When substituting the outdoor unit, should the indoor unit also be changed? The answer is, most likely yes. Here's why. Matched System Design: All air conditioner and heat pump outdoor units are specifically fabricated to work with a matched indoor unit for optimal efficiency and performance. The outcome of this matched equipment is a harmonized, top-performance team that ensures dependability and efficiency. Air conditioner and heat pump outdoor units may "work" with indoor units other than those for which they have been precisely intended; however, the consequence would be a definite compromise in system performance. Design Advances: As research has improved, indoor blower coil units have undergone numerous design advances — especially in the areas of air handling performance, filtering efficiency and operating sound levels. The latest outdoor unit will also incorporate 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 give you improved efficiency compared to the old system, but it will not be as efficient as it was designed to be, and your savings will be lower than with a matched system. Equipment Age: If an air conditioner or heat pump is 10 years old or more and is ready for replacement, the indoor unit is likely just as old and has been subjected to the same amount of degradation. Changing both units means you won't have to replace the indoor unit in a short time period… you'll have many years of efficient service with both units. New Warranty: A new system also delivers you a new product warranty. Changing the indoor unit together 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, changing only the outdoor air conditioner or heat pump may appear to be a savings. 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 the result is efficiency, reliability and comfort for years into the future.