When you replace one unit but not the other, your system is considered mismatched. Your older equipment will limit the newer, more energy-efficient unit. Therefore, your cost savings from the upgrade will be limited. A mismatched central cooling and heating system often consists of a new AC and an older furnace. The blower in the furnace drives the air for both heating and cooling. So, even if you invest in an AC with an exceptional Seasonal Energy Efficiency Ratio (SEER) rating, your monthly utility bills will likely remain high because it’s using an older blower. By replacing both together, you can ensure that you have a matched system: an AC and furnace with comparable efficiency ratings. When new equipment is appropriately matched, your whole system runs more efficiently, and energy savings can help offset its cost.