These are shown in Fig. 10-6 and are commonly used in electric furnaces to stage heat. In most residential units, resistive heat strips are placed in the evaporator section in five kW increments. For instance, there can be two 5-kW heaters giving the total capacity of heat of 10 kW. The proper way to energize the units and not cause a severe power drain, is to start each with a delay. Imagine this operation with three strips, you can see there would be a tremendous power demand if all three units were energized at the same time. The sequencer, or time delay switch, helps to eliminate this condition. These can also be used with an appropriate thermostat and sub-base to stage the heating system. All strips do not have to be energized if the thermal demand isn’t present. This too provides an energy savings. On larger units that are found in the commercial and industrial line, the heat strips have much higher kilowatt ratings. Usually in a residential application the strips can be acquired in sizes of 5, 7.5, and 10 kW. A kWh (kilowatt-hour) of electricity will yield approximately 3,400 Btu (British thermal unit). You can see that it will require much more energy to heat the same size area, that is being cooled, with an electric furnace.
The sequential switch operates with bimetal switches. The first switch to close is actuated by the thermostat within the conditioned space. The first bimetal switch will flex causing a current flow in the second switch which will heat the bimetal and cause it to flex causing the third switch to start heating the bimetal, and so on. When the control voltage is removed, the bimetal switches begin cooling and drop out the resistive heating coils in the reverse order.
