Temperature Measurement

Thermostat Working Principle

What is Thermostat?

Thermostat Working Principle 1
  • A Thermostat is a device that is used to regulate and control the temperature of a heating or cooling system in a building or other controlled environment. 
  • A thermostat maintains the temperature in a controlled environment by comparing the current temperature to a user-defined setpoint and using that comparison to control a heating or cooling system. 
  • Thermostats are commonly used in homes, offices, industrial facilities, and various other settings to ensure comfort and energy efficiency.
  • The thermostat works on the principle of feedback control, where it continuously monitors the temperature and adjusts the heating or cooling system as needed to maintain the set point temperature.
  • The basic working principle of a thermostat involves sensing the current temperature and comparing it to a desired or set temperature, then taking action to maintain the desired temperature by controlling the heating or cooling system.
  • This basic principle is used in various types of thermostats, from simple manual ones to advanced programmable or smart thermostats that offer additional features and capabilities for energy efficiency and comfort control.

The Science of Bimetallic Thermostat

Setting the Temperature

A dial allows us to select the temperature at which the heater is required to turn on or turn off.

Bimetal Strip: 

The key component of a bimetallic thermostat is the bimetallic strip, which is usually made by joining two different metals with different coefficients of thermal expansion. 

The Bimetal Strip is composed of two metals such as iron and brass bolted together. 

When heated Iron doesn’t get much heated compared to brass, so the strip bends inward when hot.

Electrical Circuit: 

The bimetal strip in the circuit is a component of an electric path. 

This bimetallic strip acts like a bridge when the strip is cool and straight and the heater is on.

As the movable contact moves, it either makes or breaks the electrical circuit with the fixed contact. When the temperature is below the setpoint, the bimetallic strip bends to close the circuit, turning on the heating system. Conversely, when the temperature exceeds the setpoint, the strip bends in the opposite direction to open the circuit, turning off the heating system.     

What are the different types of Thermostats?

Thermostats are classified into  

  1. Mechanical Thermostats
  • Bimetallic Strip Thermostats
  • Liquid-Filled Thermostats
  1. Electronic Thermostats
  • Digital Thermostats
  • Programmable Thermostats
  • Smart Thermostats
  1. Hybrid Thermostats
  2. Line Voltage Thermostats
  3. Low Voltage Thermostats
  4. Pneumatic Thermostats
  5. Wi-Fi-Enabled Thermostats
  6. Touchscreen Thermostats

Typical working of Thermostat

Thermostat Working Principle 2

Temperature Sensing

  • The thermostat is equipped with a temperature-sensing component or temperature sensor, such as a thermocouple, thermistor, bi-metallic strip, or electronic sensor. 
  • This component measures the current temperature of the area or environment being controlled.

Set Point or Desired Temperature

  • The user sets the desired or target temperature on the thermostat which is referred to as the set point.
  • This is the temperature at which they want the environment to be maintained.


  • This thermostat element continuously compares the current sensed temperature (measured by the sensor) to the desired temperature (set point).
  • The thermostat initiates corrective action if the sensed temperature deviates from the set point, 

Control Action

  • Based on the comparison of the current temperature and the desired temperature set point, the thermostat generates a control signal to take action to either turn on the heating element or turn on the cooling system.
  • This control action can be achieved through various mechanisms:

For Heating Systems

  • If the temperature is too low or falls below the set point, the thermostat will send a signal to activate the heating system, such as a furnace or electric heater to increase the temperature. 
  • Once the temperature reaches the set point, the thermostat will turn off the heating system.

For Cooling Systems

  • If the temperature is too high or rises above the set point, the thermostat will send a signal to activate the cooling system, such as an air conditioner or a refrigeration unit, to reduce the temperature. 
  • When the temperature reaches the set point, the cooling system is turned off.

Feedback Loop

The thermostat is built with a feedback loop to ensure that the temperature remains very near the set point. 

