Using window fan in finished attic to exhaust hot air
We live in a large (3000sq ft), old (1907) home in Northern Kentucky. Our house has three finished levels and an unfinished basement. In the summer, our A/C (Daikin Fit heat pump) does a pretty good job of cooling the first floor. The second floor is reliably 5* warmer than the first. The the third floor can get very warm with all of the heat accumulating there and the heat pump sizing ignored the third floor for due to a lack of registers up there. I have been wondering whether it would improve the overall circulation of conditioned air through the home if I put a window fan (like a Vornado Transom) in the window at the top of the stairs leading to the third floor and set it to exhaust the accumulated hot air out of the third floor. My hope would be that this helps to slowly pull cool air up from the first floor (and maybe even some cool air from the basement) to the second floor. My goal is not to cool the third floor though. We have a window unit and some fans we use when we want to be on the third floor, but otherwise the third floor is largely unconditioned in the summer. Would a window fan provide assistance the A/C in moving air to the second floor and cooling it and also to assist in removing the accumulated hot air as day turns to night. I could not find any discussion directly on point with a bit of internet sleuthing.
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This is sort of like the attic fan discussion with a small twist. The short answer is that, yes, the window fan will make the second and third floor cooler. However, it does that by stealing makeup air from all of the gaps and openings in the home. Whenever you have a negatively pressurized home makeup air is coming in from somewhere. Sometimes it can come from some really nasty places like crawlspaces, attics, spray-foamed fill walls, etc and bring some nasty stuff and odors along with it. Not to mention the overall energy penalty you will take because this unconditioned makeup air now has to be conditioned.
So it's not ideal from an energy use perspective, but if you need the 2nd/3rd floor cooler yes this will work. A better option would be to somehow open up more connected pathways between the floors - possibly with a large jumper duct or a more serious floor plan change like opening up ceilings, etc. Any convective air movement between floors will help mix the air like ceiling fans and the like as long as the two floors are well connected.
Edit: actually an even better solution may be to take the same fan concept you have but instead of dumping the conditioned air out of the house somehow pump it down to the basement and have another fan pumping air from the basement back up to the 3rd floor. More fans needed for this setup, but less energy lost to the outside.
Understand every cubic foot of air the fan expels will be replaced with a cubic foot of unconditioned air pulled into your home thru the gaps.
It seems to me HVAC equipment you own is under sized for your home and pulling in more unconditioned air will only make the undersized equipment work harder.
Walta