<P> Domestic immersion heaters, usually rated at 3 kilowatts and on a 1.5 - inch British Standard Pipe screwplug in the UK, run on the normal domestic electricity supply, but users may also take advantage of a cheaper, off - peak electricity tariff such as Economy 7 (in the UK). In a typical off - peak installation, a lower immersion heater is connected to the separately switched off - peak heating circuit and an upper heater is connected to the normal circuit via its own switch . The consumer then has the option to top up the available hot water supply at any time, rather than waiting for the cheaper supply to turn on (typically after midnight). A poorly insulated hot water cylinder will increase running costs because a consumer must pay for electricity used to replace lost heat . </P> <P> Electric shower and tankless heaters also use an immersion heater (shielded or naked) that is turned on with the flow of water . A group of separate heaters can be switched in order to offer different heating levels . Electric showers and tankless heaters usually use from 3 to 7.5 kilowatts . </P> <P> Industrial immersion heaters can be either screwed or flanged . Screwed industrial immersion heaters, in the UK usually on a 2.25" British Standard Pipe are usually only rated up to approximately 24 kW, with 6 kW being considered the very top end that can be accommodated safely on a single - phase supply . Flanged immersion heaters (such as those used in electric steam boilers) can be rated at up to 2000 kilowatts, or more, and require a three - phase supply . </P> <P> Electrical immersion heaters may heat water immediately adjacent to the heating element high enough to promote the formation of scale, commonly calcium carbonate, in hard water areas . This accumulates on the element, and over time, as the element expands and contracts through its heating cycle, the scale cracks off and drops to the bottom of the tank, progressively filling up the tank . This reduces the tank's capacity and, where the immersion heater is secondary to the heating of the water by a coil fed from a gas or oil - fired boiler, can reduce the efficiency of the primary heating source by covering that other coil and in turn reducing its efficiency . Regular flushing - out of accumulated sediment can reduce this problem . </P>

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