How Does a Split System Air Conditioner Work?

A split system air conditioner moves heat rather than making cold air. Refrigerant circulates between the indoor and outdoor units, absorbing heat from the air on one side and releasing it on the other. This is called the vapour compression refrigeration cycle. In cooling mode heat is taken out of your room and released outside, and a reverse cycle system can run the same process the other way to heat your home.

The refrigerant (for example R-32, R-410A or R-22) changes state from a liquid to a vapour and back to a liquid again as it moves around the system, and these changes of state are what allow the air conditioner to absorb and reject heat.

The four main components

The diagram below shows a typical vapour compression refrigeration system, with the main components numbered:

1. Condensing coil and fan (outdoor unit)
2. Expansion valve (inline component of the refrigeration system)
3. Evaporator coil and fan (indoor unit)
4. Compressor (inline component of the refrigeration system)

The refrigeration cycle step by step

1. Superheated vapour and the condenser

We start as refrigerant leaves the compressor (number 4 on the diagram) as a superheated vapour, shown in red and flowing to the left. Superheated means the refrigerant is at a temperature higher than its boiling point, in the same way steam can be hotter than boiling water.

The superheated vapour moves into the condensing coil (number 1), a long serpentine coil with a fan blowing over it. As air passes over the coil, heat is released from the refrigerant into the outside air, and the refrigerant changes state from a superheated vapour to a subcooled liquid.

2. The expansion valve

The liquid refrigerant then reaches the expansion valve (number 2). The diagram shows a thermal expansion valve (TXV), which uses a sensor at the evaporator to work out how much refrigerant is needed to hold the temperature of the air passing over the evaporator coil. Based on the heat load, the valve opens and closes to regulate how much liquid refrigerant flows into the evaporator. Many modern split systems use an electronic expansion valve that does the same job.

As refrigerant passes through the valve it experiences an abrupt pressure drop. Some of the refrigerant flash evaporates, which lowers its temperature, and it leaves the valve as a cold, low pressure mix of liquid and vapour.

3. The evaporator (indoor unit)

As this cold refrigerant travels through the evaporator coil (number 3), the indoor fan draws room air over the coil. The refrigerant absorbs heat from the air, boils and evaporates into a vapour, while the air itself is cooled. The cooled air is then blown back into the room through the louvres.

This is why refrigerants are used in air conditioners. R-32, a common split system refrigerant, boils at around -52°C at atmospheric pressure, so it readily absorbs heat from the air passing over the coil.

4. The compressor

The compressor (number 4) draws the vapour out of the evaporator and compresses it to a higher pressure and temperature, ready to be released back into the system as a superheated vapour. Five types of compressor are commonly used in air conditioning systems:

• Reciprocating compressor
• Scroll compressor
• Screw compressor
• Rotary compressor
• Centrifugal compressor

There is no simple way to say one type is better than another, as each manufacturer uses the compressor type that suits its own technology and the capacity of the system.

The cycle continues

The superheated vapour travels back to the condenser and the cycle repeats. There are surprisingly few components inside the refrigeration system itself. If this is your first time reading about refrigeration and it does not all make sense yet, that is completely normal, so feel free to come back to it.

How reverse cycle heating works

In a reverse cycle system, a reversing valve changes the direction the refrigerant flows. The indoor coil then acts as the condenser and releases heat into your room, while the outdoor coil absorbs heat from the outside air. Read more in what is a reverse cycle air conditioner.

Further reading

• Split system refrigerants: R-32, R-410A and R-22 explained
• What is an inverter air conditioner?
• Wikipedia: Refrigeration
• Wikipedia: Vapour compression refrigeration

Frequently asked questions

Does a split system bring fresh air in from outside?

No. A split system moves heat using refrigerant that circulates in sealed pipes between the indoor and outdoor units. No air is transferred between inside and outside in this process.

Why does the outdoor unit blow warm air in summer?

In cooling mode the outdoor unit releases the heat that the indoor unit has removed from your home, so the air coming off the outdoor unit is warm.

What refrigerant does a split system use?

Most new split systems use R-32. Older systems may use R-410A or, in much older units, R-22. The refrigerant type is shown on the rating plate of the outdoor unit.

Why does my split system produce water?

As room air passes over the cold indoor coil, moisture in the air condenses and drains away. In heating mode the outdoor unit produces water instead, so both units need a compliant drain.

Related guides

Keep reading with these related EZYAIR guides:

• What Is a Split System Air Conditioner?
• What Is a Reverse Cycle Air Conditioner?
• What Is an Inverter Air Conditioner?
• Split System Refrigerants: R-32, R-410A and R-22 Explained
• Correct Termination of Air Conditioner Condensate Drains

Talk to EZYAIR

Ezy Air Conditioning & Heating Pty Ltd (EZYAIR) has been helping Melbourne homes and businesses with heating and cooling since 2007. If you have a question about how split systems work or would like a quote, contact our team or call 1300 399 247, Monday to Friday, 9am to 5pm.

Our head office and display centre is at 49 Huntingdale Rd, Burwood VIC 3125.