outdoor unit of an air conditioner, the condenser
outdoor unit of an air conditioner, the condenser

An HVAC compressor is an integral part of an air conditioner. If an air conditioner’s compressor begins to fail, the air conditioning system won’t function correctly. If the compressor breaks down, the entire system will stop running. Various issues can cause compressor failure. Luckily, there are usually some indications that an A/C compressor is failing, and homeowners can troubleshoot some compressor problems.

What is an HVAC compressor?

An HVAC compressor is a vital component of the AC system, often referred to as the “heart” of the air conditioner. It pumps refrigerant through the air conditioning system to cool air inside the home. A compressor can last 10-15 years with proper care.

Where is the HVAC compressor located?

The compressor is located in the outdoor air conditioning unit known as the condenser. The unit resembles a large metal box with vents or fins along the sides and is typically located at the back or side of the house.

How does an A/C compressor work?

The compressor applies energy until the refrigerant becomes a hot, high-pressure vapor. The refrigerant must be heated to a temperature higher than the outside air to release heat as it travels through the condenser’s coils. Pressurization allows the refrigerant to flow smoothly through the coils.

Main Components of an Air Conditioner illustration

Illustration: Chris Philpot

Main Components of an Air Conditioner

There are five significant parts of an air conditioning system. The compressor, condenser, refrigerant, refrigerant line set, and evaporator coils. Each of these components functions in sync with each other and has a specific job to keep your air conditioner running like a well-oiled machine.

  1. Refrigerant is the chemical that cools the air by absorbing the home’s heat and humidity. The refrigerant flows through the air conditioning system in copper or steel lines; the temperature, pressure, and state of the refrigerant change throughout the process.
  2. The refrigerant travels outside to inside the house and vice versa through the refrigerant line set. Cold liquid refrigerant flows into the home to the evaporator via one line, and warm refrigerant vapor flows out of the house to the condensing unit via the other line.

  3. The refrigerant absorbs heat and humidity from the home’s air at the evaporator coils. The evaporator coil is attached to the furnace or contained within the air handler. Cold refrigerant flows through the metal tubing while warm air from the house passes over the coil. The refrigerant absorbs the heat and humidity, and a fan blows the cooled air into the home through the vents. The warm refrigerant then flows outside to the compressor.

 

Heat is removed from the high-pressure refrigerant as it flows through the coils in the condensing unit. The compressor releases hot, high-pressure refrigerant vapor into the condenser. The refrigerant moves through the condenser coils while the fan supplies cool air. The fan’s air cools the refrigerant in the coils, and the metal fins conduct heat away and into the outside air. The refrigerant condenses as it cools, becoming a liquid.

Types of HVAC Compressors

Five types of compressors are used in air conditioners: scroll, rotary, reciprocating, screw, and centrifugal.

how a scroll HVAC compressor works in an air conditioner illustration

Scroll Compressors

The scroll compressor is the newest type of compressor and is gaining popularity. It is used in residential, light commercial, and commercial air conditioners and refrigeration applications.

How does a scroll compressor work?

A coil, or scroll, resembling a spring, is fixed in the center. Another scroll rotates around the fixed scroll, pushing refrigerant towards the center and compressing it. The refrigerant then moves to the condenser.

Benefits of a scroll compressor

Scroll compressors are more reliable, efficient, and quieter than reciprocating compressors due to fewer moving parts.

how a rotary HVAC compressor works in an air conditioner illustration

Rotary Compressors

The rotary compressor is used in appliances and residential air conditioners

How does a rotary compressor work?

The compressor contains a shaft with attached blades or vanes. The blades push refrigerant through the cylinder and as the shaft rotates, compressing the refrigerant. Then, the refrigerant flows to the condenser.

Benefits of a rotary compressor

Rotary compressors are reliable, small, quiet, and vibrate less than other compressors. They are often used in applications where noise is a consideration.

how a reciprocating HVAC compressor works in an air conditioner illustration

Reciprocating Compressors

The reciprocating compressor is the most common type of compressor. It is used in residential, light commercial, and commercial air conditioners, as well as in refrigeration and industrial applications.

