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Illustration of a tankless coil water heater. The heater is box-shaped, and has two pipes sticking out one end: one a cold water inlet, and one a hot water outlet. These pipes lead into the heater to a cylindrical coil called a heat exchanger. Long tubes

Tankless coil and indirect water heaters use a home or building's space heating system to heat water as part of an integrated or combination water and space heating system.

How Tankless Coil and Indirect Water Heaters Work

A tankless coil water heater uses a heating coil or heat exchanger installed in a main furnace or boiler. Whenever a hot water faucet is turned on, the water flows through the heat exchanger. These water heaters provide hot water on demand without a tank, like a demand water heater, but because they rely on the furnace or boiler to heat the water directly, tankless coil water heaters work most efficiently during cold months when the heating system is used regularly.

That's why they can be an inefficient choice for many homes and buildings, especially for those in warmer climates.

Illustration of an indirect water heater, in which a boiler with an oil burner provides heat to the water storage tank through the coiled heat exchanger inside the storage tank. The water storage tank also has a drain at the bottom of the tank.

Indirect water heaters offer a more efficient choice, even though they require a storage tank. An indirect water heater uses the main furnace or boiler to heat a fluid that's circulated through a heat exchanger in the storage tank. The energy stored by the water tank allows the furnace to turn off and on less often, which saves energy.

Indirect systems can be fired by gas, oil, propane, electric, solar energy, or a combination of any of these. Tankless systems are typically electric or gas-fired. Also, these integrated or combination water heating systems not only can work with forced air systems but also with hydronic or radiant floor heating systems.

More Information

Learn more about the selection, installation, and maintenance of tankless coil and indirect water heaters for homes, as well as strategies for reducing hot water use.