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How does Air Conditioning Work?

Air conditioning is a marvel of modern comfort, ingeniously designed to extract heat from your space, leaving behind a cool, refreshing environment. It operates on the principles of evaporation and condensation, using a refrigerant to transfer heat outside. Intrigued by this clever system? Discover the intricate dance of components that keep you cool on sweltering days by diving deeper into the science behind it.
Michael Pollick
Michael Pollick
Michael Pollick
Michael Pollick

Contrary to popular perception, Air Conditioning (AC) is not about adding cool air to the room, but more about drawing heat away from it. The end result is a space with significantly less heat, which makes it feel cooler to occupants. Air conditioning takes advantage of the effects of evaporation, much like a swab of alcohol makes a person's skin feel cooler as the liquid evaporates. The alcohol doesn't lower the person's skin temperature, but rather draws away heat from the air as it turns to a gas.

Air conditioning units contain a special chemical called a refrigerant, which has the unique ability to change from a gas to a liquid in a short amount of time. A refrigerant called freon is commonly used in AC units, although there are other commercial refrigerants available. The refrigerant is pumped into the unit at the factory, along with a small amount of lubricating oil for the compressor.

Air conditioning works by drawing hot air away from a room.
Air conditioning works by drawing hot air away from a room.

The parts of a typical AC unit usually form a closed system consisting of a compressor, a condenser, an expansion valve and a thermostat. Motorized fans help to circulate the conditioned air, while thin metal fins allow heat to dissipate quickly. The heaviest part of a typical air conditioning is often the compressor, since it must be strong enough to withstand a significant amount of pressure.

A thermostat connected to an air conditioning system.
A thermostat connected to an air conditioning system.

The process of cooling an area begins with the refrigerant entering the compressor, usually located at the bottom of the unit. At this point, the refrigerant is a cool gas. As the gas enters the compressor's inner chamber, the compressor squeezes the refrigerant and the gas becomes a very hot gas under high pressure. This hot gas goes through a series of condensing coils placed outside of the room being cooled. The heat dissipates into the outside air, much like a car's radiator dissipates heat from the engine coolant. Once the refrigerant reaches the end of these coils, it is significantly cooler and in liquid form.

A central air conditioning unit.
A central air conditioning unit.

This liquid is still under high pressure, like the contents of an aerosol can. In the case of air conditioning, the liquid refrigerant is forced through a very tiny opening called an expansion valve. The liquid refrigerant comes out of the other end of the expansion a very small amount at a time. Because the refrigerant evaporates at a much lower temperature than water, it begins to evaporate while traveling through another set of coils. It is this evaporation action that draws heat out of the surrounding air, including the air contained in the room. The unit's fan blows across metal fins placed over these coils, causing the sensation of cooling in the room.

A refrigerant called freon is commonly used in AC units.
A refrigerant called freon is commonly used in AC units.

At this point, the liquid refrigerant has become a cold gas again and re-enters the compressor, where the entire process begins again until a thermostat registers a specific temperature and shuts off the compressor. When the room warms up, the thermostat senses the added heat and the compressor kicks back on to create more of the hot pressurized gas. At some point, the temperature of the room may equal the cooling power of the air conditioner and the compressor will shut off again. The air conditioning systems of most houses do benefit from energy-saving steps such as using window shades and keeping doors closed, since they don't have to work as hard to keep the room at an acceptable level of cool.

Michael Pollick
Michael Pollick

A regular AboutMechanics contributor, Michael enjoys doing research in order to satisfy his wide-ranging curiosity about a variety of arcane topics. Before becoming a professional writer, Michael worked as an English tutor, poet, voice-over artist, and DJ.

Learn more...
Michael Pollick
Michael Pollick

A regular AboutMechanics contributor, Michael enjoys doing research in order to satisfy his wide-ranging curiosity about a variety of arcane topics. Before becoming a professional writer, Michael worked as an English tutor, poet, voice-over artist, and DJ.

