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Thermostats are intriguing devices. You spend so much money on HVAC components like boilers and air conditioning, and yet their operation depends on this little device. It is also the part of your HVAC system that you interact with the most.

Hence, it's only natural to want to understand what goes on behind the thermostat, and how it functions in an HVAC system. Let's start with the basics: what is a thermostat, really?

What is a Thermostat?

A thermostat is a device, typically mounted on a wall, that enables you to control the temperature in your house. It uses temperature sensors to sense the current climate and then operates the HVAC system accordingly to raise/drop the temperature to the required levels. It is the point of control for the entire HVAC system and has mechanical or digital interfaces which humans can use.

Difference between a Thermostat and a Thermometer

Though the words thermostat and thermometer sound similar, they are actually quite different and not interchangeable at all. A thermometer is a device you use to measure the temperature. You might have come across one at the doctor’s office when you had a fever. Thermostats control and manage the temperature of an area.

Basic Working Principle

Your thermostat is connected to your heating and cooling system. Using the buttons, display, or a phone app, you can determine the desired temperature of the house. Then, the thermostat will measure the temperature of the air (using built-in or additional sensors) and trigger heating or cooling to turn on in order to match your desired temperature. 

If you’d like to turn the heat up, the thermostat will instruct the boiler to send more hot water to the radiators and warm up the house. If you’d like some cool air, the thermostat will instruct the boiler to turn off and, in some cases, turn the air conditioning unit on. 

Types of Sensors

Mercury Thermostats

Mercury thermostats measure and control the temperature in the house through the use of mercury. The way it works is that mercury is responsive to temperatures. Changes in the temperature lead to mercury contracting or expanding. When that happens, it activates a switch that signals to the HVAC system to either turn the heat up or down. A mercury thermostat can have a number of switches, depending on the number of systems you’re working with.

Bi-metallic Mechanical Thermostats

A bi-metallic thermostat operates through two pieces of metal. The strip bends or deforms in relation to the temperature of the room. Each temperature corresponds to a particular shape of the strip. For most thermostats, the hotter the temperature is, the more the strip bends. As the strip bends, the thermostat is triggered and it operates the HVAC system accordingly.

The strip actually consists of two metals that expand/contract at different rates with changes in temperature. For example, if the temperature of the strip rises, one metal will expand more than the other. As both metals are stuck together, the strip bends to accommodate the uneven expansion of both metals.

Metallic Disc Thermostats

Thermostats with bi-metallic disks work in the same way as those with bi-metallic strips. The design is a bit different, but they operate on the basis of two metal objects heating up and cooling down. As you can imagine, metallic discs have a different shape. However, they contract and expand with temperature changes just like a metallic strip would. When the circuit is completed, the disc “snaps” and changes shape. 

Modern Electronic Sensors

Digital electronic thermostats have a different way of affecting your HVAC system and reading the temperature. The thermistor allows the thermostat to measure the temperature in the house. The thermostat then compares this temperature to the temperature set by you. If there is a difference, it sends a signal to the heating or cooling system.

Many modern electronic thermostats are also “smart”. The majority of the newer models feature a clock, which lets you program your thermostat for the future to fit your schedule. A smart thermostat can be controlled remotely using a phone app.

HVAC Operation

The thermostat is the heart and soul of your HVAC system. Without it, the HVAC system itself would not be very useful, as it wouldn’t be able to sense the temperature of the air, receive a signal from you, and adjust the temperature accordingly. 

Your thermostat senses the air temperature in the house and sends signals to the HVAC system to regulate it in accordance with your settings. For example, let’s say you’d like it to be x degrees in your house. When the air temperature falls below that number, the thermostat sends a signal to the heater to start pumping more hot water until the desired temperature is reached. Then, if it gets too hot, the thermostat signals the heater to stop.


If the air was too hot to begin with, the thermostat will signal to the air conditioner to start working and blow cool air until the desired temperature is reached. Then, the whole HVAC system works on maintaining the desired temperature at the smallest cost to you.

To put it simply, the thermostat is right in the middle of your HVAC system. It receives signals from users, assesses the environment, and communicates further signals to the boiler or AC unit. Different types of thermostats may have different components, but all work in a similar manner. 

Smart thermostats take it one step further and let you plan your heating and cooling needs in advance. Using a good thermostat can shave some money off your electricity bill.

Modern Smart Thermostats

Modern smart thermostats make the newest addition to the thermostat family. Though they sense the temperature and communicate with the rest of your HVAC system the same way non-smart thermostats do, they have some distinctive features that set them apart from all the other models.

For starters, smart thermostats usually come with a phone app. You can use that app to control your HVAC system remotely. Let’s say you’re at work and you want your pet to be warm and cozy, so you can crank up the heat at home through your phone. If you’re coming back home after a hot day and need some fresh air, turn on the AC while you’re on your way!


Another feature of smart thermostats is that they are programmable. Say you wake up at 6 am every day, leave for work at 8, and come back home at 6. You know you tend to get cold in the mornings, so tell your thermostat to start turning the heat up at around 5. There is no reason for the heat to be on when you’re at work. You can schedule for the heat to turn off during that part of the day. Finally, program the thermostat to start turning the heat back up at around 5, right before you come back home. 

That way you can save money on your electricity bill and always be comfortable in your home. Saving energy is a big part of the appeal of smart thermostats. 

The Verdict

Thermostats are not complicated, they are specially designed so that they are easy to use, and can perform multiple functions. If we could sum up everything in this article, we’d say the thermostat is the brain of your HVAC system. It senses the climate, figures out what is needed, and operates the HVAC devices accordingly.


Nest Thermostat

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