Since the 1970s, central air conditioning systems have become a staple in American homes, offering whole-house comfort with the convenience of a single thermostat. But, what is central AC? A central air conditioning system is a type of HVAC setup that cools and dehumidifies air from a central location, then distributes it through ductwork to each room in your home.
These systems are made up of several key AC components that work together to circulate cool, dry air throughout your home. Understanding the parts of an air conditioning system can help you maintain your AC, spot trouble early, and ensure optimal performance for years to come.
Parts of Your Central Air Conditioning System
Your central AC is a complex machine. Whether you’re trying to solve an issue, replace a part, or just want to learn how it all works, this section offers a clear breakdown of the most essential air conditioning unit parts and their functions.
Outdoor Condenser Unit
Your central AC system has an outdoor condenser, often located on a concrete pad outside your home (or sometimes on the roof). This metal unit has slats or “fins” that help dissipate heat. As the evaporator cools warm air, refrigerant absorbs the heat and carries it to your condenser. Here, the heat is released into the outdoor air, and the refrigerant is pumped back into your home to begin the cooling process again.
Maintenance Tip: The best way to keep your condenser working properly is to clear the area around it of debris and keep landscaping trimmed back for at least three feet in all directions. You can clean your condenser using a gentle setting on your hose, or have your HVAC technician perform this task during regular air conditioning maintenance visits.
Indoor Evaporator Unit
Inside your home, the air conditioner’s evaporator unit draws air across the evaporator coil. This coil contains the refrigerant, which absorbs heat from the air to cool it. Once the air has been cooled, it is blown through your ducts by an air handler. If you also have a furnace, your HVAC system will share the same air handler.
Pro Tip: The evaporator should be cleaned by your HVAC technician during your seasonal tune-up. This is especially important before turning your AC on after winter.
Compressor
Inside the outdoor condenser unit is the compressor, often called the "heart" of your AC system. The compressor pressurizes the refrigerant gas, increasing its temperature and pressure before sending it to the condenser coils. This process is essential for moving refrigerant through the system and enabling heat exchange. Without a functioning compressor, your AC system cannot cool your home.
Watch Out For: A failing compressor might make clunking or rattling sounds and could cause the system to trip the circuit breaker. Because it’s a sealed part that handles refrigerant under high pressure, replacement is labor-intensive and costly—often prompting homeowners to consider full system replacement.
Blower Motor
The blower motor is located in the air handler and circulates air through the system. Once the evaporator coil cools the air, the blower motor pushes it through the ductwork and into each room of your home.
There are two main types of blower motors:
- Single-speed motors, which operate at full capacity when on.
- Variable-speed motors adjust their speed to match your cooling needs, offering better energy efficiency and comfort.
If the blower motor fails, your AC won't be able to distribute cooled air—even if other components are working properly. The blower motor also runs during heating cycles if you use a gas furnace.
Maintenance Tip: Keep up with air filter changes and consider annual HVAC inspections to ensure your blower motor stays in top shape.
Refrigerant Lines
A key component of your home AC, these insulated copper or aluminum lines connect the indoor and outdoor units. One line delivers the liquid refrigerant to the evaporator, while the other carries the gas refrigerant back to the condenser. Leaks in these lines can drastically affect cooling performance.
Is your AC blowing warm air? You might have a Freon leak. Leaking refrigerant is not only bad for performance, but it can harm the environment and may require a recharge or part replacement to fix.
Expansion Valve (or Metering device)
The expansion valve is a lesser-known but vital component that controls how much refrigerant flows into the evaporator coil. As the high-pressure liquid refrigerant passes through this valve, it drops in pressure and temperature, allowing it to absorb heat more efficiently in the evaporator coil. A malfunctioning expansion valve can lead to poor cooling performance or system freeze-ups.
Did You Know? If your AC cycles on and off rapidly or your evaporator coil keeps freezing, a faulty expansion valve could be to blame.
Drain Line and Pan
As your AC cools the air, it also dehumidifies it. That moisture collects in a drain pan and exits via a condensate line. If the line clogs, you risk water damage or system shutdown.
