Just How Does a Thermostat Work?

Most of us maintain the central heating and cooling in our homes with a thermostat. Depending on your thermostat’s age, it may be mechanical, digital or even smart.

Whatever its age, however, it uses some cool technology.

Most thermostats have two essential components—a thermometer and a switch. The thermometer is responsible for detecting the temperature in the room and relaying it to a switch, which turns the heating or cooling system on or off as needed to maintain the desired temperature.


A mechanical thermostat (see diagram) is often found in older homes. A common mechanical thermostat has two pieces of different metals bonded together to form a bimetallic—or bimetal—strip.

The metals expand at different rates when heating, causing the strip to bend, since metal expands when warm and contracts when cool.

The motion of the metal determines whether the thermostat’s electric circuit is open or closed, and in turn, whether the heat—or air conditioning—turns on or off.

The advantage of using a bimetallic strip is that it is accurate and can be finely calibrated to maintain a specific temperature.

The disadvantage is that it takes a bit longer to react to a change in the room temperature.


Modern thermostats are usually digital and can do a few things a traditional, mechanical thermostat cannot.

One of their most useful features is programmable settings. For example, in the summer you can program a digital thermostat to automatically turn up the air conditioning around the time you usually get home from work.

A digital thermostat uses a simple device called a thermistor to measure temperature.

This is a resistor that experiences electrical resistance changes with temperature. The microcontroller in a digital thermostat can measure the resistance and convert that number to an actual temperature reading.

Thermostats Get Smarter

As technology improves, so do thermostats. Talking thermostats allow you to control temperature with voice commands.

A telephone thermostat connects your heating and cooling system to your phone line. You simply call your property and enter a password on a touch-tone phone to access the controls.

Wi-Fi-based smart thermostats allow you to control your home’s thermostat with your smartphone. Some are advanced enough to “learn” the settings you like at different times of the day and simply adjust the heating or cooling on their own.

Sources for this article include accessheating.com, howstuffworks.com and explainthatstuff.com.

Cutting Costs

If you turn down the heat 1 degree for eight hours a day, you can save about 1% of your heating costs. Turn it down 10 degrees to save about 10%.

The same goes with your air conditioning. Turn the temperature up 10 degrees for eight hours a day to save about 10% on your bill.