Why Do Coils Freeze?

For many homeowners, air conditioning (AC) is essential for staying comfortable during warm weather. However, one common issue that can compromise an AC system’s performance is evaporator coil freezing. Frozen coils prevent the AC unit from cooling effectively and can cause catastrophic damage to the system if left unaddressed.

Here are some photos of what frozen coils look like in an HVAC system.

What Are Evaporator Coils Anyway?

For most homeowners, the HVAC system can seem pretty mysterious. This magic box, often tucked away in the closet, attic, or garage just keeps the house comfortable year round. Most people don’t give their systems much thought until they realize their house has gotten uncomfortable. Understanding the key components of the system is critical to understanding why coils freeze and why that’s a bad thing.

Here’s a diagram showing the major components of an air conditioning system with ‘A’ being the evaporator coil. For those interested in learning more about evaporator coils and how air conditioning, check out this great explainer video on YouTube.

Source: U.S. Department of Energy — Energy Saver 101 Infographic

An HVAC system typically consists of an indoor system and an outdoor system connected via refrigeration lines. The outdoor system (the condensing unit) compresses refrigerant and cools this hot, high pressure refrigerant into a liquid. This high pressure refrigerant flows through the pipes into the home and enters the evaporator coil where the refrigerant expands. Just like how a can of compressed air gets cold when you pull the trigger, the expansion of the refrigerant drops the temperature of the evaporator coil. When air from inside the home is pulled over the evaporator coil, the air cools down and is returned back throughout the home. When everything works as intended, everyone stays happy.

Evaporator coils are critical for cooling the air that circulates through your home. These coils rely on a steady flow of warm air and adequate refrigerant to function properly. When that airflow is restricted or refrigerant has leaked out of the system, the temperature around the coils drops too low, leading to ice formation. Understanding why coils freeze and how to prevent it can help keep your air conditioning system running smoothly, ensuring a comfortable indoor environment and avoiding unnecessary expenses.

So Why Do Coils Freeze?

Unfortunately, from time to time and often when most inconvenient, something in this system breaks down. When the system is low on refrigerant (often due to a leak) or airflow is restricted, the temperature of the air around the evaporator coil falls below 32°F and freezing begins. Understanding these two common causes can help you understand which is causing issues in your system.

Refrigerant Issues

When refrigerant expands in an evaporator coil, the refrigerant and coil get cold. These HVAC systems are closed loop and rely on precise levels of refrigerant to operate properly. When refrigerant leaks out, often due to the age of the system, the refrigerant pressure falls too low in the evaporator coil. Just like the body needs sufficient blood to maintain blood pressure, the HVAC system needs sufficient refrigerant to operate properly. When enough refrigerant has leaked out, the evaporator coil will freeze and cease to function properly.

Source: Choate’s Air Conditioning, Heating, Plumbing and Generators

Airflow Restrictions

Proper air flow is crucial to maintaining airflow and preventing coil freezing which is why measurement of the filter’s pressure drop is so important. The air flowing through an HVAC system can have a lot of contaminants in it. With a proper air filter installed, the air filter will collect these contaminants and keep the rest of the system clean. Without an air filter, the evaporator coil itself will collect contaminants and will have to eventually be cleaned to keep the system working. Filters are designed to collect contaminants, not coils.

Source: Adams Air Conditioning

While filters are designed to collect contaminants, they need to be changed out when they are sufficiently dirty. As filters become clogged with dust and particles, airflow can decrease significantly. Without measurement, it can be difficult to tell whether a dirty filter caused the freezing or something more serious.

Source: Made with Grok

The Consequences of Evaporator Coil Freezing

When evaporator coils become a block of ice, the HVAC system can’t keep the rest of the home cool. The block of ice physically prevents air from moving through the system. Left unaddressed, frozen coils can also damage the compressor, one of the most expensive parts to repair or replace. The compressor can overheat as it tries to maintain system pressure despite ice buildup, which can lead to mechanical failure. Frozen coils can put strain and do damage to one of your home’s most expensive investments which is why it is a critical issue that must be addressed quickly.

Preventing Coil Freezing with Regular Maintenance and Quality Air Filtration

The simplest way to prevent coil freezing is through monitoring filter pressure and regular HVAC system checkups. Because filter life is dynamic, monitoring is critical. Time of the year, pets, construction, humidifier usage and many other elements play a role in how long a filter will last. If you don’t measure, there’s no way to truly know.

High-quality filters like Woosh’s MERV 13 filters are designed to pull the bad stuff out of the air while reducing the load they add to the HVAC system supply fan. With the built-in pressure sensors, Woosh filters are continuously monitored to determine how full they are and notifying the user when it is time to change them out to a clean one. Beyond filter changes, make sure ducts and vents are clear and schedule professional tune-ups annually to ensure your AC system is in optimal condition.

Source: Made with Grok

Protecting Your AC with Proactive Maintenance and Smart Technology

Evaporator coil freezing is a preventable issue that impacts comfort, energy costs, and system lifespan. Consistent maintenance, quality filters, and airflow monitoring can make all the difference. High-quality filters like Woosh’s MERV 13 keep your air cleaner and help prevent freezing issues. Explore Woosh’s smart filter options to maintain healthier air and keep your system running smoothly.