Think of your air conditioner and furnace as part of a total system. That system will work better, longer and more efficiently if all of its components are matched in capacity and efficiency. By the same token, a mismatched system—one that mixes old technology with new—could decrease system performance and overall comfort. For example, your new air conditioner will be capable of reaching a certain SEER efficiency rating. Because SEER is determined by a complete system combination, if your existing furnace and indoor coil are older or don’t match in efficiency and capacity, you might not get the efficiency level you paid for or be able to receive accurate information on what your rating actually is. Plus, an older indoor coil may even be dirty and clogged with particles, meaning more energy will be required to push air through the system.