Why Does Ice Form in Mini Split AC?

Although it’s not as common as, say, a heat pump freeze up, ice formation in the indoor air handler can happen without notice. It often begins inside the air handler, meaning that there could be a block of ice inside when you notice ice beneath the air handler.

It’s why you’re always encouraged to check the air handler (and entire air conditioner) at least once every few days to make sure that everything is okay. You’d be able to catch the icing issue before it becomes a severe problem.

5 Reasons for Ice Formation in Mini Split AC Indoor Unit

If you’re wondering why ice would form in the air handler of all the places, the following are a few common reasons;

Restricted Airflow Issues

The first and most common cause of air handler icing is restricted airflow. In regular operation, air flows freely from the room/home into the air handler via the return vents, gets cooled, and flows back into the home/room – again freely. It’s an effortless process that goes on smoothly and repeatedly to reach and maintain the thermostat setting.

Cooling happens at the evaporator coil, where coolant material absorbs heat from the return air, resulting in a substantially cooler air supply.

However, the process can fall apart if a blockage constricts the smooth flow of air in and out of the air handler. Cool air might linger around the evaporator coil and fins too long. Since more cool refrigerant keeps cycling through the evaporator coils, the extremely cold conditions may eventually cause the air to freeze up.

The freezing often starts as a minor issue but may grow into a block of ice that eventually causes the air handler to freeze. So, what causes restricted airflow in the air handler?

Blocked Air Filters

All air conditioners are equipped with an air filter at the return channel to trap dust and other particles that might enter the air handler and the evaporator fins. Some AC units even have dual-filtration systems. If the filter is blocked, airflow through the AC can be constricted, potentially resulting in a frozen air handler.

Solution: Ensure to clean the air handler filters regularly. You want to take them out and thoroughly wash them at least once every 1-2 weeks. Also, remember to change the filters every 1-2 months or more frequently as advised by the manufacturer.

Clogged Evaporator Coils

Aside from the filters, the evaporator coil itself can become clogged and blocked. This often happens if you fail to clean the filter as required. In addition, some of the dirt trapped in the filter may find a way into the evaporator unit and the coils. Over time, the dirt can build up, constricting the air passage.

Solution: There are spray-type coil cleaners that you can purchase from the hardware store and use to clean clogged evaporator coils safely. If the spray doesn’t work, you have no other choice but to call an HVAC technician to clean the coils.

Fan/Blower Issues

Finally, the AC air handler’s constricted airflow can also result from a compromised fan or blower system. Remember that the fan is responsible for drawing air into the AC, forcing it through the condenser coil, and blowing it out at the other end of the AC and into the house. A damaged fan may not be powerful enough to perform this task effectively.

Solution: The first step is to ensure there’s no obstruction impeding airflow into the air handler. Also, make sure the thermostat is not set lower than 70˚F during the summer. After that, inspect the fan for damages. Did the motor malfunction? Did some of the fan blades break? Is the fan correctly wired? These are just some of the issues that may cause poor fan function.

Low Refrigerant

If you’ve checked and can’t find any signs of airflow constriction, then the ice-formation is likely the result of low refrigerant.

The refrigerant in an air conditioner is responsible for transporting heat into the house during heating and out of the house during cooling. It absorbs heat from the return air and dumps it outside or extracts heat from outdoor air and brings it inside the house, depending on whether you’re cooling or heating.

As such, problems arise if refrigerant levels are low. Coolant material absorbs very little heat at reduced pressure. This means that the amount of heat at the evaporator coils will drop.

The evaporator unit depends on the heat extracted from return air to keep moisture around the evaporator in gaseous form. Thus, a pressure drop can cause the moisture to freeze and eventually form ice.

Low refrigerant levels in the air conditioner can be caused by many issues, ranging from a leak in the refrigerant lines to factory defects. However, the leading causes are;

Poor Flare Connections

Flares are the special bolts that connect refrigerant lines to the compressor unit outside the house and the air handler inside the house. If you use the wrong flare or over-tighten the connection, it may cause a leak. Corrosion and natural wear and tear can also cause Freon leakage.

Solution: Flare connections and related issues are best handled by a licensed HVAC professional. Don’t attempt a DIY.

Line Set Damage

Line set damage refers to any damages that may pierce or crack the line set. The damages tend to happen more outside the house and may include scratches from lawnmowers and piercings during general home repairs. A tree branch accidentally falling on the line set may also bend it and cause cracks.

Solution: Always cover the line set to minimize the risk of damage. However, if a leak occurs (in most cases, you’ll hear a hissing sound), turn off the AC and call an HVAC professional immediately.

Beware of the Danger

The formation of ice in the mini split indoor unit may seem like a harmless issue. However, it can quickly turn into your worst nightmare.

Unless the issue is remedied immediately, you’re staring at the potential water damage and damages to some air handler components. In worst cases scenarios, you may lose both the air handler and the compressor.

