My house is split level with a single AC system.  The air intake is located in the central stairwell at about the 50% elevation level.  
The temperature differential is extreme in the summer.  I've tried adjusting the registers to bias airflow but it's nowhere near enough.  
The AC runs a lot in the summer afternoon/evenings because the upstairs is so hot, and the lower levels get very cold.  
For reference, hot summer afternoon/evening temperatures upstairs about 1-2ft below the ceiling are frequently almost 90 degrees while the downstairs is often in the low-60 degree range.

While looking at whole house fans I found "remote mount" whole house fans where the inlet is a grille sized to fit between rafters and a duct connects to the fan which you mount at a convenient location in the attic.  
Has anybody used one of these fans to recirculate air vertically?  ie. suck in hot air from upstairs and blow it out downstairs where it's cold?  
Assuming these fans move enough air (the ones I saw were around 18 inch diameter) I think this could be a very effective fix.  Suggestions?

I had a 2 story house in Delaware that had the same issue. Hot upstairs, and comfy downstairs. There was a ceiling exhaust fan upstairs that exhausted outside through the attic. It made a tremendous difference.

What about extra attic insulation and high efficiency windows?  If you aren't insulated and sealed up, you won't win the battle.

As mentioned above you need proper amount and type of insulation, and if direct sunlight is entering rooms some energy efficient windows.  Do you have a good amount of attic vents/soffets?  Maybe and attic fan as well?

In the summer you can close the vents to each room in the downstairs and leave the upstairs room vents fully open, and reverse in the winter time.

I would suggest ceiling fans.  They are quiet and more energy efficient than what you are doing.  In addition to mixing the stratified air mass, they add a small degree of convective cooling, evaporative cooling for the occupants.

Yep, and cheap and easy to install.

FYI, the post above is from Trollslayer not from xeeoneyz.  I'm not sure why the web site has attributed the post to some other name.

Relatively new double pane insulated windows and only 2 are in sunlight, the rest are shaded.  Three large gable vents as well as ridge vents.  
There could stand to be more insulation between the top floor and attic, but it's not too bad.  
The heat is from the attic mostly, which is extremely hot because the entire (black shingle) roof is in full sun all afternoon until sunset.  

It seems to me that recirculating the air (say 80deg air from upstairs) is more efficient than drawing in fresh 100+ degree air from outside.  

I've considered a small diameter ceiling fan in the stairwell above the return vent, it would be the easiest option and I believe it would help greatly.  
But a ceiling fan won't exchange the air from  the coldest room to the hottest like a ducted whole house fan would.

Cold air sinks and hot air rises.  You need a return in the ceiling upstairs and a return at floor level downstairs in order to properly mix and distribute the air.  Running you furnace fan constantly may also help.  Once you have two returns you could consider adding a zoning system, but if you want a good one (American Standard or Trane) you may need to replace your existing system to get the most out of it.

It is very difficult to know what to recommend, what would work and what would not without seeing the situation(s).  We don't know your duct work, you noise tolerance, the size of the space, ...

All we know is you are dealing with stratified air.  You need to mix it up (the air, that is).  

Get the cold air and pump it upstairs.  Let it sink back down on its own.  

Alternatively, get the hot air and pump it downstairs.  Let it rise back up on its own.

Mix the hot air upstairs with cold air down stairs using a pressure differential (fan or fans).  I've designed and built refrigerated and heated enclosures.  It can be made to work either way because your house is a closed system.  

Another way to do it is to close off the upstairs from the downstairs (think, a closed door).  This makes two separate zone that won't stratefy as easily.


Honestly, your posts read like you already know what you want.  If so, just do it!

I would say if all the heat is coming from your attic like you said, you would need more insulation to cut that down.   Then you can work on the circulation issues.

More insulation would decrease the magnitude of the differential by an unknown amount, not eliminate it.  

I'm going to try a small diameter ceiling fan at the top of the stairwell, it's much faster and cheaper than the recirculating idea.

Turn your thermostat fan to the on position during the hottest times. If that doesn't circulate good enough then there is more info needed and need to start looking at the system size and setup possibly...

Split levels suck to cool
I lived in one once and couldn't get below 75 upstairs, wouND up hanging a sheet over the foyer at the top of the steps. 
With out the sheet it'd be 75-80 upstairs, 60-65 down stairs.