air conditioner coils frozen on outdoor unit
ice on air conditioner ac line cause of extra cooling performance in hot temperature

Why is my AC freezing up? 7 Reasons and Troubleshooting Tips

Your trusted air conditioning unit that provides relief from the scorching heat can build up frost and become non-functional. An AC freezing up is a common problem, but few people understand its cause. Moreover, this can happen to the best air conditioning units from the biggest brands and at any time of the year. Usually, the problem creeps up without noticeable signs. It can often lead to costly repairs due to ice formation on the evaporator coil or in the form of a damaged blower fan.

A central AC does more than cool the room's interiors. A properly functional air conditioner should also lower humidity, and many air conditioning units are as adept at removing dust particles via advanced filters. However, ice buildup is a problem associated with all types of AC models, and it affects the compressor, impairing the airflow. If this problem persists for too long, the damage can be severe.

What causes an air conditioner to freeze up?

An AC freezing up happens when there is ice buildup on the unit, a frozen AC line, or when ice forms on the evaporator coil. The first two problems can generally be identified with a visual check. However, determining whether your evaporator coil is frozen is a bit tougher and often requires professional help. You have to open the panel door on the AC unit and inspect for ice buildup—though, even in the absence of ice buildup the evaporator coil might still be frozen.

Knowing how to spot and address the signs of a poorly functioning air conditioning system can help you protect the unit from further (and costlier) damage.

1. Dirty Air Filter

The primary function of an air filter is to capture the air pollutants like mold, dust, and dirt, keeping them away from the AC ducts. The constant airflow keeps the evaporator coil free from humidity. The filter has a slightly porous structure for better filtration performance. Still, the buildup of pollutants on its surface can restrict and reduce the airflow to the HVAC system, allowing humidity and, eventually, freezing to build up.

Cleaning or changing a dirty air filter is easy but with some basic precautions.

  • First, turn off AC's thermostat cooling
  • Turn the fan on to blow warm air on the coil to accelerate the defrosting. Caution: Don't use hot water to accelerate the defrosting. Pouring hot water can cause irreparable damage.
  • Now, check the air filter

Do you know? The United States Department of Energy states that replacing dirty air filters can help improve the energy efficiency of an AC by 5-15%

dirty air conditioner filter

How to Fix the Problem

Most technicians recommend changing the air filter every three to four months. However, some homeowners might need an early replacement, depending on the type of air filter, the number of people living in the house, climatic conditions, amount of smoke within the room, everyday AC usage, and the presence of household members living with a health condition such as asthma, allergies, COPD, or congestive heart disease. A good test to see if you need to replace your air filter is to notice if light can shine through the filter or not.

man holding tools to measure the refrigerant levels of an air conditioner

2. Low Refrigerant

The second most common reason for your AC freezing up is low refrigerant levels. An AC's ability to maintain a consistent temperature is dependent on the refrigerant. Most refrigerant leaks go unnoticed for long periods and can be found in the refrigerant lines, compressor, or evaporator coil. Low refrigerant levels can affect the evaporator coil's performance, resulting in moisture accumulation. Over time, this can cause the coil to freeze up.

Delay in restoring the necessary refrigerant levels can cause severe damage to the compressor, and replacing or repairing a faulty compressor can be very expensive. People who have an older AC unit with a damaged compressor are likely to consider purchasing a new unit because of the costly replacement part.

How to Fix the Problem

If you do suspect a leak, turn off your air conditioner immediately. Contact an HVAC professional to perform a leak check.

3. Damaged Blower Fan

Drawing warm air into the ducts and circulating cool air throughout the house is the primary function of a blower fan. However, this heat transfer process can halt if the blower fan is damaged or malfunctions. Too much condensation can build up on the coils if the blower fan is not functioning correctly. Without enough airflow, the coils and refrigerant line may freeze over.

You might need a blower fan replacement or repair if you suspect any of the following issues:

  • Reduced blower fan speed
  • Difficulties in starting the AC unit
  • Constant whirring or humming sound
  • A severe reduction in warm air blown externally

How to Fix the Problem

Contact your local HVAC technician to check the age and overall wear condition of the blower fan and its motor. They should also check the fan's rotation speed and power output to ensure maximum functionality. Depending on the extent of damage, it may require repair or replacement.

air conditioner unit blower fan location  illustration
white floor vent cover in home on wood floor and blue wall

4. Blocked Vent Returns and Registers

HVAC vents and registers are designed to pull in the warmer, humid air and carry it to the handler where it gets cooled and dehumidified. Many people aren’t aware that vent blockages or too many vents being closed off can lead to an AC freezing up. When not enough air circulates over the coil, moisture condenses and freezes.

How to Fix the Problem

  • Turn the thermostat to the “off” or “fan” position and allow the coil to thaw
  • Move furniture and other objects away from any returns
  • Don’t close more than 2-3 supply vents

5. Air Duct Issues

Ducts are passageways for delivering and removing air from an HVAC system. Leaks and obstructions impede the proper flow of air, which prevents warm air from reaching the coils, causing them to freeze.

How to Fix the Problem

  • Check for dust and obstructions, such as curtains, furniture, near air vents
  • Make sure that you have routine maintenance performed
  • Contact an HVAC professional for an airflow inspection
dirty HVAC air ducts
dirty and clogged air conditioner evaporator coil

6. Dirty and Clogged Coils

Your evaporator coil captures heat from the air inside the home and turns this heat into cooler air. In other words, it is one of the primary components of the heat exchange system. Because the coil is damp, it easily catches dust and debris. Dirty coils restrict airflow.

How to Fix the Problem

The first thing you need to do is to turn the AC unit off to prevent further damage. When it comes to cleaning dirty evaporator coils, you want to enlist the services of an HVAC expert.

7. Thermostat Levels

Most air conditioning systems have a predetermined temperature range, and they can't function if the outside temperature falls below 60°F. As a result, many homeowners experience frequent freezing because the low evening temperatures prevent the air conditioner from working correctly. It also causes lower pressure inside the unit, resulting in ice buildup.

How to Fix the Problem

  • Turn off the system and let it thaw
  • Avoid running your air conditioner overnight
  • Only place your thermostat at low levels during heat waves
  • If the problem persists, take the help of an HVAC technician
african american woman adjusting air conditioner thermostat on wall of home

How do I keep my air conditioner from freezing up?

Detecting early signs of your AC freezing up can be challenging. However, if you are watchful, there is every chance of noticing some initial signs of the problem. This includes a reduction in blower fan speed and problematic, noisy air filters. Many AC freezing-up problems can be quickly resolved with regular maintenance and timely tune-ups. An experienced HVAC technician can identify the earliest signs of such problems, preventing mechanical failures. Regular inspections by a qualified AC technician help to diagnose dirty coils or obstructed ductwork in time, preventing costly replacements.

In addition, follow these tips to keep your AC working in top-notch condition:

  • Change the air filters every few months
  • Keep the blower and fan clean and dust-free
  • Ensure proper venting and ducting
  • Schedule frequent inspections if you use the AC regularly

Preventing your AC from freezing up 

If you catch it in the early enough stages, you may be able to handle an air conditioner freezing up yourself. Still, if the freeze-up results from a refrigerant leak or a damaged evaporator coil, it may be time to consider purchasing a new air conditioning system.

The Furnace Outlet offers the lowest prices on residential HVAC supplies online. Please get in touch with our experienced technicians with any questions or concerns you may have, and we will be happy to help.

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1 comment

Ricardo Montalvo

Ricardo Montalvo

Very helpful

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