Instantaneous (Tankless) Water Heaters
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.
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.
(Tankless) Gas Water Heater
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.