Tankless water heaters need a dedicated gas line — most homes lack this

A standard tankless unit draws 150,000-199,000 BTU, roughly 3x what a 40-gallon tank uses. Most homes have 3/4" gas lines sized for a tank heater. You need a minimum 3/4" dedicated line from the meter or a 1" main line upgrade. Failure to upsize causes low flow codes (usually error 11 or 12 on Navien/Rinnai). Always run a gas load calculation per IFGC before installing.