The outside unit of an air conditioning system, commonly referred to as the AC condenser or AC compressor, is an important part of any air conditioning setup. This exterior component is responsible for removing heat from the air inside your home and dispelling it outdoors. Understanding what this outside AC box is, what it does, and how it works can help homeowners better maintain their cooling systems.
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Names for the External AC Unit
The external part of a split air conditioning system has several common names, including:
- AC condenser
- AC compressor
- Condenser unit
- Outdoor unit
- External unit
While these terms refer to the same external component, there are some minor differences between them:
- AC condenser – Highlights that this unit condenses refrigerant gas into liquid as part of the AC process.
- AC compressor – Draws attention to the compressor inside the unit that pressurizes the refrigerant.
- Condenser unit – Focuses on the condenser function of the external unit.
- Outdoor unit – Describes where the unit is located outside the home.
- External unit – Indicates the unit is located outside the building.
While these terms refer to the same box-like device outside your home, AC professionals most often use the terms AC condenser or outdoor unit. The remainder of this article refers to it as the outdoor unit, condenser unit, or external unit for simplicity.
What Does the Outdoor Unit Do?
The outdoor AC unit has several important jobs that are vital to cooling your home:
- Contains the compressor that pressurizes refrigerant gas as part of the cooling cycle.
- Houses the condenser coil that converts hot pressurized refrigerant gas into cooler liquid refrigerant.
- Contains a fan that pulls in outdoor air across the condenser coil to expel heat.
- Provides a place for accumulated condensation to drain safely.
Without the outdoor unit condensing refrigerant gas into liquid, the indoor evaporator coil would not be able to provide cool air. The outdoor unit is essential for heat removal and proper AC functioning.
How Does the Outdoor Unit Work?
The outdoor AC unit relies on several components and refrigeration principles to cool your home:
Compressor
The compressor is the device that pressurizes the refrigerant gas, raising its temperature. It works similarly to a pump, using a piston system and valves to draw in low-pressure vapor from the evaporator coil and compress it into a hot, high-pressure gas.
Condenser Coil
The hot compressed refrigerant gas then flows into the condenser coil. As cooler outdoor ambient air passes over the coil, it absorbs heat from the refrigerant gas and condenses it into a liquid.
Condenser Fan
The condenser fan pulls in outdoor air and blows it across the condenser coil to provide cooling. It typically runs whenever the AC is operating to maximize heat transfer.
Expansion Device
After condensing into liquid, the refrigerant passes through an expansion device like a valve or orifice tube. This causes the refrigerant pressure and temperature to drop rapidly.
Evaporator Coil
The now cooler, low-pressure liquid refrigerant flows into the indoor evaporator coil. A blower fan blows indoor air across the coil, where the refrigerant absorbs heat and evaporates into a gas.
This cycling of refrigerant between gas and liquid states while absorbing and expelling heat is the essential process that allows an AC system to cool your home.
Types of Outdoor AC Units
While all outdoor AC units work similarly, there are a few types to be aware of:
Split System
A split system is the most common type of air conditioner in residential homes. It has an outdoor unit connected to an indoor unit via refrigerant lines.
Packaged System
A packaged AC system has the evaporator, compressor, and condenser all in one large outdoor cabinet. The thermostat wiring and ductwork run directly to this single external unit.
Mini Split
A mini split has one outdoor unit connected to one or more smaller indoor evaporator units. The indoor units mount on walls or ceilings.
While less common in homes, other types of outdoor AC units are used for commercial buildings like multi-zone, VRV, VRF, and chillers.
Outdoor Unit Size
Outdoor AC unit sizes are rated in tons of cooling capacity. One ton equals 12,000 BTU of heat removal per hour. Common residential sizes include:
- 1-2 tons – Small homes or condos
- 2-4 tons – Average size homes
- 4-6 tons – Large homes
Oversizing or undersizing the outdoor unit relative to a home’s needs can lead to comfort and efficiency issues. Proper sizing requires an AC load calculation by HVAC professionals.
Outdoor Unit Operation
The outdoor unit runs in conjunction with the indoor blower fan according to thermostat signals. Here is a typical sequence of operation:
- Thermostat senses rising indoor temperatures.
- Thermostat signals AC system to run.
- Indoor blower fan turns on.
- Outdoor unit starts up – compressor pressurizes refrigerant and the condenser fan starts blowing.
- Cooled refrigerant circulates indoors to the evaporator coil.
- Evaporator coil absorbs heat from the indoor air.
- Cooled supply air blows out into the home.
- Thermostat reaches set temperature and system shuts off.
The outdoor unit will run as needed to maintain indoor temperatures according to the thermostat setting. Outdoor unit operation indicates your AC is working to cool your home.
Outdoor Unit Maintenance
Routine maintenance is important for keeping an outdoor AC unit operating efficiently. Recommended maintenance includes:
- Replacing or cleaning air filters every 1-3 months.
- Having a professional technician perform an annual check-up and service the unit.
- Cleaning debris and leaves from around the unit.
- Straightening bent condenser coils fins.
- Checking refrigerant charge levels.
- Verifying electrical connections are tight.
- Cleaning condenser and evaporator coils.
Neglected maintenance can lead to loss of cooling capacity, higher energy bills, and even complete unit failure over time. Follow the manufacturer’s maintenance guidelines for your specific outdoor AC unit.
Signs of Outdoor Unit Problems
Watch for the following signs that may indicate issues with your outdoor AC unit:
- Unit not turning on when AC is activated
- Strange noises or grinding sounds
- Refrigerant leaks detected
- Reduced cooling airflow indoors
- Frozen evaporator coil
- Condenser coil clogged with debris
- Corroded refrigerant lines
- Overgrown vegetation near unit
- Electrical problems or burnt wiring
The most common AC repairs involve the compressor, refrigerant charge, electrical components, coils, and condenser fan motor. Professional diagnosis is recommended if you suspect your outdoor unit needs service.
Conclusion
The outdoor unit is an essential component in any air conditioning system. This exterior AC box houses the compressor, condenser coil, and condenser fan responsible for pumping heat out of your home. Proper maintenance and care is required to keep this outdoor equipment operating efficiently for cooling comfort. Watch for signs of potential problems and contact an HVAC technician to service your external AC unit.