Gas Line Sizing for a Tankless Water Heater

A tank heater stores hot water and reheats it slowly with a small burner. A tankless heater stores nothing, so it has to heat water from tap temperature to shower temperature in the few seconds it takes to pass through the heat exchanger. That takes a very large burner, and the burner only fires at full input when the demand is high.

Natural gas carries roughly 1,000 BTU per cubic foot, so a 199,000 BTU unit at full fire needs close to 200 cubic feet of gas an hour. The tank it replaced needed about 40. The number to size from is the maximum input on the unit's rating plate, not the average.

Gas lines are sized for everything that can run at once. Make a list from the labels:

• The tankless unit, at its maximum input
• The furnace, often somewhere in the range of 40,000 to 100,000 BTU
• A range and oven
• A dryer
• Gas logs, a grill, a pool heater or a generator, if you have them

The fuel gas code sizing tables give the gas a pipe can carry by its size, its material and the length of the run, with a small allowed pressure drop. The longer the run, the less a given pipe carries. A half-inch line that fed a 40,000 BTU tank across a basement is nowhere near big enough for a tankless unit at the same spot, and the trunk line it tees into may be undersized for the new total as well.

The manufacturer adds a requirement of its own: a minimum gas inlet pressure, in inches of water column, measured at the heater while it runs at full fire alongside the other appliances. A line can pass a static pressure check with everything off and still fall short once the burner opens up. A proper install measures it under load.

CSST and black iron have different sizing tables, and CSST in particular is sized by its own maker's charts. Mixing them on one system is common and fine, as long as each piece is sized for what it carries.

Your meter and service line have a rated capacity, and many homes have a residential meter rated around 250 cubic feet an hour. A furnace, a range, a dryer and a 199,000 BTU tankless heater can add up past that. When the total is over the meter, it is the utility's call, whether that is Piedmont Natural Gas or Greer CPW. They may set a larger meter or change how gas is delivered to the house.

Utility work runs on the utility's schedule, so the load question should be settled before the unit is bought and the old tank is pulled.

A tankless heater that cannot get enough gas tells you in a few ways:

• Ignition or flame-failure error codes, especially when the furnace is running
• Water that goes hot, then lukewarm, then hot again during a shower
• Plenty of hot water at one tap, but not at two at once
• Furnace or range flames that shrink or flutter when the tankless fires
• A unit that runs fine in summer and falls short on cold January mornings, when it needs every BTU

The usual answer is a new, properly sized line from the meter, or from a point near the meter where the pipe is big enough, to the heater. On some houses the trunk line toward the meter is upsized so the whole system can carry the new total. Where the meter is the limit, the utility handles the meter. In some cases, a lower-input unit sized for how many showers you actually run at once is the better fit.

Whatever the fix, it is new gas piping, so it is permitted, pressure-tested and inspected, and the heater's inlet pressure is checked at full fire before the job is done.