I have a 2100 sq ft, two story colonial built in 1800. The oil furnace and blower push plenty of warm air thru the ductwork to both floors-- it heats up FAST.

Now I want to install central air, hoping it would be easy to add a coil above the furnace and set a 2 1/2 or 3 ton condenser on a pad 4 just feet from the basement furnace.

But more than one salesmen has told me I won't be able to cool the 2nd floor. I have NO returns on the 2nd floor, just the stairwell. The one return I do have is on the 1st floor near the bottom of the stairs.

After considering several costly options, including an attic AC unit, I'm wondering: can't I direct enough cool air to the 2nd floors by closing off first floor vents, or the use of a damper near the head of the ductwork? The next, least costly option would be to add one return from the 2nd floor, though it wouldn't be in the optimum location. First I'd try the system without it.

I've had a two story home before with a single central AC unit, so I know the 1st floor needs to be chilly to keep the 2nd floor comfortable -- but that house did have a 2nd floor return. Anybody with a historic home can understand why I'm trying to make the least invasive (and least expensive) plan work. I can only spend $4000-$6000.

I'm hoping to hear that one unit will be ok, even if not ideal.

thanks