Continuous Monitoring

  • The thermostat continuously monitors the temperature of the room or environment as the temperature approaches the desired set-point, the thermostat makes minute adjustments to the heating or cooling system as required maintaining the desired temperature.
  • The thermostat may also use a hysteresis band to avoid frequent on-off cycling when the temperature is close to the set point.

Maintenance of Set-point

  • The feedback loop of the thermostat element maintains the room temperature within a specified range of the set point. 
  • If the temperature falls below or rises above this range, the thermostat activates the heating or cooling system again to correct it. 
  • The heating or cooling system responds to the control signal and begins heating or cooling the room. As the system operates, it affects the room temperature.

User Interaction

  • Most modern thermostats have advanced technology that allows users to set the desired temperature set point, program schedules, and make adjustments as necessary. 
  • Some thermostats come with more advanced features such as programmable schedules, remote control through the smartphone app, or integration with other smart home systems.

Many thermostats allow users to manually override the setpoint for temporary adjustments. For instance, users can set a lower temperature during periods when they’re away from home to save energy or a higher temperature for added comfort.     

Advantages of Thermostats

  1. Precise temperature control
  2. Energy efficiency
  3. Convenience and easy adjustment
  4. Integration with other systems
  5. Can lead to significant energy and cost savings
  6. Maintains consistent and comfortable temperatures.
  7. Smart thermostats offer remote control via smartphone apps
  8. Prevents unnecessary strain on HVAC equipment.
  9. Programmable and smart thermostats can automate temperature adjustments.
  10. Some smart thermostats can learn user preferences for improved efficiency.

Disadvantages of Thermostats

  1. Complexity and higher cost
  2. Compatibility issues with heating and cooling systems
  3. Dependence on power (electricity)
  4. Potential for inaccurate readings
  5. Maintenance and possible battery replacements
  6. Some older or lower-quality thermostats may not provide precise temperature control, leading to temperature variations.
  7. Smart thermostats with complex features may have a learning curve, requiring time to set up and fully utilize their capabilities.
  8. Selecting the right thermostat may require compatibility with your specific HVAC system, and some older systems may not work well with newer thermostats.

Applications of Thermostats 

  1. Residential heating and cooling systems
  2. Commercial building climate control
  3. Automotive cooling systems
  4. Industrial temperature regulation
  5. Refrigeration systems
  6. Greenhouses
  7. Aquarium temperature control
  8. Medical equipment temperature regulation
  9. Cooking appliances like ovens and grills
  10. Water heating systems
  11. Research and medical laboratories use precision thermostats for temperature-sensitive experiments and equipment.
  12. Thermostats are employed in ovens, stoves, and grills to maintain cooking temperatures.
  13. Thermostats are used in heating systems, such as boilers, to maintain the desired water or steam temperature.
  14. HVAC Systems: Heating, ventilation, and air conditioning (HVAC) systems in commercial and residential buildings rely on thermostats for temperature control and energy efficiency.
  15. Data Centers: Precise temperature control is crucial in data centers to protect servers and equipment from overheating.
  16.  Thermostats are used in solar water heating systems to control the circulation of heated water.
  17.   In manufacturing and chemical industries, thermostats are used to regulate temperatures in various processes.
  18.   Portable heaters and fans often include thermostats for temperature regulation.
  19.  Smart thermostats are integral components of energy management systems in smart homes and buildings.

Frequently Asked Questions

How Does a Thermostat Work?

The thermostat works on the idea of “Thermal Expansion.”

What are the functions of Thermostats?

  • Turning on the timer heating
  • Programming work according to a certain schedule

Is the thermostat a sensor?

A thermostat is a contact-type temperature sensor consisting of a bi-metallic strip with two dissimilar metals such as a combination of aluminium, nickel, copper, or tungsten.

What are the different types of thermostats?

There are two different types of thermostats: 

  1. Line voltage thermostats.
  2. Low voltage thermostats.

How a thermostat does indicate the temperature?

The thermostat consists of a sensor mounted that determines & indicates temperature through infrared radiation.

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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