Hi JH, as far as bringing up cool air, in the past I’ve used in-line fans that really work well. If you can put some kind of ducting in place, I think that would work. As we all know, cold air falls to the lowest floor, nothing to worry about. the fans usually come in all sizes. Maybe an 8 inch would work for you if at all?
If it sounds interesting, I will send you a link :-). Daren.

Thanks, Daren, for your suggestion. I have not heard of "in-line fans." Would you send a link? I've heard of vent covers with small fans to draw air through the ducts. But doubt it would really be effective.

No basement here now, and