What is a Furnace Thermostat?

Adam Hill
Adam Hill
A digital furnace thermostat.
A digital furnace thermostat.

A furnace thermostat is a control unit for a central heating system such as those found in homes and businesses. The thermostat’s purpose is to maintain a temperature that is near the desired temperature, called a set point. To do this, the thermostat turns a furnace on and off remotely as needed to maintain the set point temperature.

Most people have seen and used a furnace thermostat many times. They come in several different varieties, but one of the most common is the mercury switch thermostat. This type of furnace thermostat is made of three layers. The top or outer layer houses the mercury switch itself, which is a glass vial containing a small amount of mercury. The vial also contains three wires, and is tilted right or left by the lever which adjusts the set point temperature.

Depending on how the vial is tilted, it will transmit an electrical current through the mercury, into one of the wires, turning the furnace and ventilation fans on or off at certain temperatures. The mercury switch is also attached to a bimetallic coil which expands and contracts with temperature changes, also tilting the vial. This mechanism is the most important part of a furnace thermostat, since it controls the main functions. The other two layers of the thermostat unit house a small circuit board and other switches to further regulate a home’s ventilation.

Homes that have been constructed relatively recently often have a furnace thermostat which also controls an air-conditioning system. In this case, the heating and cooling systems share the same ventilation ducts. The thermostat is also slightly more complex, but operates on the same principles as a simple furnace thermostat.

When planning home construction, the placement of the thermostat is an important factor to consider. It must be located away from heating and cooling vents and be exposed to the general ambient temperature of the house. Location in a high-traffic area such as a main hallway is also desirable to give the residents easy access.

Some newer thermostats are digitally programmable to help the homeowner use less energy and save money on heating and cooling bills. When used as intended, these designs automatically reduce heating and cooling when it’s not needed as much. The goal is to do this without sacrificing comfort. Manual thermostats can also be used towards the same purpose, but naturally without the convenience of programmed settings.

You might also Like

Readers Also Love

Discussion Comments


My landlord checked my furnace which wasn't working and figured out that it was the wiring to the thermostat. He has now hooked the thermostat directly to the furnace and it's now sitting on top. How will this affect my heating?

Post your comments
Forgot password?
    • A digital furnace thermostat.
      By: Dave Friedel
      A digital furnace thermostat.