Get Optimal Performance from a Tankless Water Heater

Tankless water heaters have caught the eye of many homeowners lately—as a way to reduce the 19 percent of total home energy use consumed by water heating. According to research sponsored by the National Renewable Energy Laboratory (NREL), heating water with a tankless system is 12 to 34 percent more energy efficient than using a conventional storage tank system. And when tankless systems are installed at each water outlet, gains in efficiency can range from 28 to 50 percent.

Others are impressed by the ability of tankless systems to deliver hot water "endlessly" in a busy household, as well as the space saved because a storage tank is no longer needed.

These factors, combined with rising energy bills, a federal tax credit for 30 percent of the total cost (up to $1,500, including installation), and greater availability, make tankless a technology well worth considering.

But although tankless systems have been used in Europe, Japan, and elsewhere for decades, they’re a new technology in North America, and homeowners describe a wide variety of experiences with them.

Some are proudly satisfied while others feel "shnookered" by delays in getting hot water to the tap and changes in temperature during hot water use.

To help you increase the odds that your tankless experience is positive, we’ve examined six commonly reported problems and gathered solutions.



Get the right size heater for your home and uses.

Tankless water heaters are rated by the maximum temperature rise possible at a given flow rate. To ensure that your heater will deliver enough hot water, determine the flow rate and temperature rise you’ll need. Find the appropriate flow rate by listing all the hot water outlets to be connected to the heater and adding up their flow rates in gallons per minute. Make sure the heater you purchase can deliver the most common temperature rise of 70 degrees Fahrenheit (39 degrees Celsius) at the desired flow rate.


Consider the minimum flow rate.

To avoid overheating, tankless water heaters require a minimum flow of water—ranging from a half gallon per minute to two gallons per minute—through the heating system. Failure to meet this requirement is the source of complaints such as "I do not like that when I wash dishes I have to keep the flow of water higher just to keep hot water going" from the Berkeley Parents Network. To avoid this problem, make sure the flow rates at outlets you’ll connect to the heater exceed its minimum requirements.


Hire an experienced plumber.

Tankless heater installation is complex, requiring proper configuration and sizing of the unit to fit the home’s plumbing, gas or electric supply, and ventilation requirements. Ask the plumbing supply house or hardware store where you’re buying the heater to connect you with three factory-trained, experienced plumbing contractors, and get bids from all three. Most manufacturers also will refer you to local plumbing contractors.

For more tips on the ins and outs of tankless installation, read homeowner Gordon Gray's 9 Best Practices for Choosing and Installing a Tankless Water Heater.


Choose a point-of-use model if you’re concerned about wasting water.

Many tankless users believe they "waste a lot of water while waiting for it to turn hot." Water waste is caused by two factors. First is the number of feet of plumbing—and thus cool or cold water—between the tap and the water heater. Because of this, whole-house tankless systems do not offer a water-saving advantage over conventional storage tank systems. However, tankless systems can be installed in a point-of-use configuration in which a smaller, dedicated heater is located very close to the water outlet.

The second cause of water waste is the amount of time required for the heater to generate hot water. For a conventional tank system with hot water in the tank, that time is zero. Tankless systems are at a disadvantage here, as explained in the next tip.


Be aware that instantaneous isn’t quite as fast as it sounds.

While tankless water heaters are often called "instantaneous water heaters," unheated water inevitably flows through the unit before sensors detect it and water heating begins.

The two best strategies for dealing with this much-discussed weakness are preheating and buffering. Both solar water heaters ($2,500–$4,500) and drainwater heat recovery technologies (about $500) increase the temperature of water entering the tankless heater, mitigating the impact of delayed heating. A 5- to 10-gallon storage tank plumbed right out of the tankless heater dilutes cold water with hot water in the tank and makes the delay less noticeable. This solution has a lower up-front cost but is less energy efficient.


Correctly ventilate the heater’s exhaust.

The tighter a building gets, the easier it is to create pressure differentials that can cause a combustion appliance to backdraft, bringing exhaust gases like carbon monoxide into the living space, according to Arnie Katz, senior building science consultant at Advanced Energy. Whether it’s a tankless or not, a gas-fired water heater should be sealed-combustion, direct-vent to prevent this situation.