AC Compressor Humming But Not Starting: Causes & Solutions

Is your AC compressor humming but not starting? Don’t sweat it, this comprehensive guide will help you decipher the causes, troubleshoot potential issues, and understand how to prevent this from happening again in the future.

AC Compressor Humming But Not Starting
Photo © Chris Allen (cc-by-sa/2.0)

Why is your AC compressor experiencing this problem? Here are the most likely reasons.

Electrical Problems

Electrical issues are often at the root of a humming but not starting compressor. These could include problems like faulty wiring, a broken capacitor, or issues with the power supply. A faulty capacitor, for example, can’t provide the necessary jolt to start the motor, resulting in a humming sound. Similarly, issues with wiring can prevent electricity from reaching the motor, causing the same issue.

Broken Capacitor

The capacitor’s role is to store and supply the electrical energy needed to start the motor in the compressor. When it’s broken or malfunctioning, it fails to give the necessary kick-start, causing the motor to hum but not start. If the humming sound happens when you first turn on the AC but then it stops, a broken capacitor could very well be the culprit.

Faulty Wiring

Faulty wiring can be another cause for a humming but non-starting compressor. Wiring provides the pathway for electricity to reach the motor. If the wiring is damaged, it could lead to an insufficient power supply to the motor, causing it to hum without starting.

Mechanical Problems

Mechanical problems can also cause your compressor to hum but not start. These issues could be due to a seized motor or low oil levels in the compressor. A seized motor, for example, can’t rotate properly, causing the humming sound without the compressor starting. Low oil levels, on the other hand, can cause excessive friction within the compressor, causing a similar issue.

Seized Motor

A motor becomes seized when its movement is blocked, usually because of mechanical failure or damage. When a motor is seized, it may try to start (causing a humming sound) but fail to turn over. The mechanical energy thus gets converted into heat, potentially causing more damage to the compressor.

Low Oil Levels

The oil in a compressor lubricates its moving parts, reducing friction and ensuring smooth operation. If the oil level is too low, the compressor parts will generate excessive friction when they move. This can cause the motor to strain and produce a humming sound.

Environmental Issues

Environmental factors like extreme temperatures or dirt accumulation can also interfere with the compressor’s operation. An AC unit is designed to operate within a specific temperature range. If the ambient temperature is too high or too low, it could prevent the compressor from starting. Similarly, dirt accumulation can block airflow and cause overheating, leading to a humming sound from the compressor.

Temperature Extremes

Extreme cold or heat can stress your AC system, leading to the compressor humming but not starting. In hot weather, the AC unit has to work harder to cool your home, which can strain the compressor. On the other hand, if it’s too cold, the oil inside the compressor can thicken and inhibit the compressor’s operation.

Dirt Accumulation

A compressor clogged with dirt and debris may hum but not start. The accumulated dirt can block the vents, restrict the airflow, and cause the compressor to overheat. The overheating compressor may then hum but fail to start.

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How to Troubleshoot a Humming AC Compressor That Won’t Start

Want to learn how to fix this problem? Of course, you do. Let’s dive in…

Safety Precautions

Before we dive into troubleshooting, it’s important to remember safety comes first. Always disconnect the power to your AC unit before you start any work on it. This ensures you won’t accidentally shock yourself while you’re trying to figure out what’s wrong. Remember, if at any point you’re unsure or uncomfortable, don’t hesitate to call a professional.

Identifying the Issue

Identifying the cause of the problem is the first step in troubleshooting. If you notice the humming sound, try to see if it’s coming from the compressor. Check the outside unit for any visible damage or debris. Listen to the sound: if it’s a continuous, steady hum, it could indicate an electrical issue. If the hum is intermittent, it could signal a mechanical problem.

Basic Fixes

For some issues, like a tripped circuit breaker or a dirty compressor, you might be able to perform the fix yourself. Resetting the breaker or cleaning the exterior of the compressor and the surrounding area can sometimes resolve the issue. However, remember that working on an AC unit can be complicated, and some fixes are best left to professionals.

When to Call a Professional

When basic fixes don’t work, or if you’re dealing with more complex issues like a broken capacitor or seized motor, it’s time to call in the cavalry. A licensed HVAC technician can accurately diagnose and fix the problem, ensuring your AC unit is back to cooling your home in no time.

Preventing Future Issues with Your AC Compressor

Regular Maintenance

To avoid future compressor issues, nothing beats regular maintenance. Regular checkups and cleaning can prevent problems from occurring in the first place. Tasks like replacing the air filters, cleaning the coils, and inspecting the overall system can help keep your AC unit running smoothly.

Understanding AC Unit’s Limitations

Just like any machine, your AC unit has limitations. Overworking your AC unit, setting the thermostat too low, or neglecting regular maintenance can lead to premature compressor failure. By understanding these limitations, you can avoid putting unnecessary stress on your compressor and prolong its lifespan.

Replacing an Old Compressor

If your compressor is old and has been giving you trouble, it might be time for a replacement. A new compressor can improve the efficiency of your AC unit and provide reliable cooling. Remember, replacing a compressor is a complex task that should be left to a professional HVAC technician.

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