What is thermostat?

A thermostat is described as an instrument that maintains the temperature of a given system, for example, an air-conditioner, refrigerator, and many others. It almost works similar to a valve whose opening and closing totally rely on the temperature function. This device can be applied for heating and cooling purposes in any close space such as a room.

Some of the essential points about thermostat are mentioned below:

  • The thermostat can control the temperature inside your house.
  • The thermostat relies on the thermal expansion principle for solid elements.
  • Resistance change could be measured by using the digital thermostat.
  • In poultry incubators, the thermostat is applied to regulate the temperature.
  • The thermostat in several circuits controls the excess flow of current.
  • In automobiles, thermostats are used for cooling purposes.
  • Thermostats don't consume much power and are economical.

What are basic types of thermostats?

There are basically four types of thermostats, which are listed as below:

  • Non-programmable thermostat
  • Programmable thermostat
  • Wi-fi thermostat
  • Smart thermostat

Non-programmable thermostat

The non-programmable thermostat is one of the primary and easy to operate thermostats among all types. This type of thermostat is widely used in the home because its performance can be adjusted quickly. A non-programmable thermostat attains excellent efficiency by working with a heating operation. Pre-programming of the thermostat is not possible in a non-programmable thermostat.

Programmable thermostat

The programmable thermostat is used in the regions where drastic variation in climate happens. In a programmable thermostat, pre-programming is possible, and this helps in the reduction of energy consumption. The temperature change takes place without manually adjusting the thermostat device. Some of the benefits of a programmable thermostat are advanced features, economical energy bills, reliable precision, system alerts, and improved temperature control.

Wi-fi thermostat

The wi-fi thermostat is one of the types of thermostats that works on the wireless network. It is quite similar to a programmable thermostat, whose benefits are included in a wi-fi thermostat. A wi-fi thermostat helps to control or maintain the temperature of a room through the internet from any position away from the room. In several industries, a wireless thermostat plays a crucial role by altering the temperature of the modern boiler.

Smart thermostat

The smart thermostat is very compatible, but the initial cost is higher as compared to other thermostats. Also, smart thermostats are not easy to operate as well as control. In this, pre-programming can be achieved on which weather conditions need not be adjusted manually. One of the disadvantages is that the smart thermostat can fail because of software issues.

Difference between thermometer and thermostat

  • A thermometer is designed for estimating the temperature of any specific component, whereas the thermostat is used to control and regulate the surrounding temperature.
  • In a given room, the thermometer will indicate whether the room is hot or cold, but on the other hand, the thermostat will make adjustments to maintain the temperature hot or cool accordingly.
  • The construction of the thermometer and thermostat are very different. Some of the parts of the glass thermometer are mercury bulb, stem, capillary tube, scale lines for reading, and expansion chamber. Some of the parts of a thermostat are the rod, charge cylinder, bypass valve, spring, and seal.
  • Some common types of thermometers are thermocouples, gas thermometers (constant volume), silicon bandgap sensors. Smart thermostats, Wi-fi thermostats, and programmable thermostats are some variety of thermostats.

Glass thermometer

The glass thermometer is represented as the instrument or sensor which indicates the temperature of a body. The range of the scale used in the thermometer is generally between -80 degrees Celsius and 300 degrees Celsius. Mostly, mercury can be used as the fluid inside this instrument rather than alcohol because its coefficient of thermal expansion is higher, and the mercury will expand or rise more as compared to alcohol. However, in cold regions, glass thermometers contain alcohol as it has a low freezing point, and it won't freeze inside the tube.

The following is the simple diagram of glass thermometer.

A glass thermometer is represented in the diagram. Bulb, capillary tube, Bore, scale are some parts of glass thermometer.
Glass thermometer

Glass thermometer has many components, which are given as below:

  • Stem
  • Mercury Bulb
  • Capillary tube
  • Expansion chamber
  • Scale lines


In a glass thermometer, the stem is the longest part amongst all, which is made up by using annealed glass. The mercury bulb on the bottom is designed in such a way that it will enter the hollow shaft (stem).

