What is a Thermostat

You’ve probably seen or used a thermostat a thousand times. This device controls the heating and air-conditioning systems in your house — the two pieces of equipment that use the most energy. In these days of rising energy prices, you might be interested to see how your thermostat works. It is surprisingly simple and contains some pretty neat technology.

In this article, we’ll take apart a household thermostat and learn how it works. We’ll also learn a little about digital thermostats, talking thermostats, telephone thermostats and system zoning. Let’s start by taking a look at the parts of a basic thermostat.

The mercury switch is a glass vial with a small amount of mercury in it. Mercury is a liquid metal — it conducts electricity and flows like water. Inside the glass vial are three wires. One wire goes all the way across the bottom of the vial, so the mercury is always in contact with it. One wire ends on the left side of the vial, so when the vial tilts to the left, the mercury contacts it — making contact between this wire and the one on the bottom of the vial. The third wire ends on the right side of the vial, so when the vial tilts to the right, the mercury makes contact between this wire and the bottom wire.

mercury switch
The mercury switch

There are two thermometers in this thermostat. The one in the cover displays the temperature. The other, in the top layer of the thermostat, controls the heating and cooling systems. These thermometers are nothing more than coiled bimetallic strips. We’ll look at them in more detail next.

Thermometers and Switches

bimetallic strip is a piece of metal made by laminating two different types of metal together. The metals that make up the strip expand and contract when they are heated or cooled. Each type of metal has its own particular rate of expansion, and the two metals that make up the strip are chosen so that the rates of expansion and contraction are different. When this coiled strip is heated, the metal on the inside of the coil expands more and the strip tends to unwind.

thermometer coil
A thermometer coil with bimetallic strips attached

The center of the coil is connected to the temperature-adjustment lever, and the mercury switch is mounted to the end of the coil so that when the coil winds or unwinds, it tips the mercury switch one way or the other.

Circuit Card and Switches
This thermostat contains two switches. The switches move small metal balls that make contact between different traces on the circuit card inside the thermostat. One of the switches controls the mode (heat or cool), while the other switch controls the circulation fan.

The two switches: The fan switch moves one metal ball,
and the mode switch moves two.

Inner Workings

When you move the lever on the thermostat to turn up the heat, this rotates the thermometer coil andmercury switch, tipping them to the left.

thermostat layer
Inside the thermostat, there are two layers of controls.
The top layer houses the mercury switch and thermometer coil.

As soon as the switch tips to the left, current flows through the mercury in the mercury switch. This current energizes a relay that starts the heater and circulation fan in your home. As the room gradually heats up, the thermometer coil gradually unwinds until it tips the mercury switch back to the right, breaking the circuit and turning off the heat.

When the mercury switch tips to the right, a relay starts the air conditioner. As the room cools, the thermometer coil winds up until the mercury switch tips back to the left.

Heat Anticipator
Thermostats have a neat device called a heat anticipator. The heat anticipator shuts off the heater before the air inside the thermostat actually reaches the set temperature. Often, some parts of the house will reach the set temperature before the part of the house containing the thermostat does. The anticipator shuts the heater off a little early to give the heat time to reach the thermostat.

The anticipator is a ring of resistive wire on the dial.

The loop of wire above is actually a resistor. When the heater is running, the current that controls the heater travels from the mercury switch, through the yellow wire to the resistive loop. It travels around the loop until it gets to the wiper, and from there it travels through the hub of the anticipator ring and down to the circuit board on the bottom layer of the thermostat. The farther the wiper is positioned (moving clockwise) from the yellow wire, the more of the resistive wire the current has to pass through. Like any resistor, this one generates heat when current passes through it. The farther around the loop the wiper is placed, the more heat is generated by the resistor. This heat warms the thermometer coil, causing it to unwind and tip the mercury switch to the right so that the heater shuts off.

Nice, Karim, and Patrick Brothers.  “How Home Thermostats Work.”  22 August 2007.  HowStuffWorks.com. <http://www.howstuffworks.com/home-thermostat.htm&gt;  30 July 2010.

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