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How does a Thermostat Work?


A thermostat is a device that is used to control a heating or cooling system so that it maintains a certain temperature or keeps the temperature within a certain range. In a home, for example, this device can automatically turn on the heating system when the temperature in the home drops or turn on the air conditioning when it gets too hot. As the warm or cool air fills the room and the desired temperature is achieved, the device then turns off the system. There are two main types of thermostats: mechanical and digital. Many digital thermostats also are programmable, which allows a user to set different preferred temperatures for different times of the day or days of the week.

Mechanical Thermostats

Nest Learning Thermostat.
Nest Learning Thermostat.

A mechanical thermostat uses physical means to gauge the air temperature and activate a switch that turns on a heating or cooling system on turns it off. There are several types of sensor technology that can be used in mechanical thermostats, such as bimetallic strips, wax pellets, bulbs filled with gas or tubes filled with air. Each of these types of sensors will react to changes in temperature, such as by expanding or contracting, and will trigger the proper switch to raise or lower the temperature. Bulbs filled with mercury were once commonly used in thermostats, but its use has been discontinued or outlawed in many places because of the dangers of mercury.

Classic round Honeywell thermostat.
Classic round Honeywell thermostat.

The most common of these technologies in home thermostats is the bimetallic strip. This technology uses two thin strips of different metals — such as copper and iron, copper and steel and brass and steel — bonded together and rolled into a coil. As the temperature changes, the different metals expand or contract at different rates, causing the strip to bend. When the strip bends enough to touch an electrical contact and complete an electrical circuit, it turns on the heating or cooling system. If the temperature changes enough to unbend the strip, contact is lost, and the system turns off.

Digital Thermostats

Digital Thermostat.
Digital Thermostat.

Many homes now have digital thermostats, which use electronic sensors instead of physical means to monitor changes in temperature. Based on the temperatures read by the electronic sensors, these devices turn on or off the cooling or heating systems as needed. A digital thermostat usually requires one or more batteries for power. Buttons and switches allow the user to control the settings, and a display screen shows the settings as well as the current temperature.

Setting a Thermostat

Programmable thermostats can be set to automatically change the temperature when a person is sleeping.
Programmable thermostats can be set to automatically change the temperature when a person is sleeping.

To set a mechanical thermostat, the user typically must turn a dial or move a lever. One common type of mechanical thermostat has a dial that has a range of temperatures printed on it, and the user simply turns the dial until a small arrow or line is on the preferred temperature. Another type is rectangular and has two levers that allow the user to set a minimum temperature and a maximum temperature. These thermostats often also have switches that allow the user to turn on or off the heating or cooling system as well as any ventilation fans that might be part of the system.

When setting digital thermostats, in most cases, the users simply look at the display screens and push buttons to input the desired settings. Some newer models feature touch screens instead of separate buttons and display screens. For a programmable model, the user could choose to have different setting for certain times, such as when the home's residents are sleeping, when they are first waking up or when they are away at work or school. Programmable settings not only can make a home more comfortable, they also can conserve energy by keeping the heating or cooling system from turning on or off unnecessarily, such as when nobody is home. Depending on the model, these programmed times might be when the device begins adjusting the temperature, or the device might begin working earlier so that the desired temperature is achieved at the programmed time.

Ideal Home Settings

Most people set their thermostats to the temperatures or ranges at which they are the most comfortable. Some people, however, prefer to conserve energy and save money by adjusting their settings by a few degrees. Many people consider the most comfortable room temperatures to be about 70° Fahrenheit (21.1° Celsius) to 72° Fahrenheit (22.2° Celsius). Most people don't notice the difference between 72° Fahrenheit (22.2° Celsius) and 74° Fahrenheit (23.3° Celsius), for example, so setting a thermostat to turn on the air conditioning at the slightly higher temperature can save energy and money without significantly affecting the comfort level in the home.

Some experts recommend setting a home's thermostat so that the heat turns on at 67° Fahrenheit (19.4° Celsius) and the cooling system turns on at 77° Fahrenheit (25.6° Celsius). These settings can be adjusted for different seasons of the year, partly because people generally wear more clothes during colder months and less clothes during warmer months, no matter whether they are inside or outside. People often are more comfortable sleeping at lower temperatures, so programmable thermostats could be set to allow the temperature to drop to about 62° Fahrenheit (16.7° Celsius) during normal sleeping hours.

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Discussion Comments


The thermostat on our outdoor furnace is reading "SP". What does this mean?


If there is not anything showing on the thermostat, does that mean it is broken? My air conditioner is not coming on this morning, and last night it was fine. When I got up this morning it was hot, so I went to check the temperature and nothing was on the screen. Can you help me? I am 70.


My co-worker sets the AC thermostat at 60 degrees when the room gets too hot to "cool it to 70 faster". He is dumb, right?


Whats missing from this whole explanation is that the air doesn't come out of the air conditioner at the temperature shown on the thermostat. The air is much colder and then switches on and off until the average room temperature is that shown on the thermostat.


I have a window type ac with a manual thermostat controller and with numbers from 1 to 10 and i cannot regulate my room temperature to cool down. my question is how, if I want my room to be more cold will i turn the thermostat up or down?


If you use two different brands of toilet paper can you make an extremely sensitive thermostat?


This article is terrible. The bimetallic strip is made of two metals, copper and iron. It is about four inches long. One side of this strip is copper, and the other side is iron. Copper expands a little bit more than iron when it gets heats up, so the copper side gets a little bigger, causing the strip to bend a little bit toward the iron side. Imagine you had two young snakes stapled together all along their lengths, and you only fed one of them. One would grow longer, but being stapled to the other snake, it would start to curl around and bend toward the shorter snake. A thermostat is made in such a way that the bending wire touches an electrical contact when it bends a certain distance, starting the air conditioning. -Rob Cockerham


can you put it in simple english? it is quite complicated. i'm only 14 years old. help me please


Okay, when I want a room to be warmer I turn on 'heat' and turn it to higher degrees. But if I want the room cooler I turn it to 'cool' but do I turn it higher or lower for cooler air? Also what does 'fan/auto' control mean to me when it is on 'cool'? Help! :)


I have a problem in that when my AC starts up the the picture on one of my TVs blanks out for a couple of seconds. This TV is connected to a cable box. I have two other TVs on cable boxes and they are not affected. I assume that this TV is in some way on the same circuit as the AC, however, the blower motor is 220VAC and the cable box is 120VAC. Could it be that the thermostat powering the motor is wired to the same circuit as the outlet supplying power to the TV box?


This is a brand new thermostat and was very expensive so I do not again understand why when set at65 and room temp. is 65 that my furnace keeps going on.


Yes; the metals have diff. linear expansion values, which causes them to bend and trigger the heating/cooling system; simply put. Digitals are far more reliable because they don't rely on this property. They are far more stable as well (physically, they can take more beating), which makes them ideal for temperature control in mobile environments.


this is very clear thank you


i don't understand. i thought something bends when it gets too hot.


A bimetallic coil has two strips of metal bonded together, sort of like 2-ply toilet paper (with one type of metal being one ply and the other type being the other). As the coil heats up, the metals expand at different rates, causing the coil to rotate.


how does the bimetallic strip work in the thermostat?

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