How Does a Digital Programmable Thermostat Work?

Mechanical Thermostats

• Before digital thermostats were invented, people used mechanical thermostats controlled by a special type of thermometer called a bimetallic strip. A bimetallic strip was actually a coil made out of two metals attached to each other. Like most other solids, metals expand when they heat up and contract when they cool down, but different metals expand and contract at different rates. This difference would cause the strip to uncoil when it heated up and coil when it cooled down. When the thermometer expanded enough, it would bump into a special switch called a mercury switch, which turns on when it is tipped over. This switch, in turn, would turn on the heater. If there was an air conditioner, it would be powered by another mercury switch, which the coil would bump into as it contracted.
  
  

Digital Improvements

• The invention of digital thermostats improved home heating and cooling in a few different ways. First of all, digital thermostats are more compact, because they don't rely on a bulky metallic coil to power them. In addition, they tend to be more accurate and affordable and have more features. Digital thermostats rely on a thermistor to measure the temperature. A thermistor is a resistor--a circuit component that resists the flow of electricity. Unlike most resistors, a thermistor changes its resistance quite a bit as it gets hotter. Thermostats have a computer that constantly checks the resistance of the thermistor and uses this value to determine the temperature in the room.
  
  

Digital Features

• The user of a digital thermostat sets a certain temperature for the heat to turn on and another temperature for the air conditioning to turn on. Because they are powered by computers, however, digital thermostats can also numerous things that mechanical models could not. Many digital thermostats come with clocks that can automatically reset the temperature at a certain time of day. You could, for example, have your digital thermostat turn off the heat for you at bedtime, warm up the house again just before it is time for you to get up, and then turn the heat off again when you leave for work after breakfast.