How air conditioning works

We’ve all come to rely on air conditioning to create a healthy and comfortable environment for our employees and customers.

During normal operation of an air conditioning system, cold air is supplied through the building’s ductwork, where impurities and contaminants are removed through the system’s filters.

But how is this air cooled, and what should you know about the operation of your air conditioning system?

Air conditioning system

Most central air conditioning systems use a “split-system.” This means there is a compressor and condenser outside the building, and evaporator coils and ducts inside the building.

These two systems are connected by two copper pipes. The smaller pipe contains high-pressure, liquid refrigerant that flows into the building to capture heat. And the larger pipe contains low-pressure gas refrigerant that flows out of the building to release heat.

It is important to understand that the refrigerant is the means by which heat is collected from the building, moved outside and released into the outdoor air.

Thermal expansion valve

The flow of refrigerant is regulated by a thermal expansion valve. A temperature sensor at the outlet of the evaporator controls the amount of refrigerant that flows through the expansion valve and ensures the liquid refrigerant is vaporized before it leaves the evaporator.

The amount of refrigerant flowing depends on the amount of heat to be removed. When there’s more heat, more refrigerant is required. Less heat, and less refrigerant is needed.


The key to air conditioning is the boiling of the refrigerant in the evaporator. This boiling occurs by passing warm air over the evaporator coils causing the cold refrigerant to boil off into a gas.

For example, if cold refrigerant (40°F) flows through the evaporator and the air moving over the evaporator is 75°F, the cold 40°F refrigerant will absorb the heat from the relatively warm 75°F air.

The heat absorbed by the refrigerant travels outside to be discharged via the condenser, and the now cooler air travels back to the building interior to help cool the space.


The next step is to release the heat from the refrigerant gas outdoors. However, we just can’t take the refrigerant and pass it through an outdoor coil. This would just cause the refrigerant to heat up and collect even more heat. The solution is the compressor.

The compressor is the heart of the cooling cycle. It draws in cool, low-pressure refrigerant gas from the indoor evaporator.

The motor-driven compressor’s sole function is to squeeze the refrigerant, raising its temperature and pressure so that it can push out a hot, high pressure gas to the condenser.

This temperature is well beyond the outside air temperature, which allows the refrigerant to dump its heat into the 90+ degree F outdoor air.


As the gas enters the condenser, the heat is carried away by air that flows over the condenser coils. The heat that was absorbed by the refrigerant through the buildings evaporator, is now discharged to the outside air.

The outdoor fan located in the condenser cabinet draws in air through the sides of the condenser coil and discharges the warmer air through the top of the cabinet, which you can feel is usually over 100 degrees F.

The gas refrigerant entering the condenser is at a much higher temperature than the outside air, so by drawing air over the condenser coils the refrigerant releases its heat and cools back to a liquid.

The process then starts all over again. Watch our animated video to see this process in action.

Now that you know the science behind air conditioning, see how you can save money on its operation and help prevent a premature breakdown.


© 2016 The Hartford Steam Boiler Inspection and Insurance Company. All rights reserved. This article is intended for information purposes only.


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