Do tankless water heaters help save energy?

Yes

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by Daniel J. Gansle

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Tankless water heaters are becoming increasingly popular due to their supposed ability to conserve energy and to provide instantaneous hot water. But just what is a tankless water heater, and how can it heat water if there's no tank? And most importantly, do they help save energy? Let's take a closer look at the ins and outs of these new water heaters.

Tankless vs. Traditional Water Heaters
Traditional water heaters collect water in a large tank and heat it. These must keep the water at a certain temperature, all day, every day; thus the heater's elements are working quite often. When hot water is used in the home, for example after a shower, the hot water heater must refill with water and heat it up again. So you can see just how much energy is consumed just by heating and maintaining hot water on a daily basis.

Maintenance is also an issue with tank water heaters. Experts recommend frequent draining of the unit in order to expel sediments that collect in the bottom of the unit. Doing so ostensibly increases the energy efficiency of the water heater, though in my own experience I'm not so sure it made much of a difference. In areas with higher mineral content water, internal corrosion of the water heater can also become a problem that reduces energy efficiency.

Tankless water heaters work by heating water as it passes through the unit, which is typically mounted on the side of the home. The energy savings are substantial as the unit turns on only when a faucet opens looking for hot water. There's no water to heat, and no continuous temperature to maintain. This Old House plumbing and heating contractor Richard Trethewey explains: "Because you are heating water as you need it, it is "tankless," also called instantaneous. They work beautifully and are made of stainless steel, so they last. The up side is that you could shower for 24 hours straight."(1)

Natural Gas vs. Electric
You can purchase either an electric or natural gas tankless water heater. But according to Richard Trethewey, "I would only recommend gas at this point. The electric would need at least 14 to 15 KW (a lot of power), and the electric elements can scale in hard water."

Energy savings for natural gas are up to 30 percent, while electric units might see a savings of up to 70 percent over traditional water heaters. One brand of tankless water heater even comes with a remote control digital thermostat which allows you to instantly adjust the hot water temperature from anywhere in the home.

Hot Water Flow Rates
In the past, flow rates for tankless hot water heaters were minimal and allowed only one hot water use in the home at a time. But these days, you can purchase a higher gallon per minute (GPM) tankless heater that instantaneously heats up to 7.4 gallons per minute; adequate for two to three hot water applications in the home at a time (e.g., two showers and the kitchen sink). If you have a smaller space such as a 2-bedroom home or a condo, perhaps a 4.2 GPM unit would do. But if you're living in a larger home, go with the higher GPM unit for sure.

Tankless hot water heaters are more expensive than traditional ones, but the energy savings over time will most certainly pay for itself.

Source Notes:
1. Richard Trethewey, "Tankless Hot Water Heaters," This Old House, 2007 Time, Inc.
http://www.thiso ldhouse.com/toh/askt oh/question/0,213064 ,00.html

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