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Furnace and Air Conditioner Thermostats

The basic function of a thermostat is to turn a furnace or air conditioner on or off to regulate the temperature of your home. When the temperature falls below a pre-defined level (called a setpoint), the thermostat signals to your heating system to begin generating heat. Alternately, if the thermostat is connected to a cooling system, it signals the system to begin cooling your home when the temperature rises above a certain level.

Common Types of Thermostats

Thermostats are classified as either mechanical or digital, depending on their internal mechanisms. Mechanical thermostats (also called electromechanical or EM thermostats) typically have a thermometer coil and a switch (often made from mercury). Changes in temperature cause the coil to expand or contract, engaging the switch when setpoint temperatures are reached. Digital thermostats measure temperature with a device called a thermistor.

One increasingly popular type of digital thermostat is the programmable thermostat. (There are mechanical programmable thermostats called "clock" thermostats, but these are less popular for residential use.) These thermostats allow you to set different target temperatures at different times of the day. For example, you could set the temperature of your house to 68° F during the day, and 58° from 10:00 pm to 7:00 am. Many claim that such thermostats can save the average homeowner substantial amounts on their energy bills by automating such changes in temperature. (For example, the US Department of Energy claims that "You can save as much as 10% a year on your heating and cooling bills by simply turning your thermostat back 10% to 15% for 8 hours".)

How to Buy a Replacement Thermostat

Installing a replacement thermostat can be a do-it-yourself task -- although you need to be comfortable working with your home's electrical system and your heating or cooling system. It's also a relatively small job for a heating contractor.

If you own a heat pump, you need to do additional research before buying a thermostat. Many programmable thermostats don't work efficiently with a heat pump in heating mode -- so you need to buy a special thermostat.

Here are a few features you may want to consider when shopping for a new thermostat:

  • How important to you is simplicity? Some programmable thermostats can be quite complicated to program.
  • Do you want an LCD, LED or analog display?
  • How many different temperature setting do you need? More sophisticated programmable thermostats will allow you to define up to six different temperature settings.
  • Can you manually override the temperatures that you have programmed into your thermostat without having to reprogram your default settings?
  • Does the thermostat have a battery backup so that you won't lose your settings if you lose power?

Who Makes Thermostats?

Honeywell claims that the T86 Round® mechanical thermostat is the most popular thermostat in the world. First built in 1953, the company has sold more than 85 million thermostats and was granted a trademark on round thermostats in 1990. Although Honeywell's current models don't include mercury, they have been criticized in the past for including mercury switches.

Manufacturers of digital thermostats include:

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