[jyl]

I want to get your thoughts on a way to cool my 3rd floor family room.

For background, it is a 100 y/o house. Basement, main floor, second floor bedrooms, third floor attic finished to be family room. Central a/c was installed about 10 yrs ago, it works well on main floor and okay on second floor, doesn't do much on third floor because the heat load is higher (roof, lots of skylights) but even more because there is only one little a/c duct there. (Retrofitting central air into an old foursquare house is hard, very little place for ducting.)

It only gets to be a problem when it is >85F outside, and that's only a few weeks or a month in the summer, so I've basically put up with it. During those periods, my third floor family room is just a hot box. Tant pis.

I've looked at various options, of course, and they all have big problems. Reroute and expand the duct? Not gonna rip the interior of the house apart. Better insulate and vent the roof? Sure, when its time for a new roof, which is not even close. Install a big window unit just for the third floor? Ugh, I hate seeing those things hanging out of otherwise pretty houses. Install a ductless split system just for the third floor? Too expensive for what's not a major problem, and I don't want even more of my minuscule yard to be occupied by the compressor. Put a free-standing a/c unit in the family room, venting out one of the windows? Maybe, maybe, but it is a 600 sq ft room, I'm skeptical that those units have enough oomph . . .

So, here is a possible creative solution I thought of. Maybe its not a great solution, maybe it is more of a temporary fix until it is more convenient to do a more major project, maybe it is a good and permanent solution, maybe it is a totally stupid idea . . .

The idea is to run a duct up the exterior of the house, like a 5" x 12" rectangular duct, painted house color, from basement to third floor, and use an inline fan to move cool basement air to the hot attic.

My logic is, the central a/c has enough power. *On the hottest day (>100F) I can easily keep the 1st floor 70F and the basement is <70F. *(The basement is cool due to earth insulation and also due to the uninsulated a/c ducts in the joists, there are a couple of registers in the basement too but I keep them closed.) *The problem is simply that there is not enough airflow to the 3rd floor/attic. *Only about a 21 square inch duct (3" x 7").

The attic is roughly 600 square feet and average 8 feet ceiling so say 4800 cubic feet volume. *Suppose I want to replace hot attic air with cool basement air 1 time per hour. *Means I need to move 1.3 cubic feet/second. *Suppose max acceptable air velocity in duct is 3 ft/sec (limited by friction, noise). *Means I can use a duct with 0.43 sq ft cross section. *(Compare to 0.16 sq ft of current a/c duct). *That is about 8" diameter round duct, equivalent rectangular duct is 5" x 12".

I would need that duct to run from basement to attic, on exterior of house, preferably north side (cooler), with a fan at the basement end (less noise in attic). *Ideally it would turn on when temperature of attic exceeds some level AND it also exceeds temperature of basement by some specified amount - or I could just control it manually.

What do you think?