Why Does Why You Have Ice Formation In Mini Split AC Outdoor Unit

It all has to do with how the air conditioner works. Mini-split air conditioners extract air from inside the house and dump it outside. The reverse is true in winter. The heat pump extracts heat from the outside air and sends it into the house to raise indoor temperatures.

When working as an AC, warm, stale air from your home passes over the evaporator coils located within the indoor air handler uni,t where the heat is extracted and carried outside by the refrigerant material. The now-cool air then returns into the house via the return vents.

It means that the evaporator coils must be very cold. This is usually facilitated by the cycling of the refrigerant. The refrigerant leaving the condenser outside the house for the evaporator unit indoors is very cold.

However, the coils are also not supposed to freeze. Otherwise, it would block airflow and compromise the functioning of the air conditioner. This process is also facilitated by the cycling of the heat exchange between the refrigerant and indoor air. The heat extracted from indoor air helps “balance” the temperatures inside the evaporator to prevent freezing.

Now, imagine a scenario where the temperature balancing process is compromised. Specifically, imagine a situation where there’s not enough heat to keep the coils from freezing! The evaporator coils would most likely freeze up.

But it’s not just the evaporator coils that would freeze up. The refrigerant would also become extremely cold, causing the refrigerant lines to freeze. In extreme cases, the freezing can extend to the outdoor condenser.  

The Two Main Causes of Outdoor Condenser Freeze-Up

So, why would there not be enough heat to facilitate the temperature “balancing” process to keep the evaporator coils frost-free? There are two common reasons;

Restricted airflow over the evaporator coils

As we’ve seen, the air conditioner depends on warm air coming from your home to keep the evaporator coils from freezing. If there’s impeded or constricted airflow, the coils may become too cold and ultimately freeze up. Restricted airflow over the evaporator coils can result from the following;

  • Clogged air filters: Air from your home passes through a set of filters when entering the air handlers. This is an important step in the cooling process as it protects the AC from dust and other particles that may cause malfunction. However, over time, the filters can build up too much dirt, causing blockage and airflow constriction.
  • Dirty evaporator coils: Although the filters do an excellent job in most cases, a compromised filter may allow smaller dirt particles to enter the air conditioner. If these particles build up on the evaporator coils, it can reduce the surface area available for heat exchange.
  • Malfunctioned blower fan: The blower fan is tasked with drawing stale, warm air into the air conditioner and pumping cool, fresh air out of the AC and throughout the room. If the fan fails to draw in enough warm air, you may not have enough heat to keep the evaporator coils from freezing.
  • Closed or blocked air vents: Finally, you may also have unusually low airflow across the evaporator coils if the air vents are blocked. Blockage at the vents can be caused by dirt buildup or a damaged vent. Or it could be a piece of furniture blocking the vent.


The first step to addressing airflow issues in your air conditioner is to ensure proper and timely maintenance. Make sure the filter, fan, and condenser coils are clean and free from obstructions. Also, make sure that there are no leaky ducts or fan malfunction. However, if the damage has already happened, act as appropriate by changing the filter or cleaning the vents accordingly.

Low refrigerant levels

Another thing that may affect the temperature balancing inside the evaporator coils, potentially resulting in freezing inside and outside the house, is low refrigerant levels.

Refrigerant is the lifeline of any air conditioning system. It’s like blood in the human body. Lose a little bit of it, and you might enter a coma. Lose too much, and you’re staring at death.

If there’s a low level of refrigerant in your AC, the capacity to absorb heat reduces drastically, often resulting in low cooling efficiency. It also means that the refrigerant may become colder, which can cause freezing inside and outside the house. Causes of reduced refrigerant levels include;

  • Refrigerant leaks: Refrigerant can leak from the line set, the condenser, or the indoor air handler. Inside the compressor and air handler, leaks are mainly due to wear and tear (that causes holes) and poor flaring. Meanwhile, in the refrigerant lines, it often happens when the lines are bent or pierced.
  • Malfunctioned condenser: The condenser unit is solely responsible for pump refrigerant around the air conditioner. It provides the pressure that keeps the refrigerant cycling. Therefore, a malfunction can cause low refrigerant levels.


Low refrigerant issues are best handled by a licensed technician for two reasons. First, you need a license to handle refrigerants. Otherwise, you may end up in jail.

Secondly, refrigerant is dangerous for your health. It causes respiratory conditions that may cause loss of consciousness and even fainting. Shut off the AC and call a technician.

Now You Know

Air conditioner ice formation can happen to anyone. However, in most cases, it happens if there’s poor maintenance. For example, you’re at greater risk if your AC filters are clogged, the vents blocked, and the fan damaged. Regular maintenance can save you from costly damages.

More importantly, though, always know when to call the pros. For example, if you notice signs of low refrigerant, it is best to shut off the unit and call an HVAC professional.