Instantaneous Electric Water Heaters Guide

These units are typically mounted directly under a sink, although because the units operate at mains pressure the location of the heater is a purely a matter of convenience. The mode of operation is much the same as a conventional electric shower, i.e. cold incoming water is heated as it flows over an electrically heated element.


The specific details details do vary a little from product to product, although they generally have a number of common features that are outlined below:

Hot Water Supply (A)

Instantaneous electric water heaters can supply one or more sinks fitted with standard taps or mixers. They do not store heated water and therefore have no standby energy losses and can be relatively small in size. They are ideally suited for light duty applications such as:

  • En-suite bathrooms or guest suites which are remote from the existing hot water supply
  • Hand basins in commercial premises
  • Cloakrooms etc

However due to the limited electrical power available from typical single phase electrical supplies in the UK they are unsuited to heavier duty applications requiring large volumes of heated water to be produced at a high flow rate. In particular they are not generally suitable for use with baths, or for larger sinks that need to be filled quickly. Whilst they can be used for multiple outlets, the temperature and flow rate of the hot water supply may vary if more than one outlet is used simultaneously. This can be a significant consideration in some applications, for example if a single instantaneous heater is used to supply more that one back-wash in a hair dressing salon.

(Consideration should be given to a storage heater if an instantaneous water heater is unsuitable).

Heater Location (B)

These units operate at mains water pressure, and are equally at home when located above or out of sight underneath the sink.

Electrical Wiring (C)

The electrical cable must be suitable for the power rating of the unit in accordance with Part P of the Building Regulations. The exact requirements for the cable will depend on the power of the unit and specific details of the installation, but typically will be in the range 6mm to 10mm. (Installation should be carried by a competent person in accordance with the relevant regulations).

Consumer Unit (D)

The unit must be wired directly to the domestic consumer unit with a dedicated fuse, usually in the range 40-50 amps.

Vent (E)

These units typically include a pressure relief valve. This valve is a safety device that remains closed during normal operation, but in the unlikely event of a fault condition developing it can release hot water and/or steam to the waste. The vent should be connected to a waste (drain) in accordance with the manufacturer's instructions.

Service Valve (F)

A service valve is normally be fitted on the inlet (cold) pipe to facilitate any future maintenance.

Manufacturers' Installation and Operating Instructions

Please note that all units must be installed and operated in accordance with the manufacturers' specific requirements. Installation and maintenace must be carried out by competent personnel.

Instantaneous Water Heater Performance

The fundamental relationship between flow rate (litres per minute), available heating power (measured in kW) and water temperature is is given by the following formula:

Power in kW to Heat Water = [ Litres x Temp Rise oC] / [ Time in Minutes x 14.3]

Ideally the flow rate of water at the required temperature would be as high as possible but in practice it is limited by the available heating power.


(Note that the actual temperature depends on the incoming mains water temperature which varies according to the season but is typically 5oC in winter and 10oC in summer. (To calculate output temperature simply add the temperature of the incoming water (oC) to the temperature rise generated by the water heater, for example if the incoming water temperature is 5oC and the heater is able to increase the water temperature by 40oC then the outlet temperature will be 45oC) .