Rather
than storing a fixed reservoir of heated water, tankless water heaters also called
on-demand water heaters, do not rely on a storage system. Instead, these units
consist of a heat exchanger coil (typically copper) and a heating element that
has the capacity to rapidly raise the temperature of the cold water feed that
is input to the unit to the desired hot water temperature. As with the
traditional tank water heater, these units can be sized to suit the hot water
consumption needs of the installed application. While tank water heaters are
rated in gallons, the tankless models are rated in gallons per minute (GPM).
Some units offer recirculation pumps, which are designed to loop the water that
sits in the piping from the output side to the outlet (faucet) and recirculate
it back to the heater to save water. This feature eliminates the need to run
the faucet initially and clear the piping of cold water until the hot water
from the heater reaches the faucet. Other units can reduce the flow of water to
the tap initially until the desired water temperature has been reached.
One
variety of tankless heater is called a tankless coil. These units operate on
the same principles with the difference being that they use an existing
combustion source, typically the heat generated in a boiler, to produce hot
water. The tankless coil is immersed within the water jacket of the boiler,
using the thermal energy of the boiler water to transfer heat to the domestic
water.