Learn more...

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Discussion Comments


Do energy savings AC units apply to dual hose portable air conditioners as well?


Installation of a room heater and an air conditioner should be done in an appropriate place in a room.


Thanks for all the great advice. I could really use air conditioning installation and this is great to know.


How is the oxygen level maintained in air-conditioned cinema halls?


Does covering the outdoor unit under a shade in order to reduce the temp around it help in reducing the power consumption?


Are there any gaseous byproducts being produced by the air-conditioner unit, such as CO2, in the heated air going out the back?


I have a doubt about HVAC equipment. What if we increase the return air flow of the equipment over the design values while maintaining the fresh air flow rate? Would there be any problem? Even though the total supply air is increased, in the evaporator coil there should be no problems or condensation or freezing.

The evaporator fan is able to work under these conditions.

As far as I know, everything will be OK but I'm not an expert. Can anyone help me?


what happens when we interchange an evaporator and a condenser coil in an air-conditioning unit?


if we place a window air conditioner on a table in a room, will the room temp increase?


How does the water pipe help the air conditioner?


@anon59455: Oxygen content does not drop very much, however, it's impossible to get a room fully closed. Infiltration air through windows and doors cracks provide the necessary make up oxygen, while in case of the room was really tightly closed (and that happens rarely) or a complete change of air room is required (hospital applications), there are special devices and machines used for that purpose called fresh air handling units.

--HVAC engineer


@anon17853: You say your AC costs you more than your neighbor's. Well there could be a thousand reasons.

For example, it depends on your AC tonnage and AC brand, because each brand differs in energy consumption.

Another very important reason is that your AC may have a low tonnage capability, so it needs to work harder to maintain the temperature you set, while in normal conditions, a modern AC works about five minutes and shuts off for another five. That's how it saves a lot of energy. This example for about a 300 square foot room, let's say. --HVAC engineer


@#37: In theory, you would be able to turn your car AC into a small portable one. But you'd need electricity to run it. Also, car AC's have been known to leak coolant. So it wouldn't be very safe for you.

@#34: Mini splits don't have TXV (expansion valves) because the amount of refrigerant in them is so low, that they aren't needed.

@#32: ACs leak condensation. If your floor is wet under your AC, then it's not pitched correctly. Put a block of wood under the face of it to raise it up. And, yes. It is normal to see water coming out of the back of a window unit.

@#30: Freon (or Puron) doesn't disappear from the units. It's a sealed system. A service tech should never have to add coolant into the system. The only way coolant could leak from a system is if there's a hole in the lines, the condenser or the indoor coil. The fact that your installer found coolant around the service ports tells me that when he braised the lines into the compressor, he never took out the schrader valves and they over heated and failed! Tell your installer that he is responsible for installing new schrader valves and responsible for adding more coolant into the system after it is evacuated.

@#29: Cars are equipped with As. If you continue to run your AC with your heat, you'll blow the compressor.

@#28: Your unit is sized wrong, or you don't have enough return air flowing over your indoor coil. Change your air filter.

@#17: There are two reasons why you would have ice on your indoor coil. One, poor air flow. Change your filter or add another return. Two, low coolant charge. Contact a service provider that is EPA licensed to come out and look at your unit.

@#13: The temperature in the room would remain the same. The heat you are removing is being expelled into the same area. Not to mention the moisture in the air is draining onto your bathroom floor. @#2: Try installing a programmable thermostat.

@#26: The air conditioning does not remove air from the room. It removes the heat and humidity from the area. The air blowing out of the outdoor portion of your AC is NOT air from inside. It is the heat being expelled from the area.

I hope I was able to answer some questions on this site. My name is Alex. I am the owner of an air-conditioning and heating business.


Help! Is it safe to use an air-conditioning when there is liquid nitrogen inside the room?


Is it possible to convert a spare car ac to a portable one?


