Some plumbers still are cold to the idea of tankless, or demand, water heaters, but the variety on the market seems to get wider every day. With help from the Energy Department and the manufacturers listed, here are specifics.
WHAT SIZE TO PICK
Tankless water heaters are rated by the maximum temperature rise possible at a given water-flow rate, as measured in gallons per minute. To select the appropriate size for you, determine the flow rate and the temperature rise you'll need, based on whether the heater will serve an entire house or just a sink in a second-floor bathroom.
Make a list: Start by making a list of the number of hot-water devices you expect to use, then add up their flow rates.
Calculating flow rates: Say you expect to run a hot-water faucet with a flow rate of 0.75 gallons per minute and a shower head with a flow rate of 2.5 gallons per minute simultaneously. The sum of those two numbers equals the flow rate needed from the demand water heater -- in this example, at least 3.25 gallons per minute.
Subtract the incoming temperature from the desired output temperature: Unless you know otherwise, assume the incoming water temperature is 50 degrees. In most cases, you'll want water heated to 120 degrees. Most demand heaters are rated for a variety of incoming (or inlet) temperatures. For example, a 70-degree water-temperature rise is possible at a flow rate of 5 gallons per minute through gas-fired demand water heaters and 2 gallons per minute through electric ones.
Faster flow rates or cooler inlet temperatures can reduce water temperature at the faucet farthest from the heater.
Some tankless heaters, however, are thermostatically controlled; they can vary their output temperature according to the water-flow rate and inlet temperature.
For homes that use 41 gallons or less of hot water daily, demand water heaters can be 24 percent to 34 percent more energy-efficient than conventional storage-tank water heaters. And they can be 8 percent to 14 percent more energy efficient for homes that use a lot of hot water, about 86 gallons per day. Even greater savings (27 percent to 50 percent) are possible if you install a demand heater at each hot water outlet.
According to government sources, electric tankless heaters generally cost $40 to $80 a year less to operate than comparable tank-type water heaters. Gas savings may be about $50 to $100 per year.
Equipment life may be longer because tankless heaters are less subject to corrosion -- 20 years compared to between 10 and 15 years for tank-type water heaters.
Buying a demand heater: They cost two to four times more than conventional heaters. Prices range from $200 for a small under-sink unit to $1,200 for a gas-fired unit delivering 5 gallons per minute. But the energy efficiency may offset a higher price.
Installation: Proper installation depends on factors including fuel type, climate, building-code requirements, and safety issues. So it's best to have a qualified plumbing and heating contractor install your demand water heater. If you can't find an installer, contact the manufacturer for names in your area.
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