What To Do When Your AC Unit Freezes Up Inside?

When your AC unit freezes inside, there are quite a few different issues that might be affecting your HVAC’s health. A few problems are relatively simple, and you can remedy those without professional assistance. However, the more complicated glitches warrant an inspection by a certified HVAC technician.

When your AC unit freezes up inside, turn the air conditioner off and the fan on to let the ice melt. Don’t operate the AC until the entire unit defrosts and dries. Meanwhile, inspect the filters, vents, dampers, ducts, coil, handler, motor, condensate line, and outdoor unit. 

An HVAC unit is a self-contained system with several interconnected and interdependent parts. Thus, a significant glitch anywhere can cause or facilitate serious problems. This guide explains what you should do when your AC unit freezes up inside, including the common and rare fixes.

8 Things To Do When Your AC Unit Freezes Up Inside

The most common cause for an AC unit freezing up inside is inadequate or no airflow. Dirty or clogged air filters on the return vents, obstructions in the ducts, a malfunctioning blower, or a broken motor can reduce and even stop the airflow into the evaporator chamber of your indoor AC unit. 

The other typical causes of an AC unit freezing up inside are a dirty evaporator coil, refrigerant leakage, or a clogged condensate line or drainpipe. Also, you may have a bad circuit board, capacitor, restricted metering device, oversized air conditioner, or undersized return ducts

So, what do you do when you aren’t sure what the problem is? I’ll walk you through the steps you should follow to diagnose (and hopefully fix) your AC unit. 

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1. Turn Off the AC and Switch Fan From Auto Mode to On

Always turn off the AC immediately if the unit freezes up inside, irrespective of the severity of ice or frost buildup. You may turn off the AC through the thermostat or circuit breaker. Now, turn on the blower. Don’t set the fan on auto, and allow it to run as long as the unit defrosts and dries. 

The AC unit may take several hours or more than a day to defrost thoroughly. Wait until the entire system is dry before turning on the AC again. 

Ideally, you should use this time to investigate the underlying issue. So, follow the subsequent steps in this guide to prevent your AC unit from freezing. 

2. Test the Blower’s Efficiency and Inspect the Motor

The blower or fan must operate efficiently throughout the defrost cycle. An inefficient blower will not circulate sufficient air through the return inlet to the AC unit

When this happens, the AC unit can’t draw in the air or spit it out, and it’ll constantly chill itself with the cold air it produces. So, a failing blower or motor can be the cause of your AC freezing up inside.

The evaporator chamber in your AC unit needs sufficient warm air to come in contact with the coils and cool down. Otherwise, the cold refrigerant in the coil will cause rapid condensation, and any moisture or humidity in the air available inside the evaporator chamber will freeze. 

Check the blower motor inside your HVAC unit. A hot motor has burnt components, so you must replace it. If the blower or motor has no power, you may have a bad control board. Also, the fan and motor may need lubrication. Preventive HVAC maintenance can prevent these problems. 

3. Clean or Replace the Return Air Filters and Others

After verifying that the blower and motor are functioning optimally, you’ll want to inspect the air return filters.

Begin with the air filter between the return duct and the HVAC unit and look for damage or clogs. If you see dust, debris, or ice, clean the filters. If they are a lost cause, just replace them. 

Then, you can check all the air filters throughout the house, whether on return or supply vents.

Also, don’t forget the filter on your air handler unit. Anything that restricts the airflow in the supply and return vents will impair the air circulation to the AC unit. Thus, the refrigerant in the evaporator coil will not have sufficient warm air to absorb heat from, and you will have a frozen unit. 

Read: Does HVAC Filter Direction Matter?

4. Open All the Dampers and Supply Vents

Next, ensure the dampers in your supply ducts are open. When checking the air filters, inspect if any supply vent is closed, entirely or partially. Be sure to check and open up the vents in lesser-used rooms in your home

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5. Clean the Ducts and Remove Obstructions From Vents

Ducts and vents should not have any obstructions. 

Check if the return vents anywhere in your house have furniture or fabrics placed in front of them that might be blocking the airflow back into the HVAC unit. Also, you may have extensive dust and debris buildup inside the ducts and vents. Be sure to clean the vents and ducts as much as possible since the dust and debris buildup impairs the airflow. 

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6. Inspect and Unclog the Condensate Line and Drain

The condensate line connecting your AC unit with the outdoor drain pipe must not have any blockage. Otherwise, the water and debris buildup will back up inside the small reservoir attached to your HVAC unit, and gradually more moisture will condense and freeze on the coils.

To check for blocks, follow the condensate line and drain to the outlet outside, where you will see water dripping out constantly as your frozen AC defrosts and dries. If there is no water dripping out of the drainpipe, you may have a clogged condensate line, which usually causes frosting. 

The small condensate reservoir of your HVAC unit has a pump. If there is no blockage in the pipes, you may have a faulty pump. Also, you may have some water pooling on your basement or garage floor during the defrost process. Use old sheets or rugs to contain the mess.  

Watch this video to clean a clogged condensate drain pipe:

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7. Check and Clean the Evaporator Coil of the AC Unit

Open the front panel of your indoor AC unit to inspect the evaporator chamber, which you may have already done to find the freezing problem in the first place. 

As the ice melts, you may find a dirty evaporator coil. Also, it is not unusual for evaporator chambers to get moldy and rusty, creating blockages.

Generally, older AC units tend to have rusty evaporator chambers. However, relatively new coils may develop rust if the unit has repeatedly frozen and defrosted. Also, any abnormal rust buildup may imply refrigerant leakage or a damaged evaporator coil. 

You may clean a dirty evaporator coil after it defrosts and dries. Alternatively, you may contact a certified HVAC technician for thorough maintenance. Damaged or malfunctioning AC parts need a more comprehensive inspection and specific repairs for a permanent solution.

Read: How To Unfreeze An AC Unit?

8. Call a Certified HVAC Technician To Fix Refrigerant Leakage

Low refrigerant is a common reason for an AC unit freezing up inside. However, you should not check for leakage nor attempt to refill the refrigerant. Only a trained & certified HVAC technician should handle refrigerants. Besides, you may have other glitches causing the freezing problem. 

An AC unit can freeze up inside if the metering device in the coil is blocked or malfunctioning for any reason. Also, you may have electrical continuity problems due to the circuit board, a broken capacitor, a failing expansion valve outside, and condenser or compressor issues. 

All such complicated issues require taking apart your indoor and outdoor units for a thorough inspection. Likewise, an oversized AC or undersized return ducts can cause the indoor unit to freeze up due to an excessively cold evaporator coil or less than required airflow in the system.

So, if you haven’t found a problem by now, it’s likely something complicated enough to warrant a call to the pros.

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Final Thoughts

Always defrost and dry a frozen AC unit before restarting it. Do not operate an AC at low temperatures to prevent freezing. Inspect all the components highlighted in this guide and choose the appropriate remedies so your AC unit does not freeze up inside again.