How Does a Furnace Thermostat Work?

How the Thermostat Controls Temperature in the Home

A furnace thermostat turns the furnace off or on to regulate the temperature of the home. It may seem that the thermostat is not doing its job properly as the temperature is often well above or below the set temperature, but it is important to remember that the temperature detected at the thermostat defines the action on the furnace.

Depending on where the thermostat is located, it may take time for the heat from the running furnace to reach it so that it can react, making some areas much warmer than the desired temperature.

Because heat rises, upstairs rooms may be warmer than the set temperature while basement rooms remain cooler. Vaulted ceilings and sunlit windows also play an important part in the temperature of areas in the building.

Elements of the Home Thermostat

A furnace thermostat is basically a switch that is temperature sensitive. The home furnace thermostat is typically made up of a metal strip or coil that determines temperature variance, a contact that acts as the switch, and a heat anticipator that will allow fine tuning of reaction time of the furnace.

Bimetallic Strip/Coil

Two types of metal, each with its own rate of expansion from heat and contraction from cooling, are laminated together and coiled. When heated, the coil unwinds because the metal on the inside expands. When the temperature at the thermostat cools, the metal contracts and causes the coil to tighten.

Mercury Switch

The ampoule is a glass vial with mercury that is attached to the bimetallic strip and acts as the switch of the simple home furnace thermostat. A wire lies across the bottom of the bottle, placed so that the mercury is always in contact with it. Wires are also attached to the right and left ends of the vial, placed so that when the mercury arrives at either end, it touches both the bottom and side wires, making an electrical contact.

As the bimetallic band expands or contracts, the mercury switch is tilted, and depending to which side the mercury flows, once it forms a contact at the end, the furnace is either switched on or off.

When the typical home furnace thermostat is adjusted, the glass vial is tilted to initiate the desired action. The actual temperature at the thermostat must cause the coiled metals to expand or contract to make the mercury move to change the action.

Heat Anticipator

Some home furnace thermostats have a small resistance wire that is connected to the bimetallic strip. This allows fine tuning if it seems the furnace is running longer than it takes to heat the majority of the home before the heat reaches the thermostat.

Resources

• Consumer Guide to Heating