Why do air conditioners freeze up?

Each year as the first warm days of summer roll in. Thousands of people across the country find themselves confronted with the same problem. As the air conditioner begins to run alot it covers in ice and begins to blow warm air. This obviously provides no relief on a 100 degree day and renders a cooling unit worthless. Many who find themselves in this predicament face the questions, "what causes my air conditioner to freeze up?" and "what can I do about my air conditioner being frozen?". These are the main reasons that your unit freezes up.

Air Conditioners Freeze Up Because of Air Flow Problems

Several things fall under this category including a stopped up filter, dirty coils and problems with the fan motor. We will begin with best case scenario and work our way toward the more major problems with regard to airflow.

Air conditioners freeze up when the air filter has not been changed

It would seem that this goes without saying, but change your air conditioner's air filter regularly. By not doing this you invite many potential problems. People frequently ask how often they should change their air filter, but there is not a hard fast answer. While every house and every filter is different, as a rule standard one inch or two inch filters should be checked every 30-60 days and four inch and five inch filters should be checked every 3-6 months. When a filter appears dark or dust covered it should be changed immediately. When a dirty air filter is not changed it causes the blower to work harder and eventually will cause the unit to trip a breaker or freeze up. Change your air filter, run the fan on your air conditioner for one hour (not in cooling mode but fan only) and see if your this fixes your problem. After running the fan for one hour, your unit should cool whether this was the problem or not. If after running for a couple of days the unit does not refreeze, the problem is solved. I the air conditioner does freeze up again perhaps your coils are dirty.

Air conditioners freeze up when the evaporator and condenser coils are dirty

Air conditioners like any piece of machinery require regular maintenance. If your unit has not received a maintenance visit in the past year then your unit may freeze up because of poor air flow. Everyday a central heating or cooling unit will circulate all of the air in a house. This means that everything in the air including dust, smoke, carpet follicles, dead skin and other contaminants travels through the indoor evaporator coil everyday. A standard air filter catches what it can but inevitably dust and dirt in small particles lodge themselves in the coil. Consider the example to the left showing an evaporator coil before and after cleaning. The air that you breathe filters though this coil every time that your HVAC unit runs. If the air has difficulty getting through this coil your air conditioner will cover in ice. If you are concerned that dirty coils are causing your unit to freeze you can contact a local HVAC contractor or you can purchase an appropriate coil cleaner and attempt to do this job yourself. If you need assistance choosing and obtaining the appropriate cleaner please contact PilotAirHVACparts.com.

Air conditioners freeze up when a fan motor is not running

Though you may have assured that the filter in your system is new and proper maintenance has been completed, it will be to no avail if your indoor blower motor is not operating properly. If your fan motor is not coming on it may be the result of a defective motor or it may simply require a replacement capacitor or relay. If an evaporator fan motor is running but is not at the proper speed or is not operating on its proper cycle, this will also cause a frozen air condition unit. A local contractor can help determine which of these problems you need to address and even fix them if you like. 

Air Conditioners Freeze up Because of Refrigerant Problems

Ideally when your air condition or heat pump unit is frozen up it can be diagnosed as an airflow problem and easily repaired. Unfortunately, this is often not the case. Refrigerant problems are the most common cause of an HVAC unit freezing up and require handling by a CFC certified professional.

Air Conditioners freeze up because of a low refrigerant level

There are a number of reasons that a unit may be low on refrigerant. If a unit is less than a year old, it is possible that the charge was never set properly upon installation. Licensed HVAC contractors are careful to set the refrigerant level according to the proper high and low side pressures, but even the best sometimes have difficulty getting the levels right on extremely hot or cold days. Another reason that the refrigerant level may be low is a cracked or leaking valve. A leaking access valve or valve stem can normally be replaced by a professional for $10-$50. Often a low refrigerant level indicates a leak in the lineset or coil. If this is the case with your system, a repair should be made immediately. While many people feel that adding "freon" or another refrigerant is a regular part of having an air conditioner or heat pump, this should not be the case. Adding refrigerant to a system with a leak is like putting air in a punctured tire; it may get you by but the problem still persists. R22 is the most common form of refrigerant in older systems and is known to cause environmental hazards when it escapes through a leaking coil. R22 can cost in excess of $100 per pound as the EPA phases out the production of this freon and newer refrigerants can also be costly. So it can become very expensive to repeatedly charge your HVAC system. If a system has a refrigerant problem, it should be addressed immediately by a professional, and the equipment may need to be replaced.

Air Conditioners freeze up because of a problem with the TXV or Piston

Most cooling systems have some way to regulate the flow of refrigerant. The most efficient systems use a TXV (Thermal Expansion Valve), but lower grade systems will often use a simple piston. Expansion valves may become stuck in the open or closed position rendering them defective and making it necessary to replace. Both pistons and TXV can become ineffective by the introduction of trash, small particles or moisture to the refrigerant lines. While some systems contain filters to avoid any such particles causing a problem, many others do not. If a piston or TXV must be replaced the bill can often run between $300 and $800. This repair, like the diagnostic of a low refrigerant level should be completed by and experienced service technician that is familiar with brazing and refrigerants.