Electrically powered furnaces normally have several heating elements. These heating elements are essentially large enclosed metal coils or rods through which electricity passes, causing them to become white hot. The heat from these rods is then blown throughout the home to warm it. When these heating elements are first turned on, they require such a great deal of electrical current to heat up that if all of them were to turn on at once, the ensuing electrical current spike would trip the circuit breakers of the home or building. A sequencer acts to stagger the rate at which each heating element receives electricity in turn.
What is a Sequencer?
The sequencer sits between the heating elements and the thermostat on the furnace. Its outer housing is a square of heat-resistant plastic through which a small heating coil passes. Stacked above the heating coil are two or more heat-reactive circuits. There is one such circuit for each heating element. While the heating coil in the sequencer's base is connected only to the thermostat, each heat-reactive circuit connects from an electrical source to a given heating element within the furnace. Inside each heat-reactive circuit are two leads set apart from one another, one leading to the thermostat and the other to the furnace heating element. Bridging these leads is a strip of copper set on a spring, so that at room temperature the strip does not touch the leads.
When the thermostat is activated, it sends an electrical current to the sequencer. The current cannot pass through each of the heat-reactive circuits just yet, as the copper strips are not in place to close these circuits. Because of this, electricity only passes through the heating coil in the sequencer's base. The heat the coil radiates passes through the first copper strip and spring nearest. According the laws of thermodynamics, heat causes molecular structures to expand. The spring expands, pressing the copper strip against the two leads of the first heat-reactive circuit, allowing electricity to pass through that circuit and into the first of the furnace's heating elements. As time passes, more heat from the sequencer's heating coil radiates further outward, causing the second heat-reactive circuit in line to activate in the exact same way. This process repeats for however many heating elements are present in the furnace.