If both units are over 13 years old or have a history of breakdowns and repairs, it is best to replace your entire heating and cooling system. Replacing both systems together is more efficient for the installer and will likely be less expensive than replacing one system now and the other down the road. Trying to install a new air conditioner or heat pump with your old furnace can cause problems since the coils in the new air conditioner may not fit your old furnace. If your old air conditioner and heat pump system use R-22 refrigerant that is being phased out, upgrading to a unit that uses R-410A refrigerant may make service and repairs less expensive. Finally, you will get the best performance and the most energy efficiency when you buy both units together.