Help! I recently installed an a.c in my room with one pipe. The room is not cooling down, I leave the thing running all day and night but it is hitting my wallet. I took the pipe and stuck it in the ceiling but that did not help.

Someone told me i should put the pipe in the fridge as that should cool any heat but that did not work either as heat was leaking out. so i decided to attach a massive balloon to end of the pipe, and now all the hot air goes in the balloon, but the only problem is i have to keep on emptying the balloon.


I'm a student in college. my lecturer asked me why an expansion valve can't be used in a split unit.


I have a question: I just put an a.c into my room but whenever i come into the room, my floor near it is really wet. Are air conditioners supposed to be wet? Also in my backyard, whenever I turn on the a.c, there is water dripping from the a.c's bottom corner. Is that supposed to happen, or should I return it?

I am really new at this so is it okay if any of you guys help me answer this question? Thanks and I am looking forward for an answer! Bye! P.S nice article --Anna S.


I never knew that air conditioning is mostly about drawing heat away more than attracting cool air! Who knew! This really helps! -Anna


I had an a/c installed a few months ago that stopped cooling. I asked the installer to take a look. He said that the refrigerant was deliberately leaked out since (1) the a/c pressure didn't change while he ran the a/c, (2) he couldn't find any bubbles after spraying the unit down, and (3) there were wet spots outside the a/c that he said was refrigerant.

He seemed so sure that there was a deliberate sabotage that he even mentioned the police. Is there another explanation for the loss of refrigerant? We're almost always at the house and everything's lighted outside our house -- can't imagine anyone being that mad at us or daring enough to try anything like that.

The other odd thing is that the weather's been 100-plus and he unplugged our a/c a week ago. Wouldn't the spilled refrigerant (r-22) have evaporated within a week, especially in the heat?


It's called an air conditioner because it removes not only heat, but more important, it removes humidity. That's why they call it air conditioning. If yours is not cooling, have it serviced by a professional. I've been doing a/c work for several years and have come across some people who have done unusual things to try to fix systems.


What is happening if the outside unit is running but the cold coil from the unit is not cold? For several days, the temp has been mid 90s with 100+ heat index. During the heat of the day it appears the unit is not cooling. But as evening approaches it does began to cool and the cold coil is cold again.


Anon83223 and anon59455: No, for the same reason why you don't die if you don't have any air conditioners or heaters but just stay inside. Inside of your house there are many different spaces where air can escape. this is done on purpose so you don't die if you don't open your windows.


How would an AC bring in fresh air in a closed room? Will the oxygen in a room not deplete, leading to suffocation if an ac does not replenish the oxygen in the room?


I have a question in view of contamination of air by exhaled carbon dioxide since the AC circulates the air within a confined room. That when too when many people present in a room the amount of carbon dioxide will be high. Please explain. Sundar


is there any way you could run an air conditioner backward, run water through it instead of freon, and heat the water? i'm in a physics class at school and need a way to heat water mechanically.


Anon46946: yes your A/C will continue working, all of its components run on electricity.

Anon37726: I believe your unit my be low on charge.

Warmunit: there are many ways to ensure your A/C unit runs smoothly. Over time your condenser coil and evap coil become dirty, and cleaning them once or twice a year improves the life of your unit and it's performance.

I have been installing walk in coolers and freezers for around four years now, I have never run into any that use a residential unit to cool.


will air conditioner work if outside temp is at subzero?


If we close the room and the air circulates in the room over and over again in air conditioning system, wouldn't that would make the oxygen in the room become less if we continuously consume the oxygen?


have a gas furnace under my house and the a/c compressor outside. my question is: will the A/C work if the gas is shut-off to the furnace? the electric power is on. Thanks


I have built a box around the back side of my window mounted ac its about


hey anon37726, clean your intake filters. I had the same thing happen and called out the service guy and charged me $95 to tell me to keep my inlet filters clean - not enough air getting to the a-coil.


