If your thermostat is set at 74º, but the temperature won’t go below 79º, here is our “Top 10” checklist of problems that could cause the a/c system to not cool enough to reach the thermostat setting: 

1) Dirty air filter - Yes, really. This is the first place to check. An air filter caked with crud severely cuts down air flow and cuts the cooling ability of the system. Replace your air filter, if necessary.

    Also, it’s possible you have a “forgotten filter.” Some homeowners have filters installed in two locations—behind the return air registers (vents) and at the air handler (inside unit of the air conditioner). If one of them is ignored, it will eventually create an almost solid barrier to air flow. By the way, double filtration is not recommended. Can’t locate your air filter? See our blog post Where is the air filter for my central air conditioner and furnace? I can’t find it?

2) Dirty evaporator coils - 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.


    You can pull out the filter located at the bottom of some air handlers 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.

   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 nearby exhaust for a clothes dryer blows lint onto the condenser coils. The building code now requires that dryer vents not terminate near an air conditioning condenser. 

3) Refrigerant leak - 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.

   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. Sometimes a partial loss of refrigerant will also cause the suction line (larger of the two copper pipes connected to the condenser outside) to freeze over, as shown below

4) Blocked condenser unit - The condenser needs a completely open area directly above it and a minimum of 12-inches cleared all around to efficiently dissipate the heat that is removed from your home into the air. A little intrusion on this zone will have only a minor effect on the system’s performance but, when when the condenser is completely surrounded by overgrown foliage, the cooling ability will be noticeably deteriorated.

5) Failing compressor - The compressor is the most important component in the condenser unit. It is also the one part of the system that uses the most energy. Compressors use more electricity as they get older, and then begin to cool less efficiently before finally failing completely. When you put your hand over the top of the condenser unit while it is running, you should feel air being blown upward that is hotter than the outdoor temperature. If not, it’s time to call an a/c service tech. 

6) Undersize system - Air conditioning systems are sized to be able to keep the house comfortable (mid-70º range) on the hottest days of summer, but will have to run continuously to do it. If you, or the previous homeowner, enclosed the garage or back porch into a family room or extra bedroom, without upgrading to a larger a/c system, then it may not be adequate to comfortably cool the additional square footage in your home on the really hot days. A larger system, sized by your HVAC contractor, is the solution.

7) Thermostat set too low - If you set the thermostat to 65º on a hot day, as noted above, your system may not be big enough to handle a temperature differential it was not designed for.
    The record high temperatures during the summer months over the last few years, exceeding 100º F in some Sunbelt cities, is past the mid-90º parameter used for sizing most systems. So on those scorching hot days many systems will struggle to cool the house, even if the thermostat is set in a normal range.

8) Bad Thermostat - This can be difficult to determine without tech tools but, with the thermostat set at your normal comfort temperature, position yourself at a location in the house where you can hear when the air handler shuts off at the completion of a cooling cycle. If it shuts down before the temperature reaches what you are accustomed to, then it is likely that a sensor or circuit in the thermostat is malfunctioning.

9) Damaged ducts - Critters that get into the attic or crawl space will sometimes chew open holes in ducts. Also, workmen crawling around can collapse or tear open duct connections. Small areas of damage will not leak enough air to make a noticeable difference, but a large gap in a main supply duct will radically reduce the air supply and temperature in your home. 

    If you open the attic hatch or door to the crawl space—depending on where your ducts are located—and it’s cooler in there than inside your home, investigate further. We don’t recommend exploring these areas unless you are familiar with the necessary safety precautions. Otherwise, call your a/c service tech. 

10) Fan set at “ON” and condenser not functional - When the fan setting at the thermostat is set at “ON,” the blower rans continuously, even if the condenser is not functional. This may make you think the system is not cooling properly when, actually, it is not cooling at all. Check your fan setting. If it is at “ON,” set it to “AUTO,” which will make the fan run only when the system is functional. Wait a few minutes. If nothing happens, or it turns on but does not cool, see our blog post My air conditioner won't turn on. What's wrong?, or call an a/c service tech.

    An 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 we suggest reading our article How can I make my Florida air conditioner last longer? 

    Also, see Frequently Asked Questions (FAQ) about Air Conditioning Troubleshooting and Why does my home still feel humid and muggy with the air conditioner on? and Why does my air conditioner keep shutting itself off?

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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? 

• Why is my air conditioner not cooling enough? 

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

   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.