How an AC Thermostat Works: Parts, Functions, and Common Issues

Published: 20 May 2025

Welcome

Why does your air conditioner not always keep the temperature as perfect as you’d like? Or maybe you’re confused about how the thermostat inside your AC controls the temperature.

Well, you’ve come to the right place! In this blog, I’ll explain everything you need to know about AC thermostats, from their parts to how they work to keep your home comfortable.

By the end, you’ll fully understand how your AC thermostat operates and be able to use it like a pro.

Please stick with me; I promise you’ll be an AC thermostat expert soon!

Table of Content

What is an AC Thermostat And How Does It Work?
What Are the Main Parts of an AC Thermostat?
What are the common faults in the thermostat?
1. Compressor Running Continuously
2. Compressor Not Running
What are the solutions for a faulty thermostat?
Conclusion

What is an AC Thermostat And How Does It Work?

An AC thermostat is a device that controls the climate in your home. It senses the room’s temperature and tells your air conditioner when to turn on or off, ensuring your home stays at the temperature you’ve set.

Think of it as the “brain” of your AC, working behind the scenes to ensure your comfort all day long.

Let’s say it’s a hot summer day, and you’ve set your thermostat to 72°F (22°C). You’re relaxing in your living room, and as the temperature climbs to 78°F (25°C), your thermostat knows precisely what to do. It sends a signal to your AC to start cooling the room. Once your room reaches 72°F again, the thermostat tells your AC to stop.

It’s like your own personal temperature controller in your house, always working to ensure your home has the right climate for you!

What Are the Main Parts of an AC Thermostat?

It’s essential to know the main parts of your AC thermostat. These parts work together to keep your home cool and comfortable.

Once I explain them to you, you’ll see how easy it is to understand how the parts of the thermostat work. Each part plays a key role in keeping the temperature right while keeping energy use efficient and low.

Let’s dive into the parts of an AC thermostat and how each helps cool your space.

1- Feeler Bulb

The feeler bulb has a special liquid called volatile liquid or refrigerant. When you turn on the AC, the room temperature is high, so this liquid gets heated, starts to boil, and turns into gas. This is why warm air comes out at first. Once the thermostat senses the temperature, it starts cooling the room.

Think of it like boiling water on a stove—the liquid inside the feeler bulb heats up and starts to change, like water turning into steam when it gets hot.

2- Sensor

Imagine sitting in your living room, and the temperature feels just right. How does your thermostat know when to keep the AC running or when to turn it off? That’s where the sensor comes in.

The volatile liquid passes through the sensor after the feeler bulb, and the sensor’s job is to constantly “feel” the room’s temperature. It keeps track of whether the room is getting too hot or too cold, and sends signals to the thermostat to adjust the cooling.

Think of the sensor like a watchful friend who watches the weather outside. When the temperature changes, your friend quickly lets you know so you can decide if you need a jacket. Similarly, the sensor keeps your AC in check, ensuring the climate is always comfortable.

3- Bellow

After passing through the sensor, the refrigerant or volatile liquid moves into the bellow. If the liquid is hot, it increases the pressure inside the bellows, causing them to expand and push the lever to the right side. On the other hand, if the liquid is cool, it decreases pressure inside the bellows, making it shrink and push the lever towards the left.

This movement helps the thermostat control the AC, keeping the room at the right temperature.

Imagine you’re in a car and adjusting the air conditioning. If it gets too hot, you turn the dial to cool it down. When it’s too cold, you turn it off. The bellow works the same way. It helps the thermostat change the AC to keep the environment comfortable, like you control the car’s air conditioning.

4- Lever

When the bellow expands due to pressure, they push the lever to the right. The lever has a contact at the top. There are two levers: a fixed lever with a fixed contact and a movable lever with a movable contact. When the movable contact meets the fixed contact, it completes the circuit, allowing voltage to flow. These voltages pass through the contacts to the compressor’s starting relay, which turns the compressor on.

Think about how a doorbell works when you press the button. The button has a movable part inside, which, when pressed, touches a fixed part. This connection completes the path, and the doorbell rings. Similarly, the lever in an AC thermostat works when the bellow moves.

5- Spring With Range Adjustment Screw

The spring’s job is to control the tension. When the temperature gets high, the bellow expands and create pressure. This pressure makes the spring tighten. As the tension in the spring increases, it moves left and right, which helps the thermostat decide whether to turn the compressor on or off.

The range adjustment screw is used to change the temperature setting manually. To increase the temperature, you turn the screw counterclockwise, which loosens the spring and raises the temperature. To lower the temperature, you turn the screw clockwise, which tightens the spring and lowers the temperature.

6- Differential Screw

Before understanding the differential screw, it’s essential to know what “differential” means. In simple words, differential is the difference between the cut-in and cut-off temperatures.

Cut-off Temperature: This is the point when the compressor turns off.
Cut-in Temperature: This is the point when the compressor turns on again.

The differential screw adjusts the gap between the moveable contact and the fixed contact. By changing this gap, you can set the difference between the cut-in and cut-off temperatures. For example, if the cut-in temperature is 24°C and the differential is set to 3°C, the cut-off temperature will be 27°C.

The differential is usually set to 3°C by default. Changing this too much can harm the compressor, so it’s essential to adjust it carefully.

What are the common faults in the thermostat?

There are two most common faults in the thermostat that cause its unserviceability

1. Compressor Running Continuously

This happens when the fixed and movable contacts inside the thermostat get welded together. When they stick together, the voltage path remains “permanently” on, keeping the compressor running constantly. This causes a high electricity bill, and if not fixed, the compressor can get damaged from running nonstop.

2. Compressor Not Running

This occurs when carbon builds up between the fixed and movable contacts. The carbon prevents the contacts from connecting properly, breaking the voltage path, and stopping the compressor from turning on. As a result, your AC won’t cool the room as it should.

What are the solutions for a faulty thermostat?

If your thermostat is faulty, the only solution is to replace it. Unfortunately, there is no quick fix for a broken thermostat. Once it’s damaged, the best option is to install a new one to ensure your AC system works properly.

Conclusion

In this article, I’ve explained how an AC thermostat works, from the feeler bulb to the differential screw. Each part, such as the sensor, bellow, lever, and spring with a range adjustment screw, plays a role in keeping your home at the right temperature.

Understanding these parts lets you control your AC more effectively and ensure it runs smoothly. Thus, you can manage your system more efficiently and keep your space comfortable.

Can I fix a broken AC thermostat myself?

In most cases, No. The thermostat must be replaced if internal parts like contacts are damaged or welded together. DIY fixes are usually temporary and can lead to bigger problems.

How often should I replace my thermostat?

Most thermostats last 10–15 years, but you should consider replacing them sooner if they malfunction or if you’re upgrading to a smart thermostat for better control and efficiency.