Mercury bulb

The mercury bulb is created by using stainless steel, and its purpose is to accumulate the amount of mercury. In response to heat, the mercury expands or contracts, and reading can be observed in the temperature scale. Mercury can be harmful to the surroundings if it discharges from a broken or damaged thermometer.

Capillary tube

The capillary tube is the cylindrical-shaped tube that is fastened to the bottom part of the mercury bulb. As the temperature rises or falls, the mercury flows up and down in the capillary tube, respectively. A scale is located on the tube for estimating the temperature.

Expansion chamber

In the glass thermometer, a large volume of mercury is formed in the expansion chamber. When the highest temperature is reached, then mercury can be filled through this chamber for further analysis. It is connected to the capillary at the top of the mercury thermometer.

Scale lines

The scale lines indicate the rise and fall in temperature of the particular body. Equal divisions of degrees are marked on this instrument, and the measurement units are degrees Celsius or Fahrenheit, relying on the type of thermometer.

Working of bimetallic thermostat

The following two figures represent thermostat switches on and off.

The illustration represents a bimetallic thermostat when the switch is off.
Thermostat at switch off
The illustration represents a bimetallic thermostat when the switch is on.
Thermostat at switch-on

Some important points related to the bimetallic thermostat are explained below.

  • Point 1 in the illustration represents an outer dial where the temperature is adjusted for switching on and off the thermostat.
  • Point 2 indicates the connection of the outer dial at point 1 and the temperature sensor within a circuit arrangement. The temperature sensor is a bi-metal strip that switches circuits on and off.
  • Point 3 indicates the bi-metal strip, which is made up of combining two disparate metal pieces. In this case, one of the metals is brass, and the other is iron.
  • Point 4 represents the expansion of the brass rod as the temperature increases. Brass expands more than iron because the forces among particles are weaker. Also, the bimetal strip moves inward and forms a curved shape due to an increase in temperature.
  • Point 5 indicates the part of the circuit formed by the strip. As the bimetal strip's temperature reduces, it becomes straight and acts as a path for electricity flow through the circuit. Now, the circuit is on and heating, this leads to bending of the strip. So, after bending, the circuit breaks, and the switch is off.

Common Mistakes

  • Students mostly get confused between thermostats and thermometers because both of them are related to temperature.
  • The smart thermostat is somehow similar to a Wi-Fi thermostat, but still, they are not identical in all aspects. Smart thermostats are very much expensive as compared to the other ones.
  • One of the misconceptions about smart thermostats is that they are easy to operate and install.

Context and Applications

The topic thermostat is very much significant in the several professional exams and courses for undergraduate, Diploma level, graduate, postgraduate. For example:

  • Bachelor of Technology in Mechanical Engineering
  • Bachelor of Technology in Civil Engineering
  • Master of Technology in Mechanical Engineering
  • Doctor of Philosophy in Mechanical Engineering
  • Diploma in Mechanical Engineering
  • Bachelors of Science in Physics
  • Masters of Science in Physics
  • Doctor of Philosophy in Physics
  • Sensors and types of sensors
  • Heating and cooling of system
  • Smart thermostat
  • Ecobee Smart Thermostat
  • Heating element in thermostat
  • Humidity control devices and remote sensors
  • Multi-stage heating and cooling thermometer
  • Line voltage and smart home devices
  • Temperature setting
  • Heat pump

Practice Problems

Q1. A constant temperature is maintained by which device in the electric heater?

a. Thermometer

b. Thermostat

c. Thermocouple

d. All of these

Correct answer: (b)

Q2. At room temperature, the melting of ice cubes occur due to :

a. heat loss

b. heat constant

c. All of the above

d. None of the above

Correct answer: (d)

Q3. In which of the following devices, the uneven expansion of bi-metal strip is utilized?

a. Piezometer

b. Thermocouple

c. Barometer

d. Thermostat

Correct answer: (d)

Q4. On which of the following principles the thermostat works?

a. Bulk expansion

b. differential expansion

c. linear expansion

d. None of the above

Correct answer: (c)

Q5. At -40, which of the following temperature scales co-exist ?

a. Kelvin and Fahrenheit

b. Kelvin and Celsius

c. Fahrenheit and Celsius

d. None of the above

Correct answer: (c)

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