Heating elements activate the electricity within their assigned part of a furnace or hot water heater, based on information from the thermostat. When the temperature of a living space or water tank falls below the desired temperature, the heating element receives a signal to send current to the heat source.
Different furnaces and hot water heaters work differently when it comes to the timing of element activation. Some have simultaneous activation, meaning that when the temperature drops below that chosen number, all of the elements turn on at once, maximizing power delivery and ensuring the fastest heating process possible. Other types of systems are non-simultaneous, meaning that if the need to heat is significant enough, all elements can turn on at once, but if the desired change in temperature is slight, only one element at a time might turn on.
One example could involve a house in which the people fall asleep with the heat off because the previous day was warm. During the night, though, a cold front blows in, and one person wakes up to see that the internal temperature of the house is 62 degrees Fahrenheit. He sets the thermostat to heat to 68 degrees, which is a fairly significant jump. The thermostat would make a clicking sound, indicating that the element has turned on to start the heating process. When the temperature gets to 68 degrees, the element turns off, and the heater stops running. If it drops to 67 degrees, the thermostat would activate the element again, but in a non-simultaneous furnace, only one element might turn on because of the smaller change in temperature.