If you’re pricing the cost of central air for a 900 sq ft house , I’m going to save you a bunch of time and frustration by saying the quiet part out loud: There are two prices for central air. The price when your ductwork already exists and is usable . The price when we have to build the lungs of the house from scratch . I learned this the hard way walking through a 900 sq ft place near downtown Lake Worth Beach. The homeowners weren’t asking me for the “best system.” They as