How to buy a tankless water heater

Here's what you need to know before purchasing one of these energy savers.

Alina Bradford CNET Contributor
Alina Bradford has been writing how-tos, tech articles and more for almost two decades. She currently writes for CNET's Smart Home Section, MTVNews' tech section and for Live Science's reference section. Follow her on Twitter.
Alina Bradford
3 min read
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In these energy-conscious times, cutting back on using the AC or saving water are common ways to save the environment and your bank account, too. But there are many other ways to conserve energy in the home. Case in point: The tankless water heater.

Tankless water heaters (or instantaneous water heaters) heat water as it is needed, instead of continuously heating a tank full of water as traditional water heaters do. The fact that they aren't constantly running saves a lot of energy. According to the US Department of Energy, tankless water heaters can be 8 to 34 percent more energy efficient than water heaters with a storage tank.

Sounds pretty great, right? Here are some things to consider when buying a tankless water heater for your home.

Continuous hot water, lower water pressure

When you turn on the hot water tap, electric or gas heating elements heat the water as it flows your way. This gives you an unlimited supply of hot water for your shower, but at a cost.

The water pressure will be much lower than when you use a tank water heater because the heating elements take a certain amount of time to heat a certain amount of water. They produce a flow rate around 2 to 5 gallons (7.6 to 18.9 liters) of hot water per minute, though some can get up to 11 gallons (42 liters) per minute.

If you're worried about your morning shower, don't be. Most shower heads top out at 2.5 gallons per minute. Your washing machine just may fill up more slowly.

Watch this: Saving Energy, Saving the Planet

Considerations for big households

While your tankless water heater can provide endless hot water for one tap, it can't also supply hot water to the washer, sinks and other showers all at once. If you typically have multiple taps using hot water at the same time, you'll want to install two or more tankless water heaters to keep up with the demand.

In fact, the US Department of Energy says that homes that install a water heater at each hot water outlet can save more energy if your family uses more than around 86 gallons of hot water per day.


Tankless water heaters take up far less room and provide water on demand.


Gas vs. electric

You can purchase either a gas or electrically heated tankless unit. There are some benefits to each. For example, tankless water heaters usually need larger gas lines than conventional water heaters. This will typically make installation more expensive than electric. 

Gas tankless water heaters can heat water faster, though, allowing them to supply higher water pressure. On the other hand, the pilot light is always on, even when you don't need hot water. This can make them use more energy than electric units.

If you really want a gas unit, but want as little waste as possible, look for those with an intermittent ignition device (IID). This way you can easily turn the pilot light on and off when you need hot water.


To figure out what size of tankless water heater you need, you'll need to do a little research. First, add up the flow rate. This is the gallons per minute of hot water you will use at any given time. This includes any  washers , dishwashers , showers, tubs and sinks that you run at the same time. A Google search of your appliances and shower heads will give you a good idea of the flow rate. 

When you add them all up, the resulting number will be the flow rate your new tankless water heater will need to produce. Write that number down before you shop. If you can't find a tankless water heater that matches your needed flow rate, then you'll know for sure that your home needs multiple units.

Another number you'll need is the is the temperature rise. This the amount the temperature of the tap water rises before it comes out of the faucet due to the tankless heater. 

I'm not going to make you go take the temperature of your cold tap water to figure this one out. Most homes will need a unit that can produce a temperature rise of 70 degrees Fahrenheit (21.1 degrees Celsius). This temperature rise is usually possible at a flow rate of 5 gallons per minute, if the unit is gas, and 2 gallons per minute with electric units.

Have a conventional water heater? Here's how to light a gas water heater.

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