Why Is Your AC Unit Humming But Not Turning On (+DIY Fix)

Is your air conditioning unit blowing hot air into the house? Is the AC unit just humming instead of turning on? If you experience any of these problems, don’t panic.

Generally, an AC unit is a collection of many parts that make it possible for it to do its work. A problem with one item might cause the unit to malfunction. Usually, when the AC unit is humming but not turning on, you have an AC capacitor problem.

What is an AC Capacitor?

It is better to understand how an AC unit works first. When the temperature goes above the set level, the thermostat clicks on. Since large outdoor units such as compressors and fans need large amounts of energy to get started, the capacitor provides the energy.

If the capacitor does not kick in, the unit will continue humming, but will not receive enough energy to start. In this case, the capacitor is the power source of the compressor and fans. The capacitor is a small metallic object located on the side of the outdoor AC unit. It stores electrical energy within the AC’s outside unit. Without it, the compressor won’t start.

The capacitor must generate enough energy burst to power the cooling cycle system and keep it running for the initiation stage.

Read Also: 7 Reasons Your Thermostat is Not Reaching Set Temperature

How to Tell If Your AC Capacitor Is Bad?

As noted, the capacitor provides the power needed to start the cooling process. If the capacitor is faulty, you are likely to notice one or more of these symptoms:

  • The air conditioner is buzzing but not starting
  • The AC fan is not running
  • Delayed cooling after turning the system on
  • The AC unit starts shutting off at random
  • The AC is refusing to turn on
  • A hike in energy bills
  • The vents release warm air while the AC is supposed to be on

Any of the above signs should alarm you. However, it is best to look out for more than one symptom to rule out other problems such as clogged vents, a faulty blower, a dysfunctional condenser, or an aging compressor. Different systems operate using different mechanisms. If your system has a heat pump, it may work differently from other types.

The most important thing to look out for is a fan. If the fan is not running and the humming noise is coming from the unit, then you can be certain that you have a faulty capacitor.

You can visually inspect the capacitor once you have identified the problem. In most cases, the capacitor will have a slightly puffed-up top if it has gone bad. It may also be rusty or leaky. Once you notice any of these signs, be assured that the capacitor is your source of trouble.

What Causes a Capacitor to Go Bad in the AC Unit and How to Prevent It?

This device is both a mechanical and an electrical device. It is, therefore, prone to mechanical and electrical damage.

The capacitor will weaken with age, losing its ability to supply the required amount of energy to power the system. Unlike filters that have specified lifecycles, capacitors can serve for many years without developing issues. However, they work best when all the units in the system are functional, such as blowers, fans, condensers, heat pumps, capacitors, and vents.

Capacitors tend to overheat when they remain in a circuit for too long. They are not designed for continuous usage or continuous cycling. If any part of the air conditioning system is not working efficiently, these problems will weaken the capacitor due to overheating.

Other problems that could cause failure or damage to the capacitor include reverse polarity, mechanical damage, insulation failure, or poor handling.

It is always advisable to check safety precautions when dealing with internal components of an HVAC system. Do so on an independent unit or part of the central heating and cooling system.

Before handling different parts of the unit, preferably take a picture to use as a guide when reassembling the unit. You don’t want to make common mistakes, such as reverse polarity or short-circuiting the unit.

Read Also: Central Air Conditioner Smells Like Chemicals

How to Repair the Air Conditioner That is Buzzing But Not Running?

Start with prevention measures, but now that you are here already, you have two options. You can go for a quick option to get the system running while looking for a long-term solution.

A Quick Fix

The quick fix is quite rudimentary because it involves manual spinning. All you have to do is start the fan manually. You can use a screwdriver or anything else that can be used to spin the fan. You will be solving the hard start problem by providing the energy that the system requires to start.

This spin is a temporary maneuver and should not be done regularly. It should be an emergency intervention aimed at kick-starting the fan’s motor. Furthermore, take caution when attempting to spin a fan. You can damage the fan or even expose yourself to shock. Handle all electrical appliances with care.

As for the capacitor, even a limping one can be lethal. Capacitors carry ridiculous amounts of energy. They are also designed to release energy in bursts, meaning that even an old and dysfunctional capacitor could be dangerous.

The Lasting Solution

The long-term solution is to replace the capacitor. Again, handling the capacitor is not for everyone. You need the right tools and the technical know-how to determine which capacitor you need for your unit.

The process is simple if you know what you are up to. Start by disconnecting the AC unit from the electrical source, remove the panel to access the capacitor, check the voltage and capacitance rating, order a replacement, discharge the existing capacitor, remove it, and replace it. Double-check your work before turning the AC on to test it.

If your unit is old, you may consider replacing the entire air conditioner, especially if it has been giving you trouble lately.

With a lifelong fascination with HVAC mechanics and a mechanical engineering degree from Berkeley, Thomas Johnson boasts years of industry expertise. He founded AirComfortLab.com to fill the void of understandable and useful online information about complex HVAC systems. As the chief editor, Thomas strives to provide accurate and engaging content to help readers make informed HVAC decisions.

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