Is it more cost efficient to buy a furnace and air conditioner at the same time?

It may or may not be more cost efficient to buy a furnace and air conditioner at the same time. It depends on what kind of setup you have and what kind of setup you want. Will they be using the same parts or the same duct work. Do you need a new furnace? Do you need a new air conditioner? If you need one will you be needing the other soon? Are you going to need to make changes for both? If you need both and both use the same duct work, the answer is yes. If both will need updated electrical and Plumbing, the answer is yes. If you are going to need to pay a double delivery charge if you get the two delivered separately, the answer is yes. Otherwise, the answer is no.