6.5.3 - Off-peak appliance circuits

All Electricity Supply Companies offer extremely economic rates for energy taken at off-peak times, usually for seven hours each night (economy 7). The supply meter is usually arranged so that off-peak inexpensive energy can only be obtained from a special pair of terminals, whilst a second pair provide energy throughout the 24 hour period, charging for it in terms of the times at which it is taken. In most cases, energy used at the cheap rate must be stored for use at other times. There are two major methods of storing energy, in both cases involving its conversion to heat.

1. - storage heaters, which are used for space heating. Circuits feeding them should always be wired radially, with only one flexible outlet for each. This will help to avoid problems in the event of the storage heater being changed for one of a different power rating.

2. - immersion heaters, the energy being stored as hot water in a lagged tank for use during the day. Since the amount of hot water used is variable, it is usually necessary to have a method of increasing the water temperature should that heated at the cheap rate be used up. This involves the use of a second immersion heater, or a single heater with a double element. Since convection, and hence water heating, takes place mainly above the active heater, an immersion heater placed low in the tank and fed by the off-peak supply will heat the whole tank, whilst a second heater, placed higher in the tank and connected to the normal supply, will be switched on when necessary to top up the temperature of the hot water stored (Fig 6.12(a)). Sometimes a top mounted dual heater is used for the same purpose as shown in (Fig 6.12(b)). The normal and off-peak heaters must be supplied through totally separate circuits. 3 kW heaters must be connected permanently to a double pole switch and not fed via a plug and socket.

Fig 6.12 - Arrangement of immersion heaters for off-peak supplies