How Does a Thermostat Work?

When it's cold, we walk over and turn up the thermostat, and when it is hot, we walk over and turn on the air conditioner to cool us down. But, how does that little box on the wall actually control the temperature of our homes or offices?

A mechanical thermostat is actually a very simple device. It is basically a thermometer attached to a switch that turns on your heater, whether your heat source is natural gas or electricity. If you remove the cover of your thermostat, you will see the inner workings and get a better idea of how a thermostat works.

The top layer holds a mercury switch and a thermometer coil. The mercury switch is just a small vial filled with the liquid metal, mercury. Within this vial are three wires: one at the bottom of the vial; one at the left of the vial; and one at the right of the vial. As the temperature rises or cools, the vial of mercury gets tipped to the right or the left making the corresponding wire come into contact with the wire that runs along the bottom. If the mercury gets tipped to the left, a connection is made that creates a current that energizes a relay, which starts the heater and circulation fan. As the room heats up, the vial levels off and once that is in balance, the connection is broken causing the heater to turn off. If the mercury switch is tilted to the right, another relay causes the air conditioner to turn on.

What tips the vial in either direction is the thermometer coil that rests against the vial of mercury. The thermometer coil is constructed of a bi-metallic strip made of two different types of metal, usually copper and iron, which are bonded together. Because the different metals respond to heat at different levels, this strip contracts and expands causing the coil to curl up or uncurl as the temperature changes. This curling or uncurling motion tilts the mercury vial, which then signals the heating source to kick off or on. When you adjust the temperature knob on your thermostat, you are actually adjusting the tightness of the coil.

Beneath the top layer of your thermostat, you will see the circuit board, which houses the wires that actually lead to the circulation fan and heat source. The circuit board is connected to the mercury switch via a metal screw and wire, which "reads" the switch and turns on the appropriate heating or cooling device.

Newer on the market are digital thermostats. These thermostats differ from the mechanical thermostats in that they use a thermistor, a resistor whose electrical resistance changes with temperature. The microcontroller in a digital thermostat measures the resistance and converts that number to a temperature reading. Digital thermostats can save energy because they can be programmed to turn the heat or air conditioning off or on at preset times throughout the day. For example, you can set the air conditioning to come on an hour before you come home from work, or have the heater remain off during the hours while you work and then turn on an hour before you get back so your home is warm and cozy when you open the door.