Installing Night Storage Heaters

Storage heaters are a relatively efficient method of supplying heat in the home; because heat is stored, you do not have to provide a continuous supply of energy. After the initial supply needed to provide the heat for storage, the electricity can be switched off and heat will still be emitted. There are various makes of storage heater available and these are of two types: storage radiators and storage fan heaters.

Storage radiators

The storage radiator is simply blocks of heat storage material into which is inserted a spiral element; the blocks are enclosed in a metal casing which comes in various finishes. Traditionally, concrete bricks have been used for the heat storage material, but now other lighter, more efficient materials are used. There are layers of insulation material between the storage blocks and the metal casing to control the amount of radiant heat emitted from the casing; this ensures the output is extended over many hours instead of being expelled in a short time.

During the day the rate of heat output gradually drops, although many models of storage radiator have a mechanical boost device which when opened (manually or automatically) in the evening lets out more heat until most of the heat has been transferred from the storage blocks into the room. Once the storage blocks have received their full charge of heat, there are no means by which you can control the output. You can, however, vary the quantity of heat charged overnight by adjusting an input control. In colder weather the input control can be left at its maximum setting, while in milder weather the input control can be turned down to reduce the heat output and save electricity.

Storage radiators are made with a number of electrical loadings ranging from 1.2kW to 3.3kW. The loading represents the amount of heat stored and therefore the number or size of storage blocks and the overall dimensions of the casing; but it is not the rate of heat output. Stored heat is conveniently quoted in kWh (kilowatt hours). A heater of 1.2kW loading can store up to a maximum of 9.6kWh in eight hours (1.2 x 8) and a 3.3kW heater can store up to 26.4kWh in an eight hour switch-on period. Smaller size heaters are suitable for the hall or small rooms and the larger sizes for larger rooms; for an average size room a 2kW storage radiator which can store up to 16kWh is suitable. The heat output rate is about half the charge ate; this is because the heater is off-charge for 16 hours.

Radiator circuit wiring

Because all the storage radiators in the house are generally switched on at the same time with no diversity of use, they are not supplied from plugs and sockets like direct acting heaters. Instead they are supplied from separate circuits switched by the Board’s time switch. These are run from a separate consumer unit from that which supplies general services such as lighting, socket outlets, cooking and water heating. A separate unit is necessary because it has to be time-controlled so the circuits to the heaters are energized only during the overnight off-peak period.

The size, or number of fuseways, of the consumer unit depends upon the number of storage heaters installed, the number of radial circuits and, in some cases, the number of heaters connected to one radial. It should be at least a four way unit and preferably a six way unit, since even if you are starting with two storage radiators you may wish to add more later. Also, if you have an immersion heater, it is usually financially worthwhile to connect this to one fuseway in the time-controlled consumer unit to take advantage of the cheaper rate of electricity overnight. You should, however, make arrangements for an optional daytime boost, which normally means you will need a second immersion heater or a dual immersion heater with one element connected to the 24-hour supply.

Each single storage heater circuit is wired in twin core and earth PVC-sheathed cable, fused and rated according to the heater loading, and terminating at a 20amp double pole switch fixed about 300mm (or 1ft) above floor level near the radiator controls. It is common practice to use 2.5sq mm size conductors protected by a 15amp fuse and taking a load of up to 3kW. The radiator is connected to the switch by three core flex passing through a cord outlet. The switch is of the same dimensions as a one gang socket and needs the same depth of box. It can be either surface-mounted, using a plastic surface box, or flush-mounted on a one gang box sunk into the wall flush with the plaster.

Connecting the box and switch

Remove a cable knock-out blank; if it is a metal flush box, you will need to fit a PVC grommet to protect the cable from rubbing against the metal. Run the cable into the box and fix the box — if it is flush-mounted, you will have to cut out the wall to sink the box flush.

Prepare the ends of the cable by stripping off the sheathing and about 9mm (or gin) of insulation from the two current carrying conductors and slipping green/yellow PVC sleeving over the bare earth wire. Connect the red insulated conductor to the mains terminal L, the black to the mains terminal N and the green/yellow sleeved earth wire to the earth terminal E in the box. If at this stage the storage radiator is in position, you can also connect its flexible cord to the switch; if not, fix the switch to the box using the screws supplied.

When you connect the flexible cord to the double pole switch, thread it into the flex outlet hole with the switch removed from its box and strip about 100mm (4in) of sheathing from its end, exposing the three cores coloured brown, blue and green/yellow. Strip off about 9mm (or tin) of insulation from the exposed end of the conductors. Connect the brown wire to the load terminal L, the blue to the load terminal N and the earth to the terminal E alongside the existing earth terminal. Then fix the switch to the box using the screws supplied.

Connecting the consumer unit

Install the consumer unit first. Remove the cover of the unit and dismantle as much of it as necessary. Fix the frame or casing to the wall fairly close to the existing 24-hour consumer unit and meter, using a backing sheet of non-combustible material if the unit is open-backed. Run the circuit cables into the unit and prepare the ends for connection by removing the necessary amount of sheathing and insulation.

Connect the red insulated wires to the fuseway terminals, the black insulated wires to the neutral terminal bank and the green/yellow sleeved earth wires to the earth terminal bank. Connect a 1m (or 3ft) length of 10sq mm red PVC-insulated single core cable to the mains terminal L, a 1m (or 3ft) length of 10sq mm black insulated cable to the mains terminal N and a 1m (or 3ft) length of green/yellow insulated 6sq mm cable to the earth terminal bank. The electricity company will connect the two mains leads to a white meter and time switch and you should connect the earthing lead to the mains earth terminal.

Installing a single storage radiator

Ideally, even one heater should be connected to the Board’s time switch, but it can be operated from any ring circuit 13amp socket. You will, however, need a time switch to limit the charge period to eight hours. This can be a plug-in time switch, which has a socket outlet into which you plug the storage heater. You can use the white meter tariff and set the time switch to coincide with the cheap rate period. The time switch has an over-ride switch which enables the heater to be given a boost charge in the evening when needed, but at a higher rate for the electricity used. This is more satisfactory than using a direct acting heater for a short period since any heat not used goes towards the night charge.

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