Upstairs, Downstairs, A/C issues: how can I help the new A/C in my old house keep me cool?
The setting: a 2 story house, about 100 years old, 1800 sq. ft., in rural NY where summers are quite hot and very humid.
The protagonist: a fussy lady who's already having hot flashes, dammit, and who gets very cranky if she's overheated.
The issue: whether and how to use window fans to exhaust hot air upstairs, with A/C running.
The details: My old house was retrofitted a few years ago with central air conditioning. We used the existing ductwork of the forced air heating system (single zone), reconfigured a bit for better efficiency by my superlative contractor. It wasn't possible to create separate air outlets for the A/C, so the cool air comes out at floor level. There's enough overall air circulation in the house (I keep the A/C fan on continuously during hot spells) that this isn't a problem at all on the first floor, which is perfectly comfy. The upper floor, though, ends up being ~4 degrees warmer than the first floor, which is too much difference for real comfort. The cool air comes out of the vents upstairs, slinks along the floor and heads right downstairs. Keeping the doors to upstairs rooms closed helps some, but that feels stuffy, and the cats do not approve.
The ceiling above the second floor is very well insulated, and the attic is ventilated. There are no ceiling fans upstairs, and the rooms up there are really too small to accommodate them gracefully (and I need a do-it-myself solution, and my self is not up to that). I've done what I can with shades and drapes to keep the sun out.
So here's the solution I'm working on, but I'm not sure it's a good idea. I put a window fan like this
(2, 7" fans) at the top of one of the double-hung windows upstairs, blowing out on low speed. I think it moves ~500 cu.ft./min. (that's a guess). That does a very nice job of blowing the hot ceiling air out, and invites the nice cool air to get up off the floor. That room feels much more comfortable with the fan on, and I thought Hurray! and bought a similar fan to put in the other upstairs room that needs help. But then I thought, with all that air going out, there's got to be more air coming in. HOT, HUMID, SWAMPY air from outside. It could come in by way of ground level vents that let air circulate through a crawl space that is connected to my basement by a door that I can have either open or closed. Or, if that door is closed, the air would have no way to come in except random infiltration.
Other options: Before I tried the window fan, I tried using a table fan in each upstairs room to just stir the air around, but that is surprisingly ineffective, and feels too windy. The window fan creates a much more comfortable situation. I could get a small room-size A/C, window or portable, for the remaining hot room, but I'm not sure the wiring in that room is up to that, and it also seems like it would be way more inefficient than the fan(s).
So my questions:
Assuming that having the window fans in 2 upstairs rooms makes the house more comfortable, is that a bad idea? Would constantly pulling in that additional amount of hot, WET air from outside make the A/C work lots harder, and cost way more when it's time to pay the bill? Or would the difference be no big deal? The A/C equipment was carefully sized for the house, and it has no trouble keeping the temperature and humidity in the house just right, except for the stratification upstairs. If I do use the fans, would it be best to run them 24/7, so the heat doesn't build up, or just at night (which is when I'd usually be upstairs)? If I use the fans, would it be better to facilitate air intake through the basement by opening the door that connects to the crawl space, or keep that closed and rely on infiltration? Is it hot in here, or it just me?