Fuel Gas Piping, Combustion, and Venting
Know the properties of natural gas and LP: natural gas is lighter than air and rises; LP is heavier and pools. Understand Btu content (roughly 1,000 Btu/ft³ for natural gas, ~2,500 Btu/ft³ for LP vapor), specific gravity, and why orifice sizing differs between fuels. Be able to size gas piping using the longest-length method with appropriate tables, accounting for pressure drop, fittings, and appliance demand in cubic feet per hour.
Combustion: know stoichiometric air-fuel ratio and that excess air is required for complete combustion. Understand CO and CO₂ relationships: rising CO₂ with falling O₂ indicates approaching stoichiometric; CO spikes signal incomplete combustion. Know draft categories (I, II, III, IV) and that Category IV condensing appliances vent with positive pressure and require sealed, sloped venting.
Traps: sizing gas pipe by adding appliance Btu without converting to cfh; ignoring the effect of a dryer or range on the same branch; and assuming a natural-draft appliance can share a flue with a fan-assisted appliance. Also watch for questions confusing dilution air with combustion air.
Rule of thumb: Convert appliance input to cfh by dividing Btu/h by the fuel's heating value (e.g., 100,000 Btu/h ÷ 1,000 = 100 cfh natural gas). Worked example: a 100,000 Btu/h furnace plus a 40,000 Btu/h water heater on one branch = 140 cfh; size the pipe for 140 cfh at the longest run length including equivalent fittings.