My Son bought a starter house, He's getting married. Its an old summer bungalow, remodeled for a year round residence,about 15ft wide and less than 60ft long with a gas fired forced air furnace in the crawl space. The inside was recently redone, but the furnace is old and my Son would like to add central air. If he had money He'd buy a larger house. My idea is to abandon in place the old heating unit. Drop on a pad a package unit gas heat/Ac (off the cuff 80k BTU/ 1.5 Ton) and run the duct into the crawl space and do the hook up (off the cuff about $3000. total). My back ground is Electrical/HVAC for retail. My son was talking to a residental HVAC contractor who complained about the idea not being efficent, and suggested taking the house appart to install a new attic unit ($$$). Since the house is 600~800 sqft I don't see the problem with 80% efficency: the payback on the difference of 90% in the ten years or less he expects to live there, is I believe, irrelevant. Comments please. Thanks Oldfoggy.