Instantaneous Electric Resistance Water Heaters | My Business | Mississippi Power | A Southern Company

Instantaneous Electric Resistance Water Heaters

Instantaneous electric water heaters use high-input heating elements to produce hot water on demand. They have little or no storage capacity. Their primary advantages are their small size, the absence of standby loss, and their suitability for installation at the points of hot water use to serve specific individual loads. Small units are often installed to serve individual lavatories or showers. Disadvantages include higher first cost, the possibility of increased electrical demand, and limited hot water delivery capacity.

Most instantaneous water heaters activate the heating elements with a flow switch and rely on a thermostat to regulate the supply water temperature. Because they heat water continuously and do not use stored hot water to meet loads, instantaneous water heaters can supply an essentially constant stream of hot water. However, delivery rates are somewhat limited. Typical small units provide hot water at rates up to about 2.5 gallons per minute.

Because fuel-fired instantaneous water heaters have higher thermal capacitance, their efficiency suffers substantially when the units cycle frequently.

Instantaneous water heaters heat water on demand rather than storing heated water in a tank. The heating elements in an instantaneous water heater are controlled by a flow switch and thermostat. The flow switch allows the elements to operate only when water is flowing through the heater. The thermostat limits the maximum output temperature to the desired limit. When a hot water tap is opened, flow begins and the heating elements are energized to heat the water. The heat input rate is usually constant; at lower flow rates, the elements cycle on and off to deliver the desired temperature.


  • In general, low-flow uses of hot water in isolated or remote locations in a building.
  • Distributed lavatories and sinks in shopping malls, offices, public facilities, schools, service stations, schools, and other building types.
  • Point-of-use applications in commercial buildings for low-flow hot water uses, especially restrooms.

Possible applications

  • Instantaneous water heaters may also be installed at the points of use as in-line heaters on the hot water line of a conventional water heater to avoid a long wait for hot water to arrive from the central water heater when a tap is opened.


The volume of hot water delivered by an instantaneous water heater is limited by the electrical input capacity of the heater and the circuit serving the heater and the incoming cold water temperature. Useful capacity declines somewhat during winter when the incoming water temperature is low. The following table illustrates the maximum temperature increase provided by typical instantaneous water heaters. Cold water inlet temperatures are below 35°F during winter in many locations, indicating that such heaters are inadequate to meet many hot water loads.

Temperature Increase (°F)
110 3 41 27 20 14 10
208/220 4.5 61 41 31 20 15
208/220 7 95 64 48 32 24
240 9.5 123 82 61 41 31