Water Heaters are the Second-Largest Energy User in Homes

 Water heaters account for more than 14 percent of national residential energy consumption, and while most building professionals and consumers are familiar with traditional tank-style water heaters, which continuously heat and reheat a limited supply of hot water, many have discovered that there is a superior way to heat water.

Tankless Water Heaters operate only when the need for hot water is detected and shut down when the demand for hot water ceases, using less energy—in some cases up to 40 percent less—than tank-style water heaters and produce an endless supply of hot water. Because tankless water heaters heat water as needed, there is no storage tank of hot water to deplete, which means a home can run several showers in a row or multiple hot water appliances at the same time with no fear of running out of hot water. 

Tankless Technology—Condensing and Non-Condensing

Non-Condensing tankless water heaters vent exhaust gasses directly from the heat exchanger and earn an Energy Factor (EF) of up to .82. EF is the ratio of useful energy output from the water heater to the total amount of energy input to the water heater. This equates to savings in gas usage—it can be compared to a vehicles “miles per gallon” measurement.

Condensing tankless water heaters, which earn an EF of up to .96, capture the latent heat of the exhaust before it escapes into the vent system. When heat is transferred from the exhaust gases to the water, the exhaust cools and “condenses” into water vapor and drains from the appliance. This is why these types of appliances are called “condensing.” 

How a Condensing Unit Works

When a need for hot water occurs the flow of the hot water is detected, which initiates the ignition sequence The burner will adjust the flame volume to match the demand of the hot water: small flame for small draw, larger flame for larger draw. 

In a condensing unit, there are two heat exchangers. The primary heat exchanger does most of the work of transferring heat from the burning gas to the water. It is positioned directly adjacent to the burner.
The secondary heat exchanger is positioned after the primary exchanger and captures heat from the exhaust. Incoming water is first pre-heated in the secondary heat exchanger. It then enters the primary heat exchanger where the water is strategically overheated slightly while the variable bypass cools to the set-point temperature to provide higher flow rates. 

Condensation from the secondary heat exchanger is directed through the condensation trap into a drain. 

When the demand for hot water ends, the gas-control valve closes and the unit goes into standby mode until the need for hot water resumes.

Have questions about condensing technology? Leave a comment in the box below.

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User Comments – Give us your opinion!
  • Tovar Escalante
    Condensing Heaters sound logical but I wonder why is the circle not FULLY closed. How much water is condensed, and why is it drained off.....should it not be directed to a tank and recycled ?
  • Ashley Murray
    The condensation is similar to what you find on a cold can of soda on a hot summer day. It comes from the moisture in the exhaust gases of the burner assembly. As they travel through the secondary heat exchanger, they begin to cool and reach the dew point. The gases then begin to phase change, going from vapor to water. They condense on the surface of the secondary heat exchanger, which has cold water flowing through it. The condensate then drips down to a collection pan inside the water heater, goes through a trap, and then exits the unit via the condensate drain. The condensate, since it came from exhaust gases and is a mixture of water and other chemicals, is slightly acidic, about the same as lemonade or orange juice. We recommend treating the condensate with a "condensate neutralizer," which is a simple container of basic (i.e., "base" vs. "acid") material like limestone that changes the pH to a more neutral state. Then the condensate can be safely drained into any drain. The amount of water is not that much and usually not worth the effort to recycle, though if you were highly committed to preserving every ounce of water, you could collect it and use it for watering plants (though in this case be sure it is neutralized!). - Trey Hoffman, global product manager at Rinnai
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Trey Hoffman
Trey Hoffman, global product manager at Rinnai America Corporation, helps deliver new products and components that enhance people’s quality of life. Hoffman, who coordinates global initiatives among Rinnai’s subsidiaries, has more than a decade of experience in product management and business strategy development and speaks fluent Japanese.
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