Fuel Gas Piping and Combustion Air
Gas Piping Fundamentals
Fuel gas piping must be sized using the code's tables based on appliance input in Btu/h, the specific gravity of the gas, the allowable pressure drop, and the equivalent length of the longest run. Know how to convert appliance input to cubic feet per hour: divide Btu/h by the heating value of the gas (about 1,000 Btu per cubic foot for natural gas). Pipe size increases with load and length.
Installation and Testing
Gas piping must be supported, protected from physical damage, and installed with approved fittings. Sediment traps (drip legs) are required at appliances, and shutoff valves must be accessible. Testing is typically done with air or inert gas at a specified pressure for a specified duration, and the test must hold without loss. Know that gas piping may not run through ducts, chimneys, or elevator shafts, and that concealed piping must be protected.
Combustion Air
Appliances need combustion, dilution, and ventilation air. Know the methods: interior air from an unconfined space, outdoor air through openings sized per Btu/h, and mechanical ventilation. Common exam traps: assuming a confined space is fine because the door has a grille, ignoring the combined input of all appliances in the space, and forgetting that a clothes dryer or exhaust fan can depressurize a space. Also watch for questions on venting categories and backdrafting.
Worked Example
A furnace rated 100,000 Btu/h plus a water heater rated 40,000 Btu/h totals 140,000 Btu/h. At 1,000 Btu per cubic foot, demand is 140 cubic feet per hour. Compare that demand to the sizing table for the longest run length and allowable pressure drop to select the gas pipe size.