Return air leakage in a hot attic will generally:

Return air leakage in a hot attic generally decreases the system's capacity. This occurs because the air that is intended to be returned to the air conditioning system comes from a heated environment, which negatively affects the overall efficiency. When warm air from the attic leaks into the return ductwork, the system has to work harder to cool that unconditioned air, ultimately resulting in a lower cooling capacity.

In a properly sealed system, return air should ideally come from conditioned spaces, keeping the temperature consistent and allowing the air conditioning unit to operate efficiently. When ambient hot air from the attic enters the return duct, it increases the load on the air conditioning system, forcing it to run longer to maintain the desired indoor temperature. This inefficiency leads to a decrease in the effective cooling capacity of the system, as the equipment is unable to cool the additional heat generated by the incoming warm air.

Maintaining proper sealing and insulation in ductwork that runs through hot spaces like attics is essential for optimal system performance and capacity.