Central air for the second floor

Question from Chris: 

I'm looking at buying a cape with a finished attic (3 bedrooms, 1 bath upstairs). The house has central A/C only downstairs. The air handler is in the basement and the vents are all on the floor in the first floor rooms. What are my options for central air upstairs? The upstairs hall goes along the width of the house at the back edge of the roof until it runs into a bedroom on each side. The third bedroom and bath are in the center in front of the hallway. There are attic access doors in the rooms and halls.

Jake's Answer: 

Chris,

Thanks for writing.  To add central air for your second floor there are three options.  

Option 1 would be to set the air handler in the basement and find a space to run two large ducts to the attic space.  This space would have to be at least 30” square, something like a common closet through the first and second floor. These common closets, or “chases”, can be difficult to find. The supply and return air ducts would run on top of the ceiling joists and each room would have a supply and return air register.  

Option 2 would be to set the unit in the attic space behind one of the second floor walls.  The supply and return air ducts would run behind the walls and feed the bedrooms and bathroom.  Most likely it would require pulling some flex type ducts through the floor joists to serve the areas on the other side of the hall.  

The 3rd option, which often is the best, is to set the unit in the attic and let the ducts supply and return the air through the ceilings and walls of the bedrooms and bathroom.  This could also be the least expensive.  Two things to remember: return air is critical for an air conditioner to work properly, and seek the advice of a qualified heating and cooling contractor to complete the work.  This type of work will require a building permit.