Under the law, a furnace has a separate credit (and is listed on a different line on the tax form) than an air conditioner. So if they both qualified, you would list them on two different lines and claim both credits. Since only one qualifies, you can claim that credit. You will need to know the cost just of the A/C unit, if you only have one price, you need to make a "reasonable allocation" and be able to defend it if questioned.
