Magic of Air Conditioning


Now that winter is approaching, and everyone is changing the switch on their Central Air system from “cooling” to “heating” we are going to discuss how the air coming out of our vents truly gets conditioned. I am proud to present the eighth post for the Video + Blog series “What the Hell is Architecture?” A series that will present Case Studies, Code Commentaries, How-To Videos, and anything else that comes to mind to try and get the word out on what Architecture, Engineering and Construction ACTUALLY is. (Something the industry has done a poor job of enlightening the public on.)

First, we must place a big disclaimer at the start of this, I am not a Mechanical Engineer. What we are going to discuss is a series of diagrams and explanations for a simple Vapor Compression system. There is a lot of science, Engineering and knowledge required to apply this (or the countless other systems) into a building. Hopefully though, after you read this when you see air coming out a vent at the temperature you wanted, you’ll have more of an understanding of how that is done.

Four Steps in the Process

To make this easier to follow, we will use a very basic Air Conditioner that is placed in a windowsill to explain how this process works. Once we have a better understanding, we will see that the same principle can be applied to a building using a Central Air system.

There are four steps in this process, and which is the order in which we will examine. It is an endless cycle, which we will discuss shortly.

  1. Compressor to Condenser Coils
  2. Condenser Coils to Expansion Valve
  3. Expansion Valve to Evaporator Coils
  4. Evaporator Coils to Compressor

Basic Principles

Before we discuss the cycle, there are a few basic mechanical principles that drive this process. The first being that when a material undergoes a phase change, (gas to liquid, liquid to gas, etc.) its temperature is either increased or decreased. The second being that nature will always try to find balance in temperature, so if two adjacent surfaces are vastly different temperatures, the coolth (this is a real word, meaning the opposite of heat) will leave one surface to lower the temperature of the other. Finally, warm air can hold much more moisture than cold air. As it lowers its temperature, moisture is removed and left on the cooler surface it is in contact with (condensation).

Compressor to Condenser Coils

The process starts with the Compressor. The Compressor not only continuously moves the Refrigerant (specialized liquid that is continuously running through the piping and unit) throughout the entire cycle, it is also where the low-pressure warm vapor (which we will discuss later) is compressed into a high-pressure vapor. From the Compressor, the high-pressure vapor then travels to the Condenser Coils.

Compressor squeezes the warm vapor into high pressure vapor.

Condenser Coils to Expansion Valve

The Condenser is simply a series of piping or metal fins that the vapor runs through, with a fan blowing air over it. The air flow over the piping removes a lot of excess heat from the Refrigerant and is vented to the exterior of the building. This is the reason for the piping to be running back and forth, to increase the surface area and time that the vapor is exposed to the fan. As the heat is removed from this vapor its temperature naturally lowers. The combination of a lower temperature and high pressure causes the vapor to begin turning into a liquid before it enters the Expansion Valve.

Fan blows air over warm vapor, cooling it down and converting it to liquid.

Expansion Valve to Evaporator Coils

The Expansion Valve is just a very small opening that forces the liquid being pushed through it at high pressure to then form a mist like vapor. (Think of squeezing the trigger on a water bottle and then the water in the bottle forcing itself through the spout as a cool mist.) This significantly lowers the pressure of the liquid, creating a low-pressure vapor to enter the Evaporator Coils.

Expansion Valve forces liquid into cool vapor.

Evaporator Coils to Compressor

Like the Condenser Coils, the Evaporator Coils are a series of piping with a fan blowing over them. In this case however, the air flowing over the piping is cooled down from the coils, thus blowing colder air into the building. As the vapor is cooling down the air blowing over it, the vapor itself starts to warm back up, thus repeating the cycle of a low-pressure warm vapor entering the Compressor.

These colder coils also provide an opportunity to dehumidify our buildings, as warm air from the building is returned to be treated, it is run over these coils, thus removing moisture from the air as the air is cooled down. This of course creates a lot of condensation on the coils, which must be removed from the building. Look underneath the window air conditioners in apartment buildings on a hot day, and you can see all the moisture cascading below them outside.

Fan blows air over cool vapor, warming it into low pressure vapor.

How This Is Applied to Our Lives

As mentioned previously, this explanation was done with a window mounted unit, but the same principle is applied to larger scale applications. Many homes have Central Air, and it operates on the same cycle as we just discussed, except the Condenser coils are moved to an exterior unit (the large, humming box that most people hide next to their porches or decks.) Many people misuse the term “Air Conditioning” as simply referring to cooling the air, however it is whenever the air is conditioned to be hotter or colder. The system described is also how your house is most likely heated, the same principles discussed in this post, but “in reverse.” Therefore, most systems can only heat OR cool, not both, as it is a single series of piping and ducts.


What we discussed was the basic principle that MOST air conditioning systems operate on. As hinted at before, there are endless variations and methods on how this process can be used. Not to mention, the hundreds of other systems that exist. This post also completely ignored the many collaboration opportunities that must take place when it comes to installing all the duct work, supply and return grilles, and other Mechanical items that require design to be properly placed.

Speak to a Mechanical Engineer and an Architect when making decisions on any Conditioning Systems, as there will be many factors to be considered for and designed.

If you feel I missed the mark or am misinformed on any aspect of what I wrote, please let me know.

If you did enjoy the post or have any ideas or questions you would like to see or hear about in the upcoming posts and videos in this series, please let me know that as well.

Thank you all and have a great day.

  • Bryan Toepfer, AIA, NCARB, CAPM
  • Principal – Architect

Lets Build Something Together

Bryan Toepfer, AIA, NCARB, CAPM
Bryan Toepfer, AIA, NCARB, CAPM

Bryan Toepfer is the Principal Architect and founder of TOEPFER Architecture, PLLC, an Architecture firm specializing in Residential Design, Construction and Virtual Reality modeling.

Taking his philosophy of the importance of education to heart, he guide clients through the process of Design and Construction with each project. He runs a blog titled “What the Hell is Architecture?” with the goal of sharing with others the many aspects of Architecture. He also coaches Intern Architects studying for their Architectural License exams, as well as teaches at a local University.

He lives in Rochester, New York with his wife, two dogs and two cats.

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