Fuel Gas Piping and Venting
Fuel gas work is about three things: sizing the pipe, proving it tight, and getting the products of combustion outside.
Pressure: natural gas is delivered near 7 in. water column (about 0.25 psi) and manifolded near 3.5 in. wc; propane at about 11 in. wc. Because these are inches of water rather than psi, questions often hinge on conversion. One psi equals about 27.7 in. wc.
Sizing uses the longest-length method. Add the nameplate input of every appliance downstream in BTU/hr, divide by the heating value (about 1,000 BTU per cubic foot for natural gas, about 2,500 for propane) to get cubic feet per hour, then read the table using the longest run in feet. Pipe size is set by load and distance together.
Installation essentials: a shutoff valve and sediment trap (drip leg) ahead of each appliance, unions, protection where pipe passes through masonry, proper support and sleeving, and bonding of the gas piping. Appliance connectors are limited in length, typically 6 ft, and cannot be concealed or run through walls.
Venting: gravity vent connectors rise toward the chimney at 1/4 in per foot with a minimum rise off the draft hood, and listed vents terminate at least 2 ft above anything within 10 ft and at least 3 ft above the roof surface.
Worked example: a furnace at 100,000 BTU/hr, a water heater at 40,000, and a range at 35,000 total 175,000 BTU/hr. At 1,000 BTU/ft3 that is 175 cubic feet per hour; enter the table at the longest run and select the smallest pipe that carries it.