Call a Professional
Before we get into why your central air conditioner is freezing up, you should know that dealing with this issue may be dangerous and should only be handled by a certified heating, ventilation and air conditioning (HVAC) professional. Contact your local HVAC company for an inspection of your central air unit to determine the cause and to safely handle the problem.
Reasons for Freezing
There are a few reasons why your central air conditioner has frozen. Each one is a possibility and should be inspected by an air-conditioning technician before proceeding with a solution.
Refrigerant Charge: If your central air conditioner's refrigerant is improperly charged or lacks refrigerant it will freeze. This typically happens when your refrigerant lines have a leak. Leaks can occur due to friction from vibration or parts rubbing together. Leaks can also be found where fittings or shoulder joints are loose or weak. Only a qualified technician should test the system and recharge the refrigerant.
Insufficient Air Flow: Air flow restriction can cause your central air conditioner's evaporative coils to become dirty and cause them to drop below freezing. This allows the humidity in the air to collect and freeze around the coils. The most common causes of insufficient air flow are overused air filters and undersized ducts.
Outdoor Temperature: Most central air conditioning units do not work well in temperatures below 60 degrees Fahrenheit. When the system does not function properly it can freeze up.
Keep It From Freezing
It is important to understand the root cause of the freezing before attempting most of the following solutions.
Refrigerant charge: To prevent your central air conditioner from freezing contact your local air conditioning expert to test your system for leaks and level of refrigerant. If a leak is found, have it sealed before your coolant is depleted and your system freezes.
Insufficient air flow: Change your air filters on a regular bases. It is the easiest and cheapest way to make sure your central air conditioner does not freeze. If your evaporative coils are already dirty they need to be cleaned. Depending on your system the coils may have to be removed for cleaning. Again, contact your local air conditioning expert for cleaning service.
Outdoor temperature: The simplest resolution to this issue is to not use your central air conditioning when the outdoor temperature is cooler than 60 degrees. Save energy, money and your system by turning off your air conditioning and opening windows to cool down your house. If you are unable to do that, you can install an ambient temperature control to keep your system from freezing instead.