The Outdoor Condenser Unit is Blocked
Careless outdoor landscaping and/or neglect of lawn care may be another reason that your house is so hot.
"Shrubs and other plant life can obstruct airflow to the [outdoor condenser unit], which relies on it for proper operation," says Dietz of the Air Conditioning, Heating and Refrigeration Institute.
If the condenser coil is covered with dirt or leaves, the heat transfer is impeded and the unit has to work a lot harder to do any cooling, leading to possible system failure [source: Rosone].
"Homeowners should take care to ensure a decent-sized berth around and on top of their outdoor unit and also periodically use a garden hose (not a pressure washer), to remove dirt and debris that might be caught in the unit's fins," says Dietz. "Lawn clippings and dirt kicked up by heavy rainfall are common culprits here."
Consumer Reports recommends allowing 2 to 3 feet (60 to 91 centimeters) of space between the unit and any plants or backyard structures, and 5 feet (1.5 meters) between the top of the unit and any trees that hang over it.