Electric Water Heaters

Electric heating of water can be carried out in a variety of ways. These include small capacity heaters fitted above the sink, instantaneous sink heaters or immersion heaters in the hot water tank.

Small Storage Water Heaters

Small storage water heaters fitted over the sink or washbasin having capacities of up to about 3 gallons and electrical loadings of 1 to 3 kW are usually supplied from the ring circuit by means of a spur. Except in a bathroom, the outlet can be a fused plug and switched socket-outlet, preferably with neon indicator.

The connection from plug to the water heater is heat-resisting 3-core flexible cord. In the bathroom, where no socket-outlets are permitted other than an approved shaver socket, a switched fused connection unit is used as the outlet. This should also have a neon indicator and be connected to the water heater by 3-core flexible cord.

The switch of the connection unit, as other wall switches, must be out of reach of a person using the bath or shower. Where this is impracticable a cord-operated ceiling switch is necessary and the outlet to the water heater should be a cord outlet unit. As there must be a fuse in the circuit, a non-switched fused connection unit can be fixed outside the bathroom at the point where the spur cable is connected to the ring circuit.

Immersion Heaters

An immersion heater is classed as a continuous load and as such should not be supplied from a ring circuit since its 3 kW loading would deprive the ring circuit of nearly half of its 7.2 kW load capacity. Instead a separate circuit is used for the immersion heater which can be wired in 1.5 mm² PVC sheathed cable from a 15 A circuit fuse, or preferably in 2.5 mm² cable from a 20 A circuit fuse.

An essential requirement for an immersion heater circuit is a double-pole isolating switch which should be fixed within reach of the immersion heater so that anyone adjusting the thermostat with the terminal cover off has access to the isolator. For a single-element immersion heater this switch is a standard 20 A double-pole plate switch available with or without neon indicator and flex outlet facilities. Where required, the plate can be engraved water heater.

A 13 A fused plug and switched socket- outlet is sometimes used instead of a double-pole switch. Isolation is effected by pulling out the plug, but with a sustained 3 kW loading, a plug and socket tends to overheat especially in the vicinity of a hot water tank and even more so when enclosed in a tank cupboard. Where, as is often the case, a tank cupboard opens into the bathroom, a socket-outlet is prohibited as it could be misused by plugging in a portable appliance.

Although the water heater switch should be within reach of the immersion heater, it is essential that it cannot be reached by a person using the bath or shower. In these circumstances, either the switch must be fixed outside the bathroom or a cord operated ceiling switch is necessary.

Dual Immersion Heater

A dual immersion heater is designed for fitting into the top of a hot water storage cylinder and consists of two elements, one short and one long. The short element is for heating a small quantity of water for normal daily use, the long element is for heating the full contents of the tank when large amounts of hot water are required for a bath or other purposes.

Some models have an integral changeover switch mounted on the heater head to switch over from one element to the other. Immersion heaters without this integral switch require an external change-over switch wired into the circuit. The switch used for this is a combined 20 A double- pole switch and change-over switch fixed in the place of the ordinary isolating switch and connected to the immersion heater by heat-resisting flex. As an extra wire is required for the second element and 4-core flex is not generally available, it is usual to have two 3-core flexible cords as though for two separate elements but connected as shown in the diagram.

Remote Control of Immersion Heater

When an immersion heater is switched on and off as hot water is needed and not in continuous operation, it is useful to have a 4 remote control operated from the kitchen.

A special 2-point control is available for this arrangement, consisting of two double- pole switches, each with a neon indicator. One switch (the master switch) incorporates an isolator and is fitted at the immersion heater end; the other (auxiliary switch) is fitted in the kitchen. The immersion heater can be switched on and off by either switch, both neon indicators going on and off when one switch is operated.

This arrangement is intended mainly for single-element immersion heater installations but can be used with dual immersion heaters with the changeover switch wired in between the master switch and the immersion heater. Another arrangement is to have a remote switch to control the long element only of a dual immersion heater so that, when extra hot water is needed, the long element can be switched into circuit from the kitchen.

Two-immersion Heater Installations

Where the hot water storage tank is rectangular or there is insufficient headroom to insert a top-entry immersion heater into a cylinder, side-entry immersion heaters are fitted.

An immersion heater fitted a few inches from the base heats the full contents of the tank but where only small quantities are normally run off during the day a second immersion heater is fitted. This is a few inches from the top of the tank, according to the dimensions of the tank and the amount of water to be heated. The top immersion heater will be fitted lower down from the top of a small-diameter cylinder than for a large rectangular tank.

Two immersion heaters operate similarly to one dual immersion heater and, if both have 3 kW loading, a change-over switch is used to change over from one to the other.

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