Home Furnace Thermostat

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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 produced 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 article covers the low voltage thermostat often made by Honeywell, White-Rodgers, Lux, Robertshaw and others. It runs off of a transformer reducing the incoming line voltage from 120 volts down to between 12 and 24 volts, depending on your furnace.

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

  • Mercury Contact (older style)
  • Mechanical Contact
  • Digital

Electronic Programmable

Another type of thermostat, the line voltage thermostat, is typically hooked up to a 120 volt or 240 volt electric resistance heating system. This thermostats work similar to a light switch dimmer, varying the amount of electricity to the resistance heater. Line voltage thermostats are not the subject of this article.

Please note that some older thermostats use mercury and they should not be put in the trash. You should take them to your local heating dealer to be disposed of properly.

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 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 trick in making it work is that the two metals expand at different rates at a given temperature (known as 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 you have 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 inside 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 and is adjustable to allow fine tuning of when the thermostat turns the furnace burner on and off.

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.

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 as Mechanical Thermostats

Mechanical Contact Thermostat

The mechanical contact thermostat can also use a simple mechanical contact instead of a mercury contact switch. As shown in the photo using 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.

Digital Thermostat

The Digital Thermostat is an electronic version of the simple mechanical thermostat. It has 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.

Electronic Programmable Thermostat

The Electronic Programmable Thermostat take 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 other custom programming. The electronic programmable thermostat can save energy too 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 and other features as may be provided by the manufacturer and model selected.

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