You should consider replacing your system if it is old, and therefore less efficient, or if it might need repair in the near future. Systems today can be 60% more efficient than systems built as little as 10 years ago. Normal wear and tear can reduce the efficiency of your system. If you have large utility bills or are if you are facing an expensive repair, you may want to consider replacing the entire system. Going through another costly season of utility bills or paying for an expensive part failure can go a long way toward the cost of a new system.