If you're like many of us, the words "tankless water heater" might sound like a contradiction in terms. But in much of the world, tankless (also called "instantaneous," "demand," or "on-demand") water heaters are the norm—they are compact, never run out of hot water, and can be more energy efficient than familiar tank-style heaters. Certain tankless models can be a good choice as a backup heater for solar hot water systems as well. If your old water heater is failing, or if you're building a new home, this increasingly popular and efficient technology might be the right choice for you.
According to the U.S. Department of Energy (DOE), water heating is an average household's third largest energy user, accounting for about 13 percent of its energy consumption. So no matter how you heat your water, efficiency is critical if you're planning on saving energy and whittling down your utility bills. A tankless water heater can help you achieve some energy savings, depending on your circumstances.
If you've ever been last in line for your morning ablutions— and the recipient of a chilly shower—the appeal of a tankless heater's endless hot water supply is seductive. (Of course, where efficiency's concerned, endless hot water isn't always a good thing. Running out of hot water is an effective deterrent to those among us who enjoy indulging in long showers.)
Besides providing lots of hot water, tankless heater systems can also be long-lived. Properly maintained heaters may last for as long as 20 years, and failed heater components can be replaced.
Tankless water heaters heat water directly and at the time of use, instead of maintaining a large amount of water at a prescribed temperature, as a tank-style water heater does. Turning on a faucet cues a tankless heater to activate—cool water enters the heater, circulates through a heat exchanger, and is sent through the hot water plumbing to your fixtures. After the initial startup, the system continues to heat water as long as the tap stays open. When you turn off the faucet, the water heater shuts down.
Because it has no tank, an instantaneous water heater eliminates "standby losses"—heat loss through the walls of a tank-style heater and, in gas-fired tanks, through both
them good partners for a solar hot water system, since with a solar storage tank, finding space to locate an additional gas or electric-fired backup tank can be a challenge in some installations.
Gas or Electric?
Tankless water heaters are available as gas (either natural gas or propane) or electric models. Larger gas tankless models can provide more hot water than electric tankless heaters because electric models are limited by the size of a home's electrical service (usually 200 amps).
Gas models with constantly burning pilot lights may undermine efficiencies expected from tankless heaters. You can avoid this energy loss by choosing a model with an intermittent ignition device. Some models use Piezo igniters, similar to those used in gas ranges, which spark the flame only when needed. Bosch offers a unit that uses a tiny hydro-electric turbine-powered igniter in place of a standing pilot light, and other models come with electronic ignitions. Finally, gas tankless heaters are technically somewhat less efficient than their electric counterparts because some of the heat generated is lost through the exhaust venting, but fuel costs (gas vs. electric) will determine which approach will shave the most off your utility bills.
Electric tankless heaters use heating elements to boost water temperature, require no venting, and can be located almost anywhere indoors. Gas tankless heaters are available as atmospheric-vented (natural draft) and power-vented.
the tank walls and the flue. The DOE estimates that standby losses for tank-style water heaters represent between 10 and 20 percent of a household's total water heating costs.
Perhaps the biggest boon for installing a tankless water heater is its small size. Most models are about the size of a suitcase, and they work well in tight spaces. This makes
If you plan to use a tankless water heater in conjunction with a solar hot water system, be sure to buy one that can sense the incoming water temperature from the solar storage tank. You won't want to heat your water with the sun, only to have your tankless heater fire up and overheat the same water! Thermostatically controlled units register incoming water temperature and apply heat as needed to reach the desired output water temperature. If the solar thermal system is producing hot water at the specified temperature, no additional boost from the tankless heater is required. Eemax and Bosch are among several manufacturers that offer thermostatically controlled tankless water heaters.
This Bosch AquaStar tankless heater is suitable for solar backup.
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