Tankless water heater

A conventional, storage-type water heater has an insulated tank and a relatively small gas burner or electric heating element (often two elements) to heat the water. Water is heated slowly and remains "thermally stratified" so that water drawn off from the top remains hot even after 90% of the hot water is used up. There is the risk of running out of hot water, however, because the rate of heating may not keep up with the hot water draw from typical uses, such as showers.



Tankless water heaters-often called demand or instantaneous water heaters-heat water as it is used. The main advantage is that hot water doesn't sit in a tank all the time, losing energy through the tank walls, which occurs even with insulation.



As with storage water heaters, tankless models can be either gas-fired or electric. For very small loads, such as a remote lavatory that has only a sink with a low-flow aerator, an electric tankless water heater can make a lot of sense since it obviates the need for running a gas line. But for more extensive hot-water needs, such as an entire home, a gas-fired tankless water heater is almost always a better choice than electric because an electric model able to supply that much hot water requires very high current draw-often 40-60 amps.



With a whole-house, gas-fired, tankless water heater, the burner is very large-typically 150,000-200,000 Btu/hour (44-60 kW), compared with 40,000 Btu/hour (12 kW) for a typical gas-fired storage water heater. This means that larger-diameter gas lines are required (usually 3⁄4" instead of 5⁄8"), and a lot of combustion air is required, necessitating large ducts and potentially resulting in significant air leakage if not properly installed.



Gas-fired, tankless water heaters used to all have pilot lights, which burned gas all the time-as much as 5,000 Btu/hour (1,500 W), nearly equivalent to the heat lost through the insulated walls of a storage water heater. Today, most gas-fired tankless water heaters have electronic (pilotless) ignition. These models offer the highest efficiency of any water heater, except electric heat-pump and solar water heaters.



A few gas-fired tankless water heaters made by the Korean companies Takagi and Navien use condensing technology with an Energy Factor (a measure of efficiency) of up to .98. Non-condensing, electronic-ignition models have Energy Factors of .82 to .87, while conventional gas-fired storage water heaters have energy factors of .58 to about .67 (up to .80 for condensing models).



While providing significantly higher efficiency, electronic-ignition tankless water heaters are a lot more expensive than storage water heaters, and they are somewhat more prone to failure (especially with hard water). They also don't work well in every application (with the most water-conserving lavatory faucets the burner may not turn on), and by providing unlimited hot water, tankless water heaters don't encourage shorter showers as do storage water heaters