When replacing the outdoor unit, should the indoor unit also be replaced? The answer is, probably so. Here's why. Matched System Design: All air conditioner and heat pump outdoor units are specifically designed to work with a matched indoor unit for peak efficiency and performance. The consequence of this matched equipment is a harmonized, top-performance team that ensures dependability and efficiency. Air conditioner and heat pump outdoor units may ”operate” with indoor units other than those for which they have been specifically intended; however, the outcome would be a definite compromise in system performance. Design Advances: Over time, indoor blower coil units have undergone a number of design advances — especially in the areas of air handling performance, filtering efficiency and operating sound levels. The latest outdoor unit will also comprise the latest design advances. Higher Cooling And Heating Efficiency: The cooling and/or heating efficiency ratings that are posted for an air conditioner or heat pump are based on their performance as matched systems. While changing only the outdoor unit may result in improved efficiency compared to the old system, but efficiency will be lacking from what it was designed to be, and your savings will be not as much as with a matched system. Equipment Age: If an air conditioner or heat pump is 10 years old or more and needs to be replaced, the indoor unit is likely just as old and has been subjected to the same amount of wear and tear. Substituting both units means you won't have to change the indoor unit in just a short time… you'll have many years of efficient service with both units. New Warranty: A new set of equipment also delivers you a new product warranty. Substituting the indoor unit simultaneously with the outdoor unit results in peace of mind, knowing that the new warranty covers the entire system for the same period of time. A Bargain: At first glance, replacing only the outdoor air conditioner or heat pump may appear to be a bargain. But, when you consider the lower cooling and heating efficiencies, decreased reliability and high cost of ownership that results, it's not such a bargain. Replacing an entire system may cost more, but you get much more efficiency, reliability and comfort for years into the future.