If your air conditioner won’t stop running, there are three likely reasons. The first two are simple and the last one is more complicated and expensive. Hopefully, your problem is #1 or #2:

1) Check the fan setting on your thermostat. There are two settings, “AUTO” and “ON.” The ON setting keeps the blower running continuously, even when the system is not cooling. It will cause an early demise of your system if used all the time, and is not recommended.  If your thermostat has been inadvertently set to ON, put it back to AUTO—which only activates the blower when the system is heating or cooling—and your problem should be solved. 

2) Check the temperature setting of your thermostat. Air conditioning systems are sized, based on the volume of air in your home and amount of installed insulation, to be able to maintain a temperature of around 72º, and on the hottest afternoons of summer the system may have to run continuously to keep up. But if you have the thermostat set well below 70º it will have to work longer all the time to maintain a temperature it was not engineered to provide. Resetting your thermostat up to somewhere around 72º to 75º F may solve the problem.

3) If setting the fan back to AUTO does not fix the problem, or if the fan is already at AUTO, and the thermostat is at a sensible setting, then the issue is that your system is not cooling satisfactorily, and it is taking much longer than it should to a maintain the desired indoor temperature. Several things can cause this, and here are the three most common:

3A) A dirty air filter severely cuts down air flow and reduces the cooling ability of the system. Check and replace your air filter, if necessary. The problem might be what we call “the forgotten air filter.” An air filter is usually placed in one of two locations: 1) inside or right next the air handler, or 2) mounted behind the return air register (duct vent). Some homeowners have filters at both locations, and regularly change one of them, but have forgotten—or never knew—about the other one. The forgotten filter clogs completely with dust over time and starves the system of air flow, which makes it struggle and run longer to cool down the home. The culprit is sometimes a filter placed on the side of the bottom of the air handler that is only accessible by the removing the bottom front panel of the unit.

3B) If the air filter has become so dirty that it has collapsed and pulled away from the filter slot, or there is no air filter in place, then the fins of the evaporator coils (the chilling surfaces that air flows over) will act as a unintended filter, collecting bits of airborne debris in the tiny gaps between them until enough crud accumulates to severely reduce air flow and act as an insulator between the cool coils and the air around them.  The photo below was taken looking up into the tent shape of an evaporator coil that had become so dirty that the coil surfaces are no longer visible, and the following photo shows what a clean evaporator coil looks like.

    In some air handlers (indoor units) you can pull out the filter located at the bottom of the unit and look up directly into the tent shape of the evaporator coil to check for any dirt/dust accumulation. If not, a service technician can open the unit to examine the coils.

   Unfortunately, cleaning a dirty evaporator coil is an expensive proposition, involving evacuating the refrigerant from the system, disconnecting the coil unit, washing and scrubbing away the dirt in a solvent solution, then reinstalling it  and reloading with refrigerant gas.

3C) Dirt accumulation at the condenser coil (outside unit) will also reduce the efficiency of your system, and the photo below shows a typical accumulation that needs cleaning. But it is rarely a big problem, with one exception: when the exhaust for a clothes dryer blows lint into the immediate area. The building code now requires that dryer vents not terminate near an air conditioning condenser. 

3D) The gradual leakage of refrigerant from the pressurized lines in the system will reduce the cooling ability down to zero over time. An average HVAC system chills the air coming out of the air handler to about 18º F cooler than the incoming air. But as the temperature split (difference between incoming and outgoing air) begins to descend, the only noticeable change at first is that the system takes longer to cool down the home. For example, if the temperature split drops to 9º F the system will still cool your home but it will run for twice as long—and use twice as much energy—to do it. One other symptom of refrigerant loss is that the suction line (the larger diameter refrigerant pipe connected to the condenser) will freeze over, as in the photo below. 

   A service technician can check the status of your refrigerant to verify if this is the problem. Because it is a sealed system, any loss of refrigerant indicates a leak; so, adding another shot of gas without locating and fixing the leak will only be a temporary solution.

   A annual maintenance service at a minimum or, as recommended by most HVAC contractors, every six months will catch minor problems before they get out of hand. We recommend it.

    Also see our blog post Why is my air conditioner not cooling enough? 

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  To learn more about heating and air conditioning systems, see these other blog posts:

• How can I find out the SEER of my air conditioner? 

• My air conditioner won't turn on. What's wrong? 

• How can I find out the size of my air conditioner? 

• How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

• Where is the air filter for my central air conditioner and furnace? I can’t find it? 

• Does an old air conditioner use more electricity as it ages? 

• How did homes stay cool in Florida before air conditioning? 

• What is wrong with an air conditioner when the air flow out of the vents is low?

• Why has the thermostat screen gone blank? 

• Why does it take so long to cool a house when an air conditioner has been off for a while? 

 • What are the most common problems with wall/window air conditioners?  

• Will closing doors reduce my heating and cooling costs? 

   Visit our HEATING AND AIR CONDITIONING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.