How Do They Do That? Tankless Water Heaters

Key Points

• Tankless water heaters save energy by eliminating the need for a water storage tank.
• Also called demand water heaters, they typically supply 2 to 5 gallons per minute.
• Tankless water heaters have been used for about 25 years in the United States.

Tankless/demand water heaters are common in Japan and Europe. They began appearing in the United States about 25 years ago. Unlike conventional storage tank water heaters, tankless water heaters heat water only as it is used, or on-demand, without the use of a storage tank. A tankless unit has a heating device that is activated by the flow of water when a hot water valve is opened. Once activated, the heater delivers a constant supply of hot water. The output capacity of the heating unit, however, limits the rate of the heated water flow.

Heating units may be electric resistance elements or natural gas burners. Typically, demand water heaters provide hot water at a rate of two to five gallons (7.6–15.2 liters) per minute. The following process describes how a tankless water heater works for both gas and electric versions.

1. A hot water tap is turned on.
2. Water enters the heater.
3. The water flow sensor detects the water flow.
4. The computer automatically ignites the burner or powers the resistive heating element.
5. Water circulates through the heat exchanger.
6. The heat exchanger heats the water to the designated temperature.
7. When the tap is turned off, the unit shuts down.

Gas Tankless Water Heater	Electric Tankless Water Heater
(Source: Takagi with permission)	(Source: ATEliving Trade and Industries Co.,Ltd with permission)

The natural gas burners are sized in the 20,000 Btuh to 200,000 Btuh range. The electric tankless resistance heating element size ranges from 18 kW to 27 kW. Electric units often have two to four smaller heating tubes connected serially (the output of one is the input to the next).

Source: http://www.questline.com/Article.aspx?userID=350085&articleID=9032&NL=5167