How Does it Work? The Mechanics of your Air Conditioning Unit

This time of year in North Carolina is when we really start to boil. Thankfully the majority of us have air conditioning units that allow us some relief from the heat.  A Parkwood, during your home inspection, we check the air conditioning to see how well it is working. An air conditioning unit is one of the most expensive components of a house and it has a relatively short life span.  Do you know how they work?


The goal of air conditioning is to take heat from the inside and transport it outside. Accomplishing this requires the use of refrigerant and coils.  The refrigerant is a noncorrosive, nonflammable material that can be manipulated with temperature and pressure to be either a gas or a liquid. The refrigerant runs a loop, passing through the indoor (evaporator) coil, through a copper pipe to the outdoors, through the outdoor (compressor) coil, and back inside through another pipe to the indoor (evaporator) coil.

So as you may have gathered, there are two coils (one inside and one outside). We run cold refrigerant through the inside coil and then blow warm house air across the coils, so the coil can grab heat from the house air.

As the refrigerant enters the evaporator (indoor) coil, it is a cold liquid. The cold liquid in the coil feels the warm house air on the other side of the coil. If the house air is warm and the refrigerant is cool, the heat is going to move through the coil and warm the liquid. As it warms up, the liquid boils off into a gas. As the refrigerant inside the coil changes from a liquid to a gas, it sucks heat out of the air. Logically, enough, that is why this coil is called the evaporator coil- because the refrigerant inside is evaporating from a liquid to a gas.

The next step is to get rid of the heat we collected from the indoors.  The refrigerant is transported outside to the second coil, called a compressor. The compressor squeezes the gas to increase the pressure and temperature.  When the temperature of the refrigerant is greater than the temperature outdoors, and air is blown across the coil, the heat is released into the outdoor air. So as the hot, high-pressure gas moves through the outdoor coil and gives off its heat, it cools to the point where it condenses back to a liquid, which is how the condenser coil gets its name.

Incidentally, an air conditioner is working properly if the air coming off the outdoor fan is even hotter than the outdoor air. That is because we are passing the cooler outdoor air across the coil containing very hot refrigerant.

After the refrigerant goes through the condenser coil, a high-pressure, warm liquid remains. The compressor pushes this liquid through a pipe back into the house towards the evaporator coil. However, this liquid is too warm to pick up heat from the house.  Before the refrigerant enters the evaporator coil, it passes through an expansion valve, which only allows a small amount of high-pressure liquid to move through the pipe at a time.  This means that on the discharge of the bottleneck, the liquid will be at a much lower pressure. As we lower the pressure, we also lower the temperature and the cycle repeats itself.

You don’t have to be an expert, but it is helpful to understand how the systems in your home operate. We are happy to answer questions anytime. We look forward to inspecting your house soon!

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