How Do I Choose a Tankless Water Heater?

Water heating uses 20% or more of the energy in U.S. homes, and is the second largest energy expense in U.S. households, according to the U.S. Department of Energy, so it's definitely worth it to invest the time and effort in researching different water heating options. In fact, the DOE points out in its guidelines for selecting a new water heater that most people don't get to do their homework before buying one, because they are usually forced to make a quick decision when their current heater fails unexpectedly. Storage tank models are the most popular water heaters in the United States today. A typical "tank" heater stores anywhere from 20 to 80 gallons of hot water. The water is heated up slowly and stored for later use. As water sits in the tank waiting to beused it often loses heat and has to be reheated to maintain a pre-set temperature. "Standby losses" occur as energy is lost from warmed water sitting in a tank and not being used. As long as there is hot water in the tank you can use hot water appliances simultaneously. However, once the tank is emptied there is no hot water available until it refills and reheats.

Tankless water heaters, on the other hand, heat water as it is needed and provide water continuously. They first began appearing in the United States about 25 years ago. Tankless models, also known as "demand" or "instantaneous," are common in Japan and Europe. As their name implies, they do not have a holding tank. The heating process begins when you turn on a hot-water tap. Cold water travels through a pipe into the unit, and either a gas burner or an electric element heats the water up quickly. And according to Richard H. Karney, Energy Star program manager for the US Department of Energy, tankless water heaters "do save energy because they eliminate parasitic energy losses in the tank."

Although, tankless water heaters range in size, they take up less space than most tanks. Larger gas models can be used to replace conventional water heaters as the main source of hot water for an entire household or smaller systems can be used to boost heat for dishwashers, washing machines, or even remote bathrooms.

Before you go out and buy a tankless water heater, though, it is important to understand the technology's limitations. While manufacturers say that tankless heaters don't run out of hot water, they can only supply a set volume of hot water at once so it's hard to run a dishwasher, washing machine, and your kitchen faucet at the same time. Each model has a "flow rate," which is measured in gallons per minute (gpm). You can get plenty of continuous hot water at one time as long as your water use is within the limits of the system's flow rate. And Karney points out that in other countries where tankless heaters are more popular the houses are much smaller and distribution is much smaller.

Tankless heaters are generally more expensive than their conventional counterparts, but are said to make up for the higher sticker price with increased energy savings (though how long it takes to make up those costs is subject to debate). You might not get as much energy savings with gas units powered by pilot lights so if operating costs are important you should consider an electronic ignition, but check to make sure it is compatible with your appliances.

Products

In 2001, tankless water heaters accounted for less than one percent of all water heaters sold. Because the technology is relatively new in this country, it is hard to get objective information about tankless water heaters and the Department of Energy's Energy Star program hasn't yet rated any water heaters, either with or without tanks. Since ground water temperatures vary from location to location and by season, when shopping it is essential that you know how many degrees you'll need to raise the water's temperature and that you ask vendors to calculate a flow rate based on that number. Each manufacturer bases its flow rates on different temperature rises, so make sure you get the correct number for your needs. The less heat needed to get to the desired temperature (usually 120 degrees Fahrenheit), the more water available at any given time. Builders Websource compares major brands and Controlled Energy , a Bosch distribu tor, has a competitive comparison chart on its website; it includes flow rate calculations for the major brands based on the same temperature rise.

When selecting a tankless water heater:
Because the cost of gas is generally lower than electricity, gas tankless systems are usually cheaper; they also tend to have higher flow rates than electric systems.

Compare and contrast flow rates of the different models you are considering. To calculate your peak demand make a list of how many hot water devices you expect to have open at one time and add up their flow rates. Figure at least:
• 2 gallons per shower
• 1 gallon per faucet
• 1 to 2 gallons per dishwasher and clothes washer

You'll also have to find out your incoming water temperature (ground water is warmer in the summer), and the desired outgoing temperature. Call a local plumber to determine your ground temperature or rely on the manufacturer. Once you figure out your peak demand, incoming water temperature, and desired outgoing temperature, choose a model that meets your peak flow rate and temperature rise needs. Remember manufacturers calculate flow rates based on different temperature rise requirements, so make sure you get the proper GPM for your temperature rise needs.

Consider the size of gas line and vents needed to run your tankless heater choices and the costs associated with conversion.

Find out what the servicing needs are, who is available to service it, and if replacement parts are readily available.

Tankless (Instantaneous) Water Heaters