Properly functioning air conditioning systems have a temperature drop of 14 to 20 degree temperature difference between the supply and return ducts when it is above 75 F degrees outside.
If this temperature drop does not exist the air conditioning system must be cleaned, adjusted, repaired and tested by a qualified person. The service person must be asked to provide certification no refrigerant leaks exist, that the system is operating effectively after the necessary corrective action has been taken, or perform the necessary replacement(s). Systems that are leaking refrigerant can not be legally refilled becaue leaking refrigerent can be hazardous to the enviorment.
Systems that are low on refrigerant can "self destruct". If a home owner tells you the system does not work because it was not filled up for the season that is an indication there are serious problems with the system (like leaks they can not fix). Keep in mind refrigerant leaks will not be found during a standard home inspection.
