Look for return air grates around the house. If you find any with filters that may be part of the problem, to many filters cutting the air flow. You might also check your attic for torn ducts, windows not closing tight, etc. If you do not have storm doors, buy them.I changed them after the first electric bill when I realized I wouldn't have any money left for guns if this kept up. I only changed the set of 3 that are actually in the unit itself. Are there usually other filters elsewhere?