Frozen AC Line? Why There’s Ice on Your Outside AC Pipe

The presence of ice on the outside AC pipe, also known as a frozen AC line, is a common issue in air conditioning systems that can disrupt cooling and cause potential damage. Here’s a summary of why your outside AC pipe may freeze and the underlying causes:

Causes of a Frozen AC Line

  • Low Refrigerant Levels: One of the primary reasons for a frozen AC line is low refrigerant levels. Insufficient refrigerant can lead to the evaporator coils becoming excessively cold, causing moisture in the air to freeze on the coils and the outside pipe.
  • Poor Airflow: Reduced airflow across the evaporator coils due to dirty or clogged air filters, blocked vents, or a malfunctioning blower fan can cause the coils to get too cold and freeze, extending the ice formation to the outside pipe.
  • Thermostat Issues: Incorrect thermostat settings or a malfunctioning thermostat can lead to the AC system running longer than necessary, resulting in overly cold evaporator coils and ice formation.
  • Refrigerant Leaks: Refrigerant leaks not only lead to low refrigerant levels but can also disrupt the temperature balance within the system, causing the coils to freeze.

What to Do When You See Ice on Your AC Pipe

  • Turn Off the AC: If you notice ice on your outside AC pipe, it’s essential to turn off the AC system immediately. Continuing to run the AC can exacerbate the issue.
  • Thaw the Pipe: Allow the ice on the AC pipe to thaw naturally. Do not attempt to chip away the ice, as this can damage the components.
  • Check Airflow: Inspect and clean or replace air filters to ensure proper airflow. Remove any obstructions or blockages from vents and registers.
  • Professional Inspection: Contact a qualified HVAC technician to diagnose and address the root cause of the issue. They can check for refrigerant leaks, assess the condition of the evaporator coils, and ensure the system is functioning correctly.
  • Regular Maintenance: To prevent future occurrences, schedule regular AC maintenance, including cleaning coils, checking refrigerant levels, and maintaining proper airflow.
Ice on outside AC pipe

But what really causes the outside air conditioner pipe to freeze? Or rather why would there be ice on the AC’s outside refrigerant lines?

Ice on the outside air conditioner pipe is a sign that there is a restricted flow of warm air to the evaporator coil. That is caused by blocked ducts, dirty evaporator coil, clogged air filter, leaking/collapsed ducts or a faulty blower fan. It could also be caused by low refrigerant levels.

When airflow to the AC’s evaporator coil is restricted, the temperature of the refrigerant falls below freezing point. When that happens, the moisture in contact with outside AC pipe freezes and that is how ice is formed on the pipes.

To unfreeze air outside conditioner pipes, turn off the AC and give the ice time to thaw. After that, turn on the fan only to circulate warm air and dry the frozen coils and pipes. When the coils and pipe are dry you can then turn on the AC.

Usually, when the outside AC lines are frozen, it is also very likely that the evaporator coil is frozen as well. Actually, the problem originates from the evaporator since it is the coil that handles the refrigerant when it is a cold liquid.

To prevent your AC pipes from freezing (and any other part of your AC for that matter), have an HVAC technician come over and inspect the ductwork for clogs or leaks, change the air filter, clean the coils, tune up the AC and check the refrigerant levels. You can however easily change the filter on your own.

If you outside AC pipe only freezes at night, there is also a chance that the night temperatures are quite low. In that case you can consider turning off the AC at night or install a programmable thermostat which will automatically turn the AC off when temperatures dip below a certain point.

Related: Why your AC unit only freezes at night.

How an Air Conditioner Works

To understand why there is ice on your outside air conditioner’s pipes, I believe it is important to first understand the parts of an air conditioner and how they work.

An air conditioner has 4 main parts:

  • Evaporator coil
  • Compressor
  • Condenser coil
  • Expansion valve

The 4 components are connected together using copper pipes. A chemical with a low boiling point and high latent heat of vaporization known as a refrigerant or coolant is then circulated through the 4 components.

It is the refrigerant that actually cools the indoor air by absorbing its heat and releasing it outside. The evaporator is located inside the house (where heat is absorbed) while the condenser coil and compressor are outside the house (where the heat is released).

how an AC works diagram

The refrigerant enters the evaporator coil as a cold refrigerant. At the same time, the coil’s fan will be pulling warm air from the house through the return air ducts.

As the warm air is pushed across the coils, the refrigerant absorbs heat from it, cooling it in the process. The cooled air is then forced out and into the house through the supply air ducts.

After absorbing heat from the indoor air, the refrigerant vaporizes and is ejected out of the coil and enters the compressor.

As its name implies the compressor compresses the refrigerant gas, increasing its pressure. When the refrigerant is compressed, it means than the molecules now have more energy which then means that their temperature increases as well.

Note: Low-pressure molecules have less energy while high-pressure molecules have more energy. This rule is very important in air conditioning.

Since the refrigerant now has a higher temperature than the surrounding air, heat can be transferred from the refrigerant to the outside air.

The refrigerant therefore enters the condenser coil as a high-pressure superheated gas. As it moves through the coil, the condenser fan blows cooler air across it which absorbs heat and releases it to the surrounding air.

By the time the refrigerant is coming out of the condenser coil, it will have condensed back to its liquid state. However, it will need to go through an expansion valve to lower its temperature even more.

As I mentioned, when the refrigerant is compressed, its molecules gain more energy and therefore have high pressure and high temperature. The reverse is also true.

The expansion valve is a small orifice which the refrigerant is forced through. As it emerges from the other side, it expands effectively lowering its pressure and temperature as well.

At that time the refrigerant is cold enough to enter the evaporator coil for another round of cooling.

