The # 1 question comes in to play: have you or a licensed technician been in your attic or under your house to check the condition of your air ducts? My bet is a big NO- because I assure you no one at my house has done it either.
An enormous waste of energy occurs when cooled air escapes from supply ducts or when hot attic air leaks into return ducts. Recent studies indicate that 10% to 30% of the conditioning air in an average central air conditioning system escapes from the ducts.
For central air conditioning to be efficient, ducts must be air tight. Hiring a licensed service technician to detect and correct duct leaks is a good investment, since leaky ducts may be difficult to find without experience and test equipment. Ducts must be sealed with duct “mastic.” The old standby of duct tape is ineffective for sealing ducts.
Obstructions can impair the efficiency of a duct system almost as much as leaks. You should be careful not to obstruct the flow of air from supply or return registers with furniture, drapes, or tightly fitted interior doors.
The large temperature difference between attics and dusts make heat conduction through ducts almost as big a problem as air leakage and obstructions. Ducts in attics should be insulated heavily in addition to being made airtight.