While your air conditioner’s refrigerant gets cold so it can absorb heat from your home’s air, it shouldn’t cause the system to freeze under normal operating circumstances. However, with certain problems, various parts of the system could freeze. This guide will discuss the science behind icing, where your system may freeze, and four common reasons that could cause it.

The Science Behind AC Icing

An air conditioner’s refrigerant gets cold because of what’s known as the Joule-Thomson Effect. This simply states that refrigerant will get cold when it expands and the refrigerant drops.

If you’ve ever looked at an ice-cold beverage on a hot and humid day, you’ve likely seen the moisture on the side of the glass or the water ring it leaves on the table. This happens because as temperature decreases, the amount of moisture the air can hold decreases. The excess moisture falls out, causing condensation, also called condensate in HVAC systems. When parts of an air conditioning system get cold enough, the condensate can freeze and form ice.

Where Your AC May Ice Up

Fortunately, only a few areas in your AC tend to ice up. The first is the evaporator coil and the air handler, located in the indoor unit. The next is at the compressor outside in the outdoor unit outside. Finally, exposed refrigerant lines leading to your evaporator or condensing coils may also ice up.

The key to remember is that to ice up, the surface must be lower than the current dew point and moisture in the air. This is why buried lines don’t tend to freeze. The following are common reasons an air conditioner may ice up.

1. Low Refrigerant

As discussed earlier, the Joule-Thomson Effect is a primary factor in how an air conditioner runs. However, it also explains why icing happens when there’s a leak and low refrigerant. With insufficient refrigerant charge, the refrigerant expands too much, dropping the temperature too far. The result is various parts of the unit will get too cold, causing the icing effect.

The best way to keep an eye on the refrigerant charge is through routine AC maintenance. During this visit, your technician will go through many steps to test and tune your system, including testing the refrigerant charge.

2. Restricted Airflow

Your air conditioner depends on its ability to circulate air through the system. When there’s insufficient airflow, the system won’t cool your home properly, an added strain causing additional wear and possibly even causing the system to ice up. This happens because the refrigerant cannot absorb heat and the system gets too cold. The following are some of the most common causes of restricted airflow through the system.

Dirty Air Filters

The air filter performs a critical job, keeping out contaminants that could restrict airflow within the system. However, when neglected, the air filter can also restrict that airflow. Unfortunately, neglected air filters are one of the most common causes of air conditioning problems, including icing.

If you notice icing, start by checking the air filter. Even without icing, change the filter on the recommended schedule. This is usually every 30 to 90 days, depending on the filter size and type and your home’s air quality.

Blocked or Closed Vents

Supply vents in central AC systems have dampers built in. While you can use these to help regulate the temperature in a room, they’re intended to help balance airflow, allowing for even cooling throughout your home. When you close or block these vents, air does not flow properly throughout your home, causing hot and cold areas. It can also cause air to be warmer or colder going into the system than what’s at the thermostat. When the air is colder, it can cause the system to ice up at the evaporator coil.

Start by checking all of your supply vents to ensure they are open. Next, check to make sure that each has enough space above and around it to allow the air to flow. The industry commonly recommends about 6 inches as a minimum. For best performance, avoid covering vents with furniture along with routinely checking and clearing anything that may have fallen over it.

Dirty Evaporator Coils

As the air conditioner runs, small particles will make their way into the system, some collecting on the evaporator coil. When this happens, especially when it combines with condensate from the cold coil, it forms a type of insulation on the coil. The result is that the coil starts getting too cold because it’s not absorbing heat effectively. However, it’s still cooling the air on the outside of the coil, which includes condensing the moisture from the air. Because it’s getting too cold, that condensate then freezes.

Annual AC maintenance is the best preventive action for dirty evaporator coils. During this visit, a technician will carefully clean the evaporator coil to ensure there’s no buildup causing icing or other problems.

Clogged Condensate Drain

With all the moisture that condenses from the air, it’s critical to have a way to drain it, which is the condensate drain. However, this drain can get clogged with dirt, mold, mildew, and more. When this happens, the condensate cannot drain off, which creates an environment with excess moisture that can freeze on various components, especially the evaporator coil.

To keep it flowing, you should plan to clean your condensate drain a few times per year. If you get routine HVAC maintenance in the spring and fall, your technician will clean the condensate drain as part of the maintenance protocol.

3. Mechanical Failure

There’s always the risk of a component failing within the system that’ll cause icing. The most common are failures with the compressor. These failures can cause a leak in the compressor, causing it to ice up.

Additionally, a failing fan motor may not spin at the proper speed to circulate enough air over the evaporator coils. Like all other airflow restrictions, this can cause icing inside your air conditioner unit.

4. High Humidity With Low-Temperature Settings

Finally, the humidity inside your home and the temperature setting on your thermostat may also cause problems. Higher humidity is less comfortable, and many people resort to decreasing the thermostat setting to solve it. However, doing this creates an environment where the coils are too cold, quickly freezing the moisture condensing out of the air.

If you have excessive moisture in your home, you likely need to add some additional form of dehumidification. You may want to consult an indoor air quality expert to determine whether a standalone unit or a whole-home unit is best for your situation.

For more than 40 years, property owners around Garland have turned to On Time Experts for HVAC and plumbing services they can trust. Our expert team provides heating and cooling repair, maintenance, and installation along with indoor air quality solutions and a wide range of residential plumbing services. Call to schedule your AC repair with one of our NATE-certified technicians to fix your system’s freezing problem.

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