Thanks very much for your thoughts. I am updating my hvac and trying to avoid installing a mini split to ensure cooling on the third floor. I have next-door neighbors with identical layouts. Both of them have the same ducting and new maintained central units. Both of them are unable to adequately cool their 3rd floor in hot weather (delaware valley). I was wondering if my blower was brand new and I left the fan run in addition to adding a peppy ceiling fan I could do better? I havent got a lot of windows or any intense sun exposure.

When you suggest "choke the return" could you explain which return is best choked and how that might improve 3rd fl cooling. Thanks again for your attention.

You might consider installing an attic fan instead.

Appreciate you suggestion. This dwelling has no attic. The third floor has a cathedral type ceiling, with just the roof above.

Ouch. Probably not enough insulation either, then.

Always wondered if building a fairly large cupola above such a roof could enable the addition of an attic fan -- worth asking, at least. Alternately, there are fans that move a lot of air horizontally.

Wondering if the room design would not be adversely impacted by creating a double wall at the top of the end(s) of the upstairs room(s) to vent thorough a grill installed on the outside. If the exterior of the gable is wood, it is not uncommon to see a wooden triangle or half circle or even an octagon shaped vent in the outside of a gable end of a house and one of these could cover a metal unit that could open/close w/a remote, perhaps w/interior door also opening and closing via remote.

https://www.amazon.com/iLiving-Shutter-Thermostat-Humidistat-Variable/dp/B07QXDM9XF/ref=asc_df_B07QXDM9XF/?tag=hyprod-20&linkCode=df0&hvadid=343189771824&hvpos=1o4&hvnetw=g&hvrand=9740432350483084096&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9011109&hvtargid=pla-950990724872&psc=1&tag=&ref=&adgrpid=67406895485&hvpone=&hvptwo=&hvadid=343189771824&hvpos=1o4&hvnetw=g&hvrand=9740432350483084096&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9011109&hvtargid=pla-950990724872

Until you resolve the air flow issue, you might consider setting a portable high velocity air moving fan atop a wardrobe or on a shelf near the ceiling to at least get the air moving. They usually have multiple speeds. Some are noisier than others so you might want to get the building supply dealer to let you listen to your choices before you select one.

Instead of a traditional ceiling fan, perhaps you could build a box (w/vents on all sides and on the bottom) in the ceiling and install a high velocity fan -- it could even be aimed up or down rather than to the side.

You have your work cut out for you and a fan is probably not going to make much difference for what you describe as the problem.

The cool air from the top floor will continue it's descent to the lower floors. The heat in the winter and summer will continue it's ascent to the upper floor.

Everyone likes these 'open style' floor plans and often times they are much harder to make comfortable, especially when they are 3 levels operating from 1 HVAC system.

The best way to solve this problem would be installing a zone system that would direct the air to each floor controlled by it's own thermostat. However, due to how you describe this structure there is probably not enough room to pull it off.

If you don't have the room to pull it off, the zone system 'could' make matters worse or no better than you have now.

Many architects just design buildings with no thought of how to make those buildings comfortable. It's either that or the builder says hey we got this land, but not much so our genius plan is to build straight up. So the architect soothes the builder and designs something that will 'fit' the land space available.

So while I don't necessarily like mini splits, they do have their uses. The best bet would be to install one on the third floor. With 1250 total sq. ft. that's only 400 sq. ft. and some change per floor.

Mini splits excel in small foot prints, because they are more or less spot cooling systems.

As far as too much heat for the third floor in the winter that would be another challenge... because heat is going to rise no matter what you do. --- Unless you start closing off the floors from one another and insulating between them to limit the flow of heat.

My choking suggestion is a poor mans way to institute limited zoning effects. It would rely on you having ample return air vents, and then choosing to position furniture in such a manner as to obstruct the highest or lowest return, depending on where you want the air not pulled from. This might only help a little, and it is only possible with a system with ample return capacity. Austin’s zoning is the correct solution, to (as he also notes) an unsolvable problem.

Open columns bisecting multiple floors are almost the worst possible situation for climate control. If there is any way to reduce the escape of heated air up or the descent of cooled air down, via drapes or such, it might help...but you architecture is against you.

Its great to get all these thoughtful responses. Thanks very much everyone. My first move will be to replace the furnace, blower and to install a new condenser, clean and tune up the ducting. From there ill determine if the result is acceptable.

You might want to look into spray foaming between your roof and top floor ceiling. We in the Florida keys have success with this to keep the summer roof heat from entering the 2-3rd floor rooms.

Thanks to everyone for sharing. In the end i installed 2x 15k mini splits. One at the top of the stairs as seen in pic. The 2nd is on the 1st floor. This strategy has been unbelievably effective. On a 95°, high humidity day just one unit will, on medium setting, cool the whole house into the low 70s in 20-30mins. I crank up both units if we have a house full of guests on a hot day.