Generally speaking, thermostats work by using a mercury switch that is in contact with a thermometer wire to trigger a temperature-adjustment lever in response to the expansion or contraction of the wires as they are heated or cooled. The expansion and contraction of these wires triggers switch relays that control heating and cooling by triggering either a circulation fan and heater or air conditioner.
Modern thermostats use at least two semiconductors, such as thermistors or resistance thermometers, to switch on the HVAC unit. One switch regulates the mode – heat or cool – while the other controls the circulation fan. Once the switch on the thermostat is moved, the thermometer coil and mercury switch rotate to the left, sending a flow of mercury through the mercury switch. This mercury flow signals a relay that turns on the heater and circulation fan. As the room warms, the thermometer coil expands, causing it to unfurl. This pushes the mercury switch in the opposite direction, breaking the relay contact and shutting off the heat. The opposing movement, in turn, triggers the start of the air conditioner. As the room cools, the thermometer coil contracts, causing it to curl up and tip the mercury switch back to the left.