Why You Have a Frozen AC Line/Pipe?

Frozen outside air conditioner pipe

As I mentioned, there are 2 main reasons why there is ice on your outside air conditioner pipe:

  • Restricted air flow across evaporator coil
  • Low levels of refrigerant

It could also be caused by the following:

  • Clogged condensate drain line
  • Low night temperatures

Let us now look at the 2 main causes in more details:

1. Restricted Airflow across Evaporator Coil

As I mentioned, the evaporator coil fan pulls warm air from the house and pushes it across the evaporator coil for cooling to happen. If the flow of air to the evaporator is restricted, the system suffers imbalance resulting in icing over on the coil and AC pipes.

What actually happens is that the refrigerant keeps on expanding, which means its pressure drops even more resulting in its temperature falling below freezing point.

When that happens, the moisture in contact with AC pipes condenses and ice starts to form. That is also happens to the evaporator coil.

So, what causes restriction of warm air from the house to the evaporator coil? Let us look at each one of the causes.

Blocked Air Ducts

Return air ducts are responsible for bringing warm air from the house to the coil. However, it is not unusual for return air ducts to clog or even collapse, preventing the air from getting to the evaporator coil.

Leaking Air Ducts

Air ducts are also prone to leaking. This is actually one of the reasons why central air conditioners are not as energy-efficient as mini-splits. More on that in this post.

When warm air from the house leaks through the ductwork, the air conditioner will not work as designed and your air conditioner will surely freeze.

Clogged Air Filter

Air conditioners are equipped with air filters which are installed in the return air ducts. The filters removes dust, lint, pollen, dander hair and other impurities from the air.

To keep your system running optimally, you need to change your air filter after every 3 months or sooner than that. Failure to do that will result in it being clogged, and restrict the flow of air to the evaporator coil.

Dirty Evaporator Coil

Although the system as I have mentioned is equipped with a filter, some particles will still move past the filter. Unless the evaporator is cleaned regularly, the particles can cake over the evaporator coil cover, preventing warm air from the reaching the coil.

Faulty Blower Fan

The evaporator blower fan is the one that actually pulls warm air from the house and moves it across the evaporator coil. If that fan is damaged, again the refrigerant pressure inside the coil will keep on dropping causing the AC pipe and coil to freeze.

2. Low Refrigerant Levels

In order for your air conditioner to work properly, the refrigerant level/pressure needs to be right. Luckily, the air conditioning system is a closed-looped system and the refrigerant level should therefore remain constant at all times.

That however does not always happen. If the refrigerant is leaking, its level will definitely be low.

It is not easy to tell if you AC refrigerant is leaking since the smell is very faint. The signs that your AC is low on refrigerant are:

  • AC is running but not cooling the house
  • The energy bills are unusually high
  • Hissing/bubbling sounds when the AC isn’t running
  • The air from the vents is warm

When the AC refrigerant is leaking, it means that its pressure will also drop. As I mentioned earlier, there is a special relationship between the pressure and temperature of the refrigerant.

When the pressure drops, the temperature will also drop. In this case, it will drop more than it should.

Note that the temperature of the refrigerant as it enters the evaporator coil is 32 degrees. When that is the case, the moisture in the indoor air (humidity) condenses on the coil and drains out through the condensate drain line.

However, when the refrigerant levels are low, its temperature will drop below 32 degrees, which is the freezing point of water. As such, any moisture that comes into contact with the coil will freeze and ice over.

The freezing works its way back towards the condenser coil through the AC refrigerant lines/pipes. Actually, if this problem is not fixed, liquid refrigerant can flow back to the compressor and damage it.

Does Your Outside AC Pipe Only Freeze at Night?

Sometimes you will notice that your air conditioner outside pipe only freezes up at night. Why is that though?

What actually happens is that some nights are quite cold and remember that air conditioners are designed to operate within a certain temperature range.

When the temperature drops at night, the pressure of the refrigerant also drops, which then means that the temperature of the refrigerant will dip below freezing point. And that is why you will notice that the AC unit only freezes up at night.

What you can do in this case is to turn off the AC at night. If the temperatures are that low it is a sign that you likely don’t need to have the AC running.

An even better idea is to have a programmable thermostat installed. The thermostat will turn off the AC when temperatures dip below the set value and turn it on when temperatures rise again.

How Do I Unfreeze My Air Conditioner Pipe?

It is not recommended to run your AC when the pipes are frozen. The first thing you should do therefore is to turn off the AC and give the ice time to thaw.

Also make sure you check if the evaporator coil is frozen as well. As I said, that is where the freezing starts. The coil will be located where the furnace is located, which is located very close to the water heater in the basement, crawlspace, and attic or in a utility closet.

When the ice has melted, you can use paper towels or just cotton towels to dry the water from the coils and AC pipes. After that turn on the AC but set the thermostat so that only the fan will be running.

By circulating warm air through the system, both the coil and AC pipes will dry completely. Only then can you turn on the entire AC system.

To prevent the air conditioner from freezing again, you will need to bring an HVAC technician to check if there a problem with any of the AC components.

Some of the things the technician will do are:

  • Inspect the ductwork for leaks
  • Clean the ductwork
  • AC tuning up
  • Fix faulty coil blower fan
  • Clean evaporator coil
  • Check condensate drain line for clogs
  • Replace air filter
  • Check refrigerant level.

Of the above, changing the filter is the easiest and you can even do it on your one in minutes. Check out this post on how to locate and change your AC filter.

Wrap Up

And basically those are the reasons why there is ice on your outside air conditioner pipes. Although it is a normal occurrence, it is completely avoidable. Scheduling regular AC maintenance is a great way to prevent your AC from freezing up.