How does a reciprocating compressor work?

A motor and crankshaft move pistons up and down inside cylinders, creating a vacuum effect. Refrigerant vapor is sucked into the cylinders when the pistons move down and compressed when the pistons move up. The compressed vapor is then pushed into the condenser. A reciprocating compressor may contain up to eight pistons.

Benefits of a Reciprocating Compressor

Reciprocating compressors are versatile and very efficient. They are easy to maintain and can work at high pressure.

how a screw HVAC compressor works in an air conditioner illustration

Screw Compressors

Screw compressors are used in commercial air conditioners, refrigeration, and industrial applications. They are typically used in large buildings with lots of air to cool and circulate.

How does a screw compressor work?

Two large helical rotors, or screws, rotate and push air from the compressor’s intake. As the space is reduced, the refrigerant is pushed through the compressor to the outtake and the condenser.

Benefits of a Screw Compressor

Screw compressors are reliable, efficient, easy to maintain, and can function in extreme weather conditions. However, they would not be cost-effective in homes and small buildings.

Centrifugal Compressors

A centrifugal compressor is the most extensive and most expensive type of compressor. Centrifugal compressors are used in large commercial HVAC systems.

How does a centrifugal compressor work?

Refrigerant is sucked into the center of a spoked disk called an impeller. The impeller spins, and centrifugal force drives the refrigerant to the outside edge of the impeller, compressing it before it’s released into the condenser.

Benefits of a Centrifugal Compressor

Few moving parts make this compressor reliable and efficient.

Common Causes of Compressor Failure

Issues that can cause compressor failure include:

  • Dirty condenser coils
  • Blocked suction lines
  • Low refrigerant charge
  • Incorrect suction line
  • Too much refrigerant
  • Electrical problems
  • Dirt and debris in the unit
  • Insufficient lubrication

Dirty condenser coils

The condenser coils release the warmth from inside the house. The coils are located in the outdoor unit so they can become covered in dirt, bird droppings, and other filth. Dirt on the coils prevents the coils from releasing the home’s heat, causing the air conditioner to run more often. The compressor may eventually overheat and shut down.

Blocked suction lines

The suction line is a hose that pulls gaseous refrigerant from the house’s indoor unit to the outdoor compressor to release its heat. The suction line is typically the more prominent, insulated hose near the condenser.

A clogged, bent, kinked, or otherwise damaged line will affect the unit’s ability to cool the house. The compressor must work harder and can overheat. Frost on the line indicates a problem with the system.

Low refrigerant charge

Refrigerant is the chemical that circulates through your AC system and cools the warm air from your house. An air conditioner requires the correct amount of refrigerant to work correctly.

The refrigerant can leak from a crack or hole in the refrigerant line, causing the level to be too low. The system has to work harder to cool the house, and the compressor could fail. Most newer AC systems contain a fail-safe switch that will turn the system off if it has too little or too much refrigerant.

Incorrect suction line size

The condenser coils release the warmth from inside the house. The coils are located in the outdoor unit so they can become covered in dirt, bird droppings, and other filth. Dirt on the coils prevents the coils from releasing the home’s heat, causing the air conditioner to run more often. The compressor may eventually overheat and shut down.

Too much refrigerant

Sometimes, technicians installing or servicing an air conditioner fill it with too much refrigerant. Too much refrigerant can cause high levels of pressure and damage the compressor.

Electrical problems

Faulty wires, blown fuses, damaged contractors, or the unit's bad capacitors or starter relays can cause electrical failures. A tripped circuit breaker will prevent the unit from turning on, and power surges can cause electrical issues.

Dirt and debris in the unit

Dirt and debris like leaves, weeds, and vines inside the outdoor unit block the warm air outflow. When the air conditioning unit can’t expel the warm air from the home, it runs more often, and the compressor can overheat. Continual overheating of the compressor will cause it to break.

