There’s at least one reason tankless water heaters have become an attractive alternative to conventional tank units: They deliver an endless supply of hot water while using a fraction of the energy. But getting the most out of these energy-efficient appliances starts at the gas line.

When builders and remodelers specify tankless water heaters for their projects, installers must factor in pressure, volume, and pipe size and length, among other considerations, to ensure the unit operates at peak performance, advises Shawn Marshall, Navien Inc.’s East Coast Training Academy trainer.

Here, Marshall offers five tips to properly size and install gas lines for your next tankless water heater project.

1. Determining drop

You’ll hear plumbers refer to the difference in pressure between two ends of a pipe carrying fluid as “drop.” When sizing pipe for the gas line, they refer to the pipe’s “drop chart,” which calculates pressure loss, measured in inches of water column. This is to make certain the gas line meets the appliance’s Btu ratings.

But the type of water heater should also be factored into the equation. Water heaters with a modulating gas valve ensure hot water supply meets demand. While these units are efficient, they have very little tolerance for pressure drop. Builders depend on their trade partners to ensure gas line sizing is done correctly. If you aren’t sure, you can always contact your local propane supplier and they can verify using the standards in NFPA 54.

2. Diameter matters

Your traditional tank water heater will likely use a 1/2-inch gas line. However, a tankless model requires more intense heat for shorter periods. That can mean upsizing to a 3/4-inch line to meet the appliance’s Btu demands.

The difference in gas line sizes is the difference between drinking a milkshake with a narrow straw or a wide straw, Marshall illustrates. The smaller line will be slower delivering the necessary Btus to heat up the water. That means the appliance will not operate properly, which can result in wasted energy and premature failure. So just like sizing wiring and other items on the jobsite, gas line sizing can’t be a one-size-fits-all approach.

In the event of a retrofit, you might be able to take advantage of newer piping materials, like CSST (corrugated stainless steel tubing) to replace an undersized line. Because of CSST’s flexible nature, it can be easier to run than rigid iron pipe. You might also be able to take advantage of an elevated pressure system that your local propane provider can help design.

This approach can be an effective workaround. Marshall recommends designing the piping runs to follow the drop chart.

3. Right pipes

A gas line could include a combination of pipe materials, including:

  • Copper
  • Black Iron
  • Underground Polyethylene
  • Corrugated Stainless Steel Tubing (CSST)

CSST size can vary by manufacturer, and each manufacturer provides line sizing charts for their product. “It can help you get around corners and bends, but you have to make sure it’s sized properly,” Marshall cautions. Also, follow manufactures requirements for fitting installation, and never mix brands of product.

Again, be mindful of the distance of run from the tank to the regulator(s) and regulator to the appliance. Each section of piping must be specifically designed to meet the system’s requirements.

Visit to learn more best practices for tankless water heater gas lines.