In many homes, specifying or installing a tankless water heater is an ideal way to improve homeowner satisfaction and reduce energy usage for water heating. Because they heat water on demand, they eliminate standby losses that occur with storage tank water heaters and provide an endless supply of hot water.

But while gas tankless units only need to run when hot water is needed, their gas usage must ramp up quickly when they’re meeting high demands for hot water. That means it’s vital for builders, remodelers, and installers to be aware of the proper procedures for installing the gas lines that supply the tankless unit. Joe Holliday, director of business and product development for tankless water heater manufacturer Rinnai, offers five tips to ensure your water heater runs at its best.

  • A half-inch or three-quarter-inch gas line? Holliday says most of the questions he hears about proper gas installation are on how to size the gas line. And while manufacturers may offer varying guidance, following the National Fuel Gas Code is the most straightforward way for builders and remodelers to ensure they’re following proper procedure.

Sizing is important to ensure your tankless water heater always runs at optimal efficiency. “If the installer isn’t doing all the applications correctly or paying attention to the whole system load, sometimes it can be undersized,” Holliday says. A builder could install a tankless unit in the summer and it would work fine. “But when winter comes, and the furnace is running, and they’ve got the stove on and everything else, maybe there’s not enough gas supply for the whole system. You end up starving the appliances for gas and they don’t run efficiently or don’t work quickly.”

To plan for correct sizing, think about these five factors:

1. Type of gas (propane or natural gas).
2. Inlet pressure.
3. Allowable pressure drop.
4. Type of gas appliances in the home.
5. The maximum gas load of the whole home.

For a handy reference guide for you and your technicians, get a copy of the Propane Technical Pocket Guide and bring it along to the jobsite.

  • You may just want to install a three-quarter-inch gas line. Calculations aside, Rinnai often recommends builders just install the larger three-quarter-inch lines. “That way you’re sure to have the gas load you need and it’s not an iffy subject or question,” Holliday says. “It’s a good practice to do especially for builders, because the difference in cost in running a half-inch versus a three-quarter-inch is quite minimal upfront.” Retrofitting later to a larger gas line, on the other hand, could cost several hundred dollars.

Under some building codes, such as California’s Title 24, builders are required to have their homes prepared for the higher gas load of a high-efficiency appliance, Holliday says, so they’ll likely need to install the larger gas lines even if they don’t immediately install the high-efficiency water heater.

  • Consider propane if the natural gas main is difficult or expensive to reach. Many builders don’t realize that propane can be available on any type of home site, making gas tankless water heaters a possibility even when natural gas isn’t available, Holliday says. “They can get their local propane company to not just bury underground tanks but, in big developments, they’ll actually put in the underground piping so that it actually works like a natural gas system. That’s a key thing for builders: propane can be readily available to get people on these high-efficiency products.”

Virtually all tankless units are field-convertible from natural gas to propane or vice versa, so builders can develop a new community with propane and then switch over to natural gas if it becomes available later.

  • Flex pipes can speed and simplify installation. You’re no longer stuck with fixed iron pipes for running gas lines. Flexible gas piping, such as corrugated stainless steel tubing (CSST), can make bends and joining a cinch.
  • Factor gas lines into your design. In many cases, there’s no need to run gas line all the way around your home to reach a tankless water heater. By installing the tankless unit on an exterior wall or even outside, builders can allow for shorter gas line runs and easier venting. “Because they’re small, you can put them just about anywhere,” Holliday says. “For people who put them outside, you can put them by the gas meter and it’s a really short run.”

Now that you’ve mastered the art of installing gas lines, become an expert on gas water heater venting options.

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Jeffrey Lee