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The terminology is a bit dated now, but many construction pros can recall a time when tankless water heaters went by a different moniker: “instantaneous” water heaters.

The problem with this term quickly became apparent, however, when unhappy customers began calling manufacturers and installers complaining that after they installed an “instant” unit, they had to wait the same amount of time for warm water to come out of the faucet.

The reason for the confusion, of course, is that while propane tankless water heaters generate hot water instantly, the hot water must still be delivered through dozens of feet of piping, a process that can take a minute or two — with cold water spilling out of the faucet the whole time.

Recirculation technology makes the dream of instantaneous hot water closer to reality. Recirculation systems quickly pump hot water from the water heater to the fixture, cutting the wait time from more than a minute to just a few seconds, without dumping that perfectly good cold water. And while the use of recirculation systems with tankless water heaters has previously been a niche application, a new integrated recirculation technology from Rinnai has the potential to bring tankless recirculation systems into the mainstream.

Retrofit ready

When a dedicated return line is not available, Rinnai’s Ultra Series RUR98i and RUR98e condensing tankless water heaters use a thermal bypass valve at the furthest fixture to recirculate water through the existing cold water line.

In the past, says Joe Holliday, director of business and product development for Rinnai, most tankless recirculation systems required a dedicated return line. A pump would push warm water to the furthest fixture, and the return line would carry the cold water sitting in the pipes back to the water heater. That’s a fairly simple solution for new construction, when it’s relatively easy to install a third line of plumbing. But only 1 percent of existing homes have dedicated return lines, so recirculation with a tankless water heater was not an option for retrofits.

Rinnai’s new tankless water heater provides a unique solution to that challenge. The Ultra Series RUR98i and RUR98e models have a built-in recirculation system with bypass technology that works with or without a dedicated return line. “So this covers the other 99 percent of homes,” Holliday says.

The tankless water heater’s built-in recirculation system can also utilize a dedicated return line.

Here’s how it works. The tankless unit includes a pump, timer, and controller on board, eliminating the need for the installer to purchase those systems separately. The unit also comes with a thermal bypass valve. When a dedicated return line is unavailable, the bypass valve can be installed at the furthest fixture, where it acts as a bridge between the hot and cold water lines.

At preset intervals according to the user’s schedule, the pump pushes warm water to the faucet. Cold water already in the pipes flows through the installed valve and recirculates back to the tankless water heater through the cold water line. Once the water at the bridge reaches 98 degrees, flow through the crossover valve will reduce to a trickle so that it won’t unduly heat the cold water line.

“It’s a very easy installation, and now all of a sudden you have recirc and you can conserve water.”

“In my house, my water heater is in the basement, my master bath is on the second floor, and it used to take about 95 seconds to get hot water,” says Holliday, who retrofitted the new recirculation system into his home. “Now it’s about six seconds.”

Having the recirculation technology built into the unit also creates significant savings for the installer, Holliday notes. “Installing a bypass valve at the furthest fixture is a 5- to 10-minute job, and your water heater is installed just like you regularly would. It’s a very easy installation, and now all of a sudden you have recirc and you can conserve water.”

Those water savings are particularly vital in drought-stricken regions such as California. In fact, some jurisdictions offer rebates for homeowners that install hot water recirculation systems. One Lawrence Berkeley National Laboratory study estimates about 20 percent of a home’s hot water is wasted. Reducing the loss of perfectly good water down the drain can go a long way toward cutting down on that waste.

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Jeffrey Lee