Tankless water heater

What It Means

A tankless water heater heats water on demand instead of keeping a tank of it hot. When you open a hot tap, cold water runs through a heat exchanger warmed by a gas burner or an electric element, and hot water comes out the other side. With no tank sitting at temperature, it avoids the standby heat loss of a storage heater, but it can only heat so many gallons a minute, which is why sizing matters.

How It Sounds at Home

• "We went tankless when the old tank gave out."
• "The installer asked how many showers we run at once."
• "There's no tank in the closet anymore."

The Breakdown

How to size one, following the makers' guides

1. List what might run hot water at the same moment, such as two showers and a kitchen faucet.
2. Add up their flow rates. The makers use about 2.5 gallons a minute for a shower and 0.75 for a faucet, so two showers and a faucet come to 5.75.
3. Find your temperature rise: the hot water setting you want, often 110 to 120°F in a home, minus the temperature of the water coming in. Incoming water runs from the mid 30s to the upper 70s°F depending on where you live, and it's coldest in winter.
4. Choose a unit rated to deliver your peak flow at your winter temperature rise. A bigger rise takes more heating power.
5. Have a licensed installer confirm the gas or electrical supplyMilk supplyHow much breast milk a parent's body makes. It's driven by supply and demand: the more often milk is removed, the more the body makes. See more the unit needs.

Try it today. Count the showers and taps your household runs at once on a busy morning, and add up the flow. That one number narrows the choice more than anything else.

Where the efficiency comes from. The Department of Energy says a tankless heater can be 24% to 34% more energy efficient than a storage tank in a home using 41 gallons or less of hot water a dayWater intakeThe total water you take in each day from plain water, other drinks and food; the National Academies' adequate intake is 3.7 liters for men and 2.7 for women. See more, and 8% to 14% more efficient in a home using about 86 gallons a day. DOE notes a gas model with a standing pilot light can waste energy that eats into that gain.

Good to Know

Tankless units need care too. Rheem and the other makers recommend flushing or descalingDescalingDissolving the mineral scale that hard water leaves inside kettles, coffee makers and other appliances that heat water. See more about once a year, more often with hard water, because minerals build up inside the heat exchanger. The efficiency figures above are DOE's ranges for what a demand heater "can be," not a promise for your home. Installation means gas or electrical work, so Buhata suggests a licensed professional, and the unit's manual comes first.

Related Terms and Categories

• Water heater flush

  Draining a water heater and running water through it to wash out the sediment and minerals that settle inside, usually about once a year.

• Anode rod

  A metal rod inside a tank water heater that corrodes on purpose, so the steel tank doesn't. Makers suggest checking it yearly or every few years.

• Water softener

  A home system that removes the calcium and magnesium that make water hard, usually by trading them for sodium in a tank of resin beads.

• Low water pressure

  Water that comes out of taps and showers weaker than it should, from a clog at one fixture or a valve, regulator, leak or supply problem affecting the whole house.

• Appliance lifespan

  How many years a home appliance usually lasts before it needs replacing, from about 6 years for a compactor to 15 for a gas range in Consumer Reports' 2009 figures.

• Licensed contractor

  A contractor who holds the license your state or local government requires for the kind of work being done, which you can check before you hire.

Home maintenance and repairs