Gas Piping and Gas-Fitting Systems
Sizing and components
Gas piping is sized by the appliance input in Btu/hr, the gas's specific gravity and heating value, the developed length of the longest run, and the allowable pressure drop. Use the manufacturer's input rating, not the output. Typical components: a shutoff valve ahead of each appliance, a union or flanged connection for serviceability, a sediment trap (drip leg) installed downstream of the shutoff and ahead of the appliance controls, a regulator where pressure must be reduced, and a manifold to distribute to burners.
Testing and venting
Low-pressure systems are commonly tested at a minimum of 3 psi (or 1.5 times working pressure, whichever is greater) for at least 10 minutes, with the gauge monitored; verify the adopted code. Appliance venting requires proper Category classification, draft, and combustion air. Regulator vents terminate outdoors, clear of openings.
Materials and bonding
Steel pipe, copper (where permitted), and corrugated stainless steel tubing (CSST) each have installation rules. CSST must be bonded per the manufacturer and code. Piping is generally prohibited in chimneys, elevator shafts, and concealed locations without protection.
Common traps
- Sediment trap installed upstream of the shutoff valve.
- Sizing from output Btu instead of input Btu.
- Ignoring the longest run and using the shortest run length.
- Terminating a regulator vent indoors.
- Failing to bond CSST.
- Using a shutoff valve that is not listed for gas.
Rule of thumb
Longest run + total Btu input = the two numbers that drive every gas sizing table.
Worked example: A 100,000 Btu/hr furnace plus a 40,000 Btu/hr water heater on a 60 ft run uses 140,000 Btu/hr at 60 ft developed length for table lookup.