Insufficient lubrication

The air conditioning system must stay lubricated for optimal performance. The compressor uses oil to lubricate its moving parts. Without lubricant, the parts wear out, come apart, and fail. This additional stress on the compressor may cause it to break down. Leakage from the evaporator, condenser, pipe connections or the compressor shaft seal causes insufficient lubrication. A technician can check the lubricant levels and oil pump.

How do I know if my compressor is bad on my AC?

Common signs of a bad AC compressor are a noisy compressor, the system not cooling efficiently, a burned-out compressor, or an overheated compressor.

A compressor that “hard starts” or stutters as it turns on and off likely has a problem with a fuse or wiring. A noisy compressor can also indicate a loose belt or bad tensioner pulley.

Another indication of a bad compressor is when the air conditioner is running, but the home is not cooling down. The unit’s pressure gauges can be used to determine whether the compressor is to blame. The owner’s manual will indicate what the pressure gauges should read and who may perform the task of checking the gauges.

Damaged or exposed wiring on the motor can cause the compressor to short or burn out. If this occurs, the compressor will not turn on at all. Call a technician to inspect the unit.

Faulty wiring, motor issues, and electrical problems can cause a compressor to overheat. A popped reset button on the motor indicates a defect. The motor should be allowed to cool down before depressing the reset button.

Side view caucasian overheated woman using wave fan suffer from heat sweating, cools herself, closed eyes feels sluggish due to HVAC compressor problem air conditioner not working at home in summer weather

Troubleshooting HVAC Compressor Problems

Homeowners can troubleshoot some compressor problems.

    • Check for tripped breakers: Circuit breakers can be tripped by operating too many appliances simultaneously. The AC’s circuit breaker may also be tripped if the unit has a problem. Check the breaker panel for a tripped breaker and reset it. If the breaker is repeatedly tripped, there is an issue with the AC. If the switch is hot, immediately call an HVAC technician.
    • Test the thermostat: Ensure the thermostat is set to “cool” and the temperature setting is low enough to turn on. Replace the batteries with new ones.
    • Listen for buzzing: Stand near the outside unit when the system starts. If your AC sounds weird, such as it's making a buzzing noise or sounds like a jet engine, its likely indicates the start capacitor is faulty. Call a technician to handle this repair since it deals with high voltages.
    • Listening for clicking: The compressor should click when it engages. No click or repeated clicking likely indicates a bad starter relay. Call a technician to replace it.

What size A/C compressor do I need?

A new air conditioning unit requires 1 ton of cooling capacity per 500 square feet or 25 BTUs per square foot. An old A/C compressor must be replaced with a compressor of the same size.

The compressor size can be found in the model number on the outdoor unit’s label. The last two digits of the model number indicate the compressor’s size in BTUs by thousand. The number 24 indicates a 24,000 BTU compressor. 1 ton equals 12,000 BTUs. Therefore, a 24,000 BTU compressor is equivalent to 2 tons of cooling capacity.

Can an A/C compressor be repaired?

There are some situations in which a damaged compressor might be repaired. However, it is often safer and more cost-effective to replace it. Some homeowners choose to replace the condenser unit housing the compressor or the entire air conditioning system.

Consider the age of your unit. Many compressors come with a manufacturer’s ten-year warranty for easy replacement. Replacing the air conditioner compressor might be worthwhile if it's a reasonably new unit or if it will cost less than 50% of a replacement unit. If the AC system is very old and outdated, you may consider replacing the entire system.

Repairing or replacing the compressor yourself is strongly discouraged. The compressor carries a high voltage so improper handling can result in serious injury or death. Only a licensed HVAC technician should inspect and service a broken compressor.

Conclusion

The compressor is the part of the air conditioner located in the outdoor unit that pumps refrigerant through the air conditioning system to cool air inside the home. The compressor applies energy until the refrigerant becomes a hot, high-pressure vapor.

Many issues can cause compressor failure. Often, the compressor will exhibit signs that it’s failing. Homeowners can troubleshoot some compressor problems, but only professional HVAC technicians should repair or replace compressors.

Compressors can be repaired, but replacing them is often safer and more cost-effective. An old A/C compressor must be replaced with a compressor of the same size.

Air conditioning

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