Watch Out For: Musty smells or water pooling near your unit could signal a clog. Read our guide on what to do if your AC smells bad.
Air Filter
Your air filter protects your AC components from dust and debris and ensures better indoor air quality. Dirty filters can restrict airflow, increasing energy use and wearing down your system. For most homes, look for filters with a MERV rating between 8 and 13. Higher ratings capture smaller particles but may restrict airflow if your system isn't rated for them.
Maintenance Tip: Replace your filter every 30–90 days. If your home includes pets or someone with allergies, you may want more frequent maintenance. For maximum efficiency, especially in peak seasons, consider a tune-up or filter replacement service.
Ductwork and Vents
Central air conditioning systems use ductwork to carry cooled air throughout your home. Cooled air moves through the air ducts, reaching each room via vents or air registers. These vents can typically be adjusted for more or less airflow.
Maintenance Reminder: You can clean your air registers using the hose attachment of your vacuum. Make sure not to block vents with furniture or other items. Avoid closing too many vents, which can overwork your blower motor. Here’s what to do if one room stays too hot.
Thermostat and Controls
The thermostat is the control center for your home’s central cooling system. Based on your temperature setting, your thermostat prompts your air conditioner to run by comparing your set temperature with the ambient temperature in your home. When the thermostat detects that your home is warmer than your preferred setting, it sends a signal to the system to begin cooling.
Pro Tip: Thermostats should be placed away from vents, direct sunlight, and heat-producing appliances to ensure accurate readings. Consider a smart thermostat to boost efficiency and control. Learn more about HVAC troubleshooting to keep your home cool.
AC Parts Repair and Replacement
Every AC system, no matter how well maintained, will eventually need repairs or replacements. Knowing which parts of your HVAC system are more prone to wear can help you plan ahead and avoid unexpected breakdowns.
Common Parts of an Air Conditioner That Need to Be Replaced:
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Capacitors: Help start the motors. Failure may prevent your system from turning on.
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Contactors: Control electricity to key components. If worn, your system may short-cycle.
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Evaporator coils: Can corrode or leak over time.
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Drain lines: May clog and cause overflow.
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Air filters: Simple but vital for efficiency.
Most Expensive Air Conditioning Unit Parts to Replace:
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Compressor: Replacing it can cost nearly as much as a new unit.
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Evaporator and condenser coils: Especially expensive if refrigerant leaks are involved.
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Blower motor: Expensive due to both the motor cost and the time required for installation.
In some cases, replacing your system is more cost-effective than repairing it. If your system needs replacement, the pros at ARS will give you honest advice on what is best for your home and your wallet. When it is time for a new unit, find out the steps your technician will take to replace your system and make your home cool again.
When to Call a Professional
If you notice your AC isn’t cooling efficiently, makes strange noises, emits odd smells, or leaks water, it’s time to call the experts at ARS at 866-399-2885.
Our certified HVAC professional can:
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Diagnose issues early
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Perform repairs or part replacements
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Conduct seasonal tune-ups
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Recommend energy-efficient upgrades
Waiting too long to address AC problems can lead to higher energy bills, poor indoor air quality, or complete system failure. Professional technicians have the tools and experience to diagnose complex issues and help you avoid unnecessary repairs. Scheduling a visit early can prevent small issues from becoming costly breakdowns later in the season.
Get Reliable AC Services with ARS
Caring for the parts of your air conditioning system means following smart cooling practices and scheduling regular professional tune-ups to maximize AC unit efficiency. ARS offers a Pro Service Plan that includes two maintenance visits per year to keep your system in great shape.
What to expect from ARS for maintenance, repairs, or installations:
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Experienced professionals: Our technicians are trained, certified, and licensed in installing, maintaining, and repairing AC systems.
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24/7 emergency services: We’re here to help when you need us the most—weekdays, weekends, or even holidays.
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Upfront pricing and financing options: We provide a written estimate that you approve before any work begins. Plus, our flexible financing options make major repairs and installations more manageable.
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100% satisfaction guarantee: We stand by our work with our Exceptional Service Guarantee.
Contact us for replacement, repairs, or book your annual air conditioning tune-up with the experts at ARS today.
