Tankless water heaters have been around for more than 90 years, but they did not become viable alternatives to a standard tank unit until the 1970s. The efficiency of tankless systems continued to increase, so Europe began adopting the technology to save energy—as well as space. Once the EPA established Energy Star in 1992, tankless water heaters started to make a larger impact in the U.S. market.
Since then, instantaneous (or on-demand) water-heating technology has incorporated a secondary heat exchanger to generate more efficient heat transfers. Some tankless models also have integrated a built-in recirculation pump to return cooled water that would normally run down the drain, saving households thousands of gallons of water per year and reducing the wait for hot water at a faucet.
Many manufacturers now offer WiFi compatibility for their tankless products as well, which enables homeowners to monitor and tweak usage from a mobile device. The wireless adaptor, when used in conjunction with an app, allows customers to create and manage their own schedules and often connects to a smart home network—such as Nest or Wink—to provide even greater convenience.
As soon as a hot water tap opens, the water flow sensor inside a tankless unit alerts the computer board, which turns on a fan to clear the vent and ensure fresh air for combustion. A flame ignites and warms the burner. Cold water flows into the unit from an inlet pipe, passing through the heat exchanger and absorbing warmth from the burner before exiting at the desired temperature.
The computer board automatically adjusts the flame output if another tap opens and the hot water flow rate changes, so that the temperature remains steady. As the hot water taps close, cold water stops flowing into the heater and the flame extinguishes. The unit will stay off until the next time a need for hot water occurs, such as turning on a shower, washing machine, dishwasher or faucet.
Combustion produces the energy required to heat cold water, but the process also creates exhaust gases that a non-condensing tankless water heater will vent immediately. The unit loses about 20 percent of the heat generated from combustion as a result, and the material used for venting must be able to withstand the release of extremely hot gases, which can reach temperatures up to 300 F
A condensing tankless water heater, however, employs a secondary heat exchanger to extract the warmth from these gases and preheat the water. This extra step can induce an efficiency rating of more than 90 percent, and the cooler exhaust gases (around 100 F) permit a much less expensive material than the special Category III stainless steel necessary for venting a noncondensing unit.
As the gases cool inside a condensing unit, though, they form condensation that can corrode heat exchangers and other critical parts. These elements must be constructed with materials capable of resisting corrosion, such as a stainless steel alloy. The condensation that has collected within the unit needs to be neutralized through special filtration or dilution before it can be drained outside.
“Tankless water heaters have come a long way since first hitting the market,” says Andrew Tran, marketing manager for Noritz (noritz.com). “Improved heat exchangers have allowed for a more efficient heat transfer. A major improvement in our [tankless] heaters’ design [has] allowed for the industry’s strongest stainless steel to be used in the heat exchanger, which improves the heaters’ longevity.
Even though tankless water heaters provide reliable performance and savings, manufacturers continually look for ways to heat the same quantity of water with a smaller amount of energy. For example, an integrated pump can recirculate cooled water languishing in a hot water pipe back to the unit through a dedicated return line, so that users in a house receive hot water quicker.The most recent product offering from Stiebel Eltron, who invented the first electrical tankless water heater in 1929, can be installed on either the inlet or outlet side of any hot water tank, gas or electric. It supplies greater amounts of water than a tank can yield by itself and reduces recovery time, says Erika Knerr, marketing coordinator for Stiebel Eltron (stiebel-eltron-usa.com). “MegaBoost is especially useful for installations where a smaller capacity tank must be used to replace a larger tank,” she adds.
AUTHORS Kyle Clapham