I have a residential central air conditioning system. On very hot days, especially later evenings nearing midnight, the unit runs steadily and will not shut off.

Also the air from the registers is not cold at this point. I have to shut the unit off.

The a-coil in the furnace plenum freezes up, then when shut off thaws and expels water.

In the morning I turn on the system again.

Why is the A-coil freezing up?


Great article very well said, don't most home units now days use capillary tubes instead of an expansion valve?


Great article. I have a question that has been bothering me for a while, since a friend of mine found a window unit air conditioner and brought it into a bathroom to check if it was working. If an air conditioner is left running in a closed room (say the ambient temp is 80°F, will the temperature go up or down? My guess is that it will go up because of the heat generated by the fan and compressor, but I never had a chance to try it out to see!


Does adding air conditioning to your home have anything at all to do with your heating unit?


We have a Florida heat pump that is an older unit but cooled adequately. It began to trip the breaker when we would adjust the thermostat. The repair company told us that the coils needed to be cleaned. We had this done and now our air conditioning bill is extremely high, more than triple previous bills.

We keep the A/C on auto at 80 degrees. Neighbors in the same building in more sun exposed units that keep their units at 78 degrees say their bills are 2/3 of ours.

What could be causing this problem ? Thank-you for your help.


great work done haven't really thought of this. but is there any local or traditional way of cooling a room? without really depending on a/cs?


I really enjoyed this article. I have never deeply thought about an air conditioner and how it works. This article is insightful and well-written. Great job!


Something else to consider that I learned about and experienced when I lived at Grand Canyon in Arizona is an evaporative cooler. This is a box with mesh fabric panels on four sides that have water dripped through the panels. A fan pulls dry hot air through the panels and blows chilled air into a room. The temperature change can be dramatic based on fan speed. A very inexpensive way to chill out!


Cool explanation! LOL


ive heard there is ways to make your ac run cooler maybe even being able to turn a cheaper ac unit into a walk in cold storage temps, any ideas?


Respected Sir /Madam,

AC is must in these days as we are facing problem of global warming .But for AC we need electricity and this is problem in most of the world country. So can we use sun power for functioning AC? In India in Nagpur city there average temperature is 36 C. In summer it reaches about 44- 47 c, this might be the source of energy for AC. Sunlight is falling straight to earth. The area over it falls some area can we used for AC. As Solar cooker is working, can we arrange AC mechanism in that way?


I'm not an expert on anything that has to do with air conditioning but I was wondering if the term air conditioning applies to cold air only. For example in my car, if I turn the A/C on with the heat on, will it make the hot air hotter as it would make cold air colder? It seems to make sense to me but I don't know much about the mechanics. Thanks


I am looking for a system or sensor that can be used to turn on the air con when a person walks in the room or conversely turn it off when they leave the room.

I have a a vacation property in Kuala Lumpur Malaysia and constantly have huge power bills as guests leave the air con on all day even if they have gone out.

We are in an awkward position where we cannot charge too much but are having unnecessary high costs.

Any help would be appreciated.




This was a very well put, and a made very clear understanding of how air conditioning works, I found it very interesting indeed and now have a much clearer and better understanding of how it works. Cheers very much!

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    • Air conditioning works by drawing hot air away from a room.
      By: sheval
      Air conditioning works by drawing hot air away from a room.
    • A thermostat connected to an air conditioning system.
      By: Dave Friedel
      A thermostat connected to an air conditioning system.
    • A central air conditioning unit.
      By: carroteater
      A central air conditioning unit.
    • A refrigerant called freon is commonly used in AC units.
      By: corepics
      A refrigerant called freon is commonly used in AC units.
    • An air conditioner does not actually work by cooling air, but rather by removing heat and humidity from the air.
      By: alwayspp
      An air conditioner does not actually work by cooling air, but rather by removing heat and humidity from the air.