Additional heating/cooling area

**suarts** · 09-17-2010, 12:03 PM

I have a 2200 sq/ft single level rancher home w/gas hot air and whole house air contitioning. The basement is approx 1700 sq/ft. The attic is about the same but is closed in somewhat by kneewalls. It has 2 rooms at either end, with doors. The larger room is approx 12x18, 216 sq/ft with an end window and skylight, both operated by cranks, and a tiny bathroom. The other end is approx 12x12(144) and has the same type window. The center area is approx 18ft. with the steps going to the main floor; a door is at the bottom. 2 vents go to the area; 1 fm a basement duct to the floor of the small room, 1 fm the basement up a duct ending at the attic floor level of the stairwell. Until now the area has been isolated by dampers in the basement ducts and tape at the top. To have the space liveable I thought to put an air conditioner thru the wall of the large, and have an inside air unit vent thru the wall of the small, less used space. An HV/AC guy's answer is instead put a driver fan in the stair duct and extend it into the large room and let the other space be heated by the opening at the stair top, with a return at the bottom of the steps. Will this be wasteful and defeat my desire for an isolated upstairs area? How could I close off the space if no one is using it?

**Dana** · 09-17-2010, 12:21 PM

Generally speaking, except in the TIGHTEST of houses, closing off ducts tends to be neutral to the heating & cooling costs at bests, and usually results in higher outdoor-air infiltration due to the induced pressure differentials from a less-balanced system, making the great-outdoors carry a fraction of the air flow. Basically, don't close it off when nobody is using it, since it's cheaper to heat/cool the unused room than take the efficiency hit of the imbalanced system. This hasn't always been well-understood, but was the clear result of real-world testing done by the utilities in CA for the Lawrence Berkeley National Labs over the past decade or so.

If it takes a booster fan to get sufficient flow to the upstairs, that too may unbalance things, and simply turning it off when the upstairs isn't in use is likely the best solution. The only way to know for sure would be to measure the pressure differences in-situ with the fan running vs. off, while the main air handler is running.