How Thermostats Control Temperature

person adjusting analog thermostat

Throughout the year, you adjust your thermostat to warm or cool your home, but have you ever wondered exactly how a thermostat works?  In order to understand the inner workings of a residential thermostat, we start with understanding a basic mechanical, non-digital thermostat.

The Mercury Switch

If you have ever lived in an older home, you may have had a thermostat that required you to move a slider at the top of the thermostat along a temperature guide. Moving the slider up or down powered the system on or off. Inside the thermostat is a small glass vial that contains mercury. You may be familiar with mercury if you have ever used a traditional thermometer for taking your temperature. Two characteristics of mercury make it a perfect option for thermostats. First, mercury easily conducts electricity, and second, the liquid state of the metal allows it to flow easily, much like water. Inside the glass vial of mercury are three very tiny wires. One wire extends the length of the bottom of the vial, so it is always in contact with the mercury. The other two wires are on the left and right of the vial, and the mercury touches one of the wires when the vial is tilted. Tilt the vial to the left, and the mercury touches the bottom and left wires. Tilt the vial to the right, and the mercury touches the bottom and right wires.

The Coiled Bimetallic Strip Thermometers

Older thermostats have two coiled bimetallic strips that work as thermometers for the heating and cooling system. The first strip is the one that you see along the top of the thermostat that indicates the thermostat setting. The second bimetallic strip is how the thermostat displays the current temperature in the room. A bimetallic strip is made of two different kinds of metals that are laminated into one strip. Each metal responds to heat and cold by expanding a specific amount based on the change in temperature. When heat is applied to the coiled bimetallic strip, it expands and uncoils. The strip shrinks and coils tighter in response to applying cold temperatures. The coil is attached to the mercury switch, so as the coil unwinds and contracts, the vial of mercury of tilted to the left or right.

Thermostat Switches

Switches inside the thermostat work to control the heat/cool setting and the circulation fan. When you move a switch from heat/cool or on/off, you are actually moving small metal balls along a channel to make contact with a specific part of the control card inside the thermostat.

Putting It All Together

Turning on the heat in your home by increasing the temperature setting moves the mercury switch and bimetallic coil to the left. Electrical current flows through the mercury inside the vial to a relay to turns on the circulation fan and heater. As the room temperature increases, the coils slowly unwinds and starts tipping the mercury switch to the right. Once the vial of mercury levels out, the electric current stops moving through the mercury, thus powering off the heater and circulation fan.

The same process is used for your air conditioning system, except that the mercury switch is tilted to the right, thus powering on the cooling components of your HVAC system.

Energy Efficiency and Digital Thermostats

Digital thermostats are more efficient than mechanical thermostats because complex algorithms control the powering of the HVAC system. The devices are also more reliable because they have fewer moving parts inside the device. If you would like to learn more about the options for digital HVAC controls, contact And Services, and we will send a technician tour home to do a comprehensive HVAC evaluation.