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Instantaneous (Tankless) Water Heaters

Instantaneous water heaters are also known as demand water heaters and tankless water heaters. Instantaneous water heaters are direct-fired water heaters that heat water only when it is called for, in other words, when there is a flow through the water heater so they never run out of hot water. Instantaneous water heaters can have a small amount of storage, but generally have no hot water storage capability. They are defined by DOE as containing no more than one gallon of water per 4,000 Btuh of heat input.

Instantaneous water heaters can be centrally located, serving several end uses, or multiple point-of-use water heaters can be installed near sinks, showers or other end use. Point-of-use models offer the advantage of eliminating energy loss through long runs of hot water pipe and minimizing the delay for hot water to the user. Instantaneous gas-fired water heaters may have a standing pilot or electronic ignition.

Instantaneous (Tankless) Gas Water Heater

How It Works
Instantaneous water heaters heat the water directly as needed. When a hot water tap is turned on, cold water travels through a pipe into the unit. Old-style instantaneous water heater models have a fixed energy output, so water temperatures vary inversely with flow rate. As the flow rate increases, the water becomes cooler; as flow rate decreases, the water becomes warmer. Newer models have modulating controls that increase or decrease energy input to maintain the selected outlet temperature despite varying flow rates and inlet temperatures. In some models, temperature is selected by remote controls. In gas water heaters, the burner is ignited by either a standing pilot or by an electronic ignition device. In electric instantaneous water heaters, an electric element heats the water.