the longest length method, the total equivalent length is 45 feet. The total load is 200,000 BTU/h. For a 0.5-inch WC pressure drop and 7-inch WC supply, refer to the gas sizing table for Schedule 40 black iron pipe. For 45 feet, a 1-inch pipe can carry approximately 205,000 BTU/h (based on typical tables for natural gas with 0.5-inch drop). So the 1-inch main is adequate for the total load. However, the branches must be sized based on their individual loads and lengths. For the range: 60,000 BTU/h at 45 feet (30+15) requires a 1/2-inch pipe? Check table: 1/2-inch pipe at 45 feet can carry about 50,000 BTU/h, which is less than 60,000, so 1/2-inch is insufficient; a 3/4-inch pipe would be needed. For the furnace: 100,000 BTU/h at 40 feet (30+10) requires 3/4-inch pipe (3/4-inch at 40 feet carries about 120,000 BTU/h). For the water heater: 40,000 BTU/h at 35 feet (30+5) requires 1/2-inch pipe (1/2-inch at 35 feet carries about 60,000 BTU/h). Therefore, the proposed 1/2-inch branches are inadequate for the furnace and range; they need to be upsized.
2. What would you recommend to correct the piping if the branch sizes are inadequate?
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The branch to the furnace should be increased to 3/4-inch, and the branch to the range should also be increased to 3/4-inch. The branch to the water heater can remain 1/2-inch. Alternatively, the entire system could be repiped with a larger manifold and shorter branch lengths to reduce pressure drop. Also, ensure that the main line is sized based on the total load and longest length; if the main is 1-inch, it is adequate. However, if the total load increases or if the pressure drop is critical, a 1-1/4-inch main might be considered. Always verify with the local code gas sizing tables and account for fittings using the equivalent length method if required.