First invented by Willis Carrier in 1902, modern air conditioning provides a welcome respite from the summer heat and is a common appliance found in many homes as part of the HVAC (heating, ventilation, and air conditioning) system. While this system is commonly used when temperatures rise outdoors, air conditioners should not be tested or used when the outside temperature dips below 60 degrees Fahrenheit. While this does not apply to all air conditioners (some industrial systems can be used in colder conditions), most residential systems should not be operated below this threshold.
Air conditioners use a thermodynamic cycle called the “refrigeration cycle” to provide cool air by changing the pressure and state of refrigerant to absorb or release heat. Using a compressor, condenser coil, and evaporator coil, the air conditioner changes the refrigerant from a gas to a liquid and back again through repeated cycles. As the refrigerant is moved between liquid and gas, it is moved across the evaporator coils where the heat inside the home is absorbed by the refrigerant. The cooled air is then blown inside the home while the heat is released to the exterior by the refrigerant, which is then returned to liquid state.
While this process works well at most higher temperatures, residential air conditioning systems are usually not designed to run when it is below 60 degrees outside. This is because this is usually below the comfort range for most people and is often unnecessary in most residential settings. Running the AC when it is below 60 degrees outside can result in failure of the compressor.
Since refrigerant is designed to transition smoothly between a gas and a liquid, if this material does not change states at the correct times, this can cause significant damage to the system. If the system is operated when it is below 60 degrees outside, this can cause the refrigerant to not change to a gas and remain liquid when it reaches the compressor. Since the compressor is designed to receive a gas rather than a liquid, the compressor is overworked and can eventually fail as a result. A build-up of ice on the exterior of the condenser is often a first sign of a problem with the system.
Air conditioners are typically composed of two parts: the heat exchange system and a circulation fan. While the heat exchange system is the main component of the system, the circulation fan can be activated without running the whole system. While this will likely take much longer to cool rooms inside the home, this will prevent damaging the compressor.
Some industrial air conditioning systems are designed to work below 60 degrees as these buildings can have equipment that give off a significant amount of heat. Having air conditioners that work year-round in these settings provide a comfortable inside temperature in these industrial buildings. Ultimately, the 60-degree limit is a manufacturer choice and not a physical limit. While some air conditioners can run below this point, residential systems should not be tested or run when it is below 60 degrees outside to prevent damaging the system.
State law specifically exempts home inspectors from testing air conditioners when exterior temperatures are below 60 degrees Fahrenheit because testing them under these conditions can permanently damage the units. If your air conditioner cannot be tested during a home inspection, you should consider getting a home warranty that will be in effect when you are able to have the unit properly tested. In addition, home sellers are legally required to disclose known defects with components in the home like the air conditioner, so definitely review the seller’s disclosure documents closely and press for more information if necessary.