How Does a Fast Recovery Electric Water Heater Work?

How Does a Fast Recovery Electric Water Heater Work?

These tankless water heaters move hot water quickly and efficiently

By Brad Painting

Cutaway diagram of an electric water heater
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Fast-recovery electric water heaters, more commonly called demand or tankless water heaters, connect directly to the plumbing to heat water as it passes. Unlike conventional water heaters, demand heaters have no storage tank. This allows for an uninterrupted supply of hot water and improved energy efficiency.


A conventional water heater either burns a fuel or uses electricity to heat a 40 to 50 gallon tank of water. The heater maintains the tank's temperature as well as possible, but when it drops it can take over an hour to recover. A tankless water heater heats water almost instantly on demand, so there is no chance of running out of hot water.


Tankless water heaters come in whole-house models and point-of-use models. A whole-house heater uses gas, propane or electricity to heat the main water supply whenever an occupant uses hot water at a shower, faucet or appliance. A point-of-use model is almost always electric and is usually used for a single shower or sink. Point-of-use models will have a shorter lag time between turning on the faucet and receiving hot water than whole-house models, which can take several minutes for large houses.


The key parts of a demand water heater include a cold water inlet, hot water outlet, heat exchanger, and flow sensor. When an occupant opens a hot water valve at a sink or shower, water begins to flow through the heater, entering through the cold water inlet and exiting through the hot water outlet. The flow sensor signals the heater to turn on its heat exchanger, which is a set of hot coils that heats the water as it passes.


Tankless electric water heaters vary in their capacity to meet hot water needs based on their sizes. Size refers to how much heat the unit produces and how quickly it can transfer it to a stream of water, given by a gallons-per-minute (GPM) rating. A whole-house tankless water heater with too low of a GPM rating will divide its heat among multiple pathways during high-demand times, providing only lukewarm water.

Energy Efficiency

Because hot water does not idly sit in a tank with demand water heaters, it has little time to lose heat, saving energy over conventional water heating. Although demand water heaters can cost several times that of a storage tank heater, a properly chosen system can reduce water heating costs by over 20 percent. Water heaters with variable temperature controls rather than simple "on" and "off" modes may be slightly more efficient.

About the Author

Brad Painting is a regular contributor to DexKnows and specializes in green building design.

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