The short answer
A gas line is sized for the total load it carries at full fire. At about 1,036 BTU per cubic foot of natural gas (EIA), a 199,000 BTU tankless unit needs about 192 cubic feet an hour; a 150,000 BTU unit about 145. That load is added to the furnace, range and dryer on the same line, and the pipe size depends on its length. A licensed gas fitter sizes it from the gas code tables and the heater's manual.
Gas use by input
| Input | Natural gas (cu ft/h) | Propane (gal/h) |
|---|---|---|
| 80,000 BTU/h | 77 | 0.9 |
| 120,000 BTU/h | 116 | 1.3 |
| 150,000 BTU/h | 145 | 1.6 |
| 180,000 BTU/h | 174 | 2.0 |
| 199,000 BTU/h | 192 | 2.2 |
Why the line matters
A line that can't deliver the full load starves the burner at high fire, so the unit can't reach its rated flow at your rise. The gas fitter checks pipe size, length and the total connected load.
Plan it
The requirement calculator turns any input into the gas a line must deliver; give that figure, with your other appliances' inputs, to the gas fitter.
Products mentioned
Questions readers ask
What size gas line does a 199,000 BTU tankless need?
It depends on the run length and the other appliances on the line; the heater's manual and the gas code tables set it. The load to plan for is about 192 cubic feet of natural gas an hour.