Sizing a Gas Run for a Shop Heater and Correcting Garage Installation Defects
A 40-foot by 50-foot detached shop has a 100,000 BTU/h suspended furnace, a 40,000 BTU/h water heater, and a new 75,000 BTU/h unit heater. The gas meter is at the front wall and supplies 7 inches water column; the longest run from the meter to the new unit heater is 60 feet. The contractor proposes 3/4-inch black steel pipe for the entire run and a 1/2-inch flexible appliance connector to the heater, with a shutoff valve and a drip leg at the heater. The heater will hang with its burner 12 inches above the floor in the shop, where the owner parks a pickup and stores gasoline in a portable can. The gas line will be bonded with the equipment grounding conductor of the circuit serving the heater. The local utility requires a pressure test at 10 psi for 30 minutes. The pipe sizing table being used gives: 3/4-inch at 60 feet, 0.5-inch WC drop = 84 CFH; 1-inch = 158 CFH; 1-1/4-inch = 300 CFH.
1. Convert the connected load to CFH and determine the minimum pipe size for the 60-foot run using the table provided.
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Use the heating value of natural gas at about 1,000 BTU per cubic foot: 100,000 BTU/h = 100 CFH; 40,000 BTU/h = 40 CFH; 75,000 BTU/h = 75 CFH. Total connected load = 100 + 40 + 75 = 215 CFH. The run is 60 feet to the farthest appliance, and the table is based on a 0.5-inch WC pressure drop.
At 60 feet, 3/4-inch black steel is listed at 84 CFH, which is far below 215 CFH. One-inch is listed at 158 CFH, still below 215 CFH. One-and-one-fourth-inch is listed at 300 CFH, which exceeds the load. Therefore the proposed 3/4-inch pipe is undersized; the minimum size from this table is 1-1/4 inch for the common run. The branch to each appliance can be reduced only after the load and length for that branch are checked, and the meter and service must also be verified for capacity at 7 inches WC. Do not size the run by the new heater alone; the existing furnace and water heater are part of the connected load.
2. Evaluate the proposed appliance connector, shutoff valve, and sediment trap at the new heater.
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A shutoff valve and a sediment trap (drip leg) are required at the appliance, and the trap must be installed so dirt and pipe scale drop out before reaching the controls. The trap should be a tee with a capped nipple at least 3 inches long, installed downstream of the shutoff and as close to the appliance as practical. The connector must be listed for gas, sized for the appliance input and length, and protected from damage.
The 1/2-inch connector may be acceptable only if its manufacturer's rating covers 75,000 BTU/h at the installed length; connector capacity falls with length, so verify the tag or listing. More importantly, the connector may not pass through a wall, floor, or ceiling. If it passes through the closet wall as proposed, reroute the piping so the connector is entirely within the same room as the appliance, or use hard pipe to the appliance location. A union or flanged connection should be provided for service, and the connector must not be used to correct misalignment or support the appliance.
3. What corrections are needed for the heater location, gasoline storage, bonding, and pressure test?
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In a garage or shop where flammable vapors may be present, ignition sources must be elevated unless the appliance is specifically listed for garage installation. A burner 12 inches above the floor is a likely violation of the 18-inch elevation requirement. Raise the heater so the ignition source is at least 18 inches above the floor, or replace it with a listed garage heater, and protect it from vehicle impact with bollards or a barrier. The portable gasoline can is a separate fire-code hazard and should not be stored in the same space; advise the owner to remove it.
Black steel pipe does not require the special bonding used for corrugated stainless steel tubing, but the gas piping must be bonded as required and the equipment grounding conductor alone may not satisfy the bonding rule. If any CSST is used, bond it per the manufacturer's instructions, typically with a minimum 6 AWG copper conductor to the grounding electrode system. For the pressure test, isolate the meter and appliances, cap the outlets, use a calibrated gauge, and hold 10 psi for 30 minutes with no drop. Test the piping before appliances are connected, and do not use the appliance gas valve as a test cap.