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Figure 1. Digital thermostats can help to reduce energy costs in a home.[1]

A thermostat is a control device that switches the heating system in a house on or off as necessary. It works by sensing air temperature, and switching the heating devices on or off depending on whether or not the ambient temperature is above or below a setpoint.[2]

Thermostats come in many different forms, but most common in modern homes is the programmable thermostat. This type of thermostat adjusts a home's temperature automatically. Programmable thermostats have clocks that can automatically turn the temperature up or down at specified times. These types of thermostats can help save money on energy bills since it turns the temperature down when people are not in the house, or at night when people are sleeping.[3] Natural Resources Canada recommends programming the thermostat for 17°C when people are sleeping or not at home, and 20°C when people are awake and at home.[4]

Although it can be beneficial to turn down the heat in a house a couple of degrees, allowing the temperature to drop too much can affect humidity and air circulation in the house. Particularly, if the temperature in a house drops 4-6 degrees below the average in the winter, a more humid environment is created and can lead to condensation and moisture issues. It can also interrupt proper air circulation as the furnace fans will not be running if the furnace itself doesn't run. Thus it can be beneficial in saving costs to reduce the temperature by 2-3 degrees, but much more than that begins to cause other issues.[3]

In order to measure the average temperature inside the house, thermostats should be placed far from doors, windows, and heater vents.

Types of Thermostats

Although thermostats perform the same basic function, there are still different types of thermostats. Namely:[5]

  • Mercury Switch: This type of thermostat is rarely used anymore as it is less efficient than digital thermometers. It is composed of a small vial of mercury with three wires inside of it. When the vial is tilted left or right, one of two connections is made. This is what turns the heater in the house on or off. By moving the switches on the thermometer, the tilting can be initiated manually. Inside of this type of thermostat is a type of thermometer known as a bimetallic strip. This is a strip of metal that expands and retracts with temperature, and it is this coiling and uncoiling based on temperature that allows the thermostat to trigger at a certain temperature point. These are less flexible than modern digital thermostats, in terms of when it changes temperature, and to what temperature it changes to.
  • Digital: This type of thermostat uses a device known as a thermistor—a resistor that allows resistance changes with temperature, to measure temperature. It then uses a microcontroller to convert the varying resistance to a temperature reading. This type of thermostat tends to be programmable, whereas mercury switch thermostats are not.

For Further Reading


  1. "Homeywell Thermostat".Licensed under CC BY-SA 3.0 via Wikimedia Commons -
  2. Home Expert. (March 21, 2015). What is a Thermostat [Online]. Available:
  3. 3.0 3.1 Natural Resources Canada. (March 21, 2015). Programming your Thermostat [Online] Available:
  4. Natural Resources Canada. (March 21, 2015). Thermostats and Controls [Online]. Available:
  5. How Stuff Works. (March 21, 2015). Thermostats [Online]. Available: