Understanding Common Furnace Thermostats

  • 01 of 05

    What is a Thermostat and How Does it Work?

    Round White Low-Voltage 24-Volt 1 Stage Heat Only Wall Thermostat, Amazon

    The furnace thermostat is the nerve center of your home heating system. It's the main control point that determines when and how much heat will be delivered by the furnace. But for all its mystery, the thermostat is actually a very simple device. In fact, it's simply a temperature sensitive switch.

    There are two main categories of ​thermostats, depending on the type of heating system: low voltage and line voltage.

    This tutorial covers the ​low-voltage thermostat, such as those made by Honeywell, White-Rodgers, Lux, Robertshaw and others. It runs off of a transformer that reduces the incoming line voltage from 120 volts down to a level between 12 and 24 volts, depending on your furnace. (Click here to learn how to repair a thermostat.)

    Within this low voltage thermostat category, there are four major types. They include:

    • Mercury contact (older style).
    • Mechanical contact.
    • Digital.
    • Electronic Programmable (see also IP Thermostat)


    Another type of thermostat, the line-voltage thermostat, is typically hooked up to a 120-volt or 240-volt electric resistance heating system, such as that used to control electric baseboard heating units. This thermostat works in a manner similar to that of a light switch dimmer, varying the amount of electricity to the resistance heater. Line-voltage thermostats are not the subject of this tutorial.

    Let's take a look at the anatomy of each low-voltage thermostat type, starting with the older style mercury-contact thermostat,  often represented by the iconic T87 Honeywell shown in the photo above.

    Continue to 2 of 5 below.
  • 02 of 05

    Mercury Contact Bimetallic Thermostat

    Mercury contact thermostat
    Mercury contact thermostat. © www.Home-Cost.com 2006

    The mercury bimetallic thermostat is a good example with which to start in describing how an analog thermostat works.

    There are three major elements to a non-digital or analog thermostat:

    • Bimetallic strip or coil.
    • Contact.
    • Heat Anticipator.​


    Bimetallic Strip/ Coil

    In order for a non-digital thermostat to work, something has to move as the temperature increases or decreases. That's the job of the bimetallic strip. "Bimetallic" is just a fancy name for two pieces of metal joined together. But the reason it works is that the two metals expand at different rates at a given temperature (known as the coefficient of expansion). So as one side expands more than the other side, the metal curves one way or the other. You then couple this little bit of physics knowledge with the exact rates of expansion for the two metals, and the result is a precise instrument that will move a specific amount at a given temperature.


    The contact is another key part of the thermostat necessary for it to work. Think of the contact like a switch for your lights. As the bimetallic strip moves, it either opens or closes the contact. In the case of a mercury thermostat like the Honeywell T87 shown above, the mercury serves as a sealed switch. The mercury is placed in a sealed glass vial, called an ampoule.

    The ampoule is attached to a coiled bimetallic strip, and as the coil moves, it tips the vial of mercury until the mercury moves and the switch contact is opened or closed.

    Heat Anticipator

    The heat anticipator is an electrical resistance wire mounted on a center disc connected to the bimetallic strip. It is adjustable to allow fine tuning of when the thermostat turns the furnace burner on and off.

    Modes/ Programming

    The bimetallic thermostat has modes for heating and cooling and fan positions for on/off and automatic. It is not programmable.

    Continue to 3 of 5 below.
  • 03 of 05

    Mechanical Contact Thermostat

    Mechanical Contact Thermostat (White-Rodgers)
    Mechanical Contact Thermostat (White-Rodgers). © www.Home-Cost.com 2006

    The mechanical contact thermostat can also use a simple mechanical contact instead of a mercury contact switch. As shown in the photo above featuring a White-Rodgers mechanical thermostat, the bimetallic coil is used to move and open or close simple mechanical contact points attached to a lever arm. It also uses a ​heat anticipator.

    Another older version of the mechanical contact thermostat uses a straight blade bimetallic strip and not a coiled strip. They are a bit less accurate but work under the same principle.

    Modes/ Programming

    The mechanical contact thermostat has modes for heating and cooling and fan positions for on/off and automatic. The thermostat is not programmable.


    Continue to 4 of 5 below.
  • 04 of 05

    Digital Thermostat

    Lux Digital Thermostat
    Lux Digital Thermostat #DMH100. © Lux Products Corporation 2006

    The ​digital thermostat is an electronic version of the simple mechanical thermostat. It uses electronic controls versus a bimetallic coil to sense temperature changes and is much more accurate. These thermostats usually have large, easy to read displays and easily replace the older mechanical thermostat you have in your home.

    Modes/ Programming

    The digital thermostat has modes for heating and cooling and fan positions for on/off and automatic. The simple digital thermostat is not programmable.

    Continue to 5 of 5 below.
  • 05 of 05

    Electronic Programmable Thermostat

    Woman adjusting digital thermostat
    Tetra Images / Getty Images

    The electronic programmable thermostat takes the digital thermostat and gives it a "brain." It has user controls that allow you to set the days of the week, time, temperature, override, temporary temperature hold, and ​custom programming. The electronic programmable thermostat can save energy by allowing you to set back your heat at night or when you are away. These thermostats allow the weekday, Saturday, or Sunday settings to be different and even tell you when you need to change your ​furnace filter!

    Modes/ Programming

    The electronic programmable thermostat has modes for heating and cooling and fan positions for on/off and automatic. It is fully programmable, with features for multi-day settings, multi-period-per-day settings, hold, override, furnace filter change reminder, and other features as may be provided by the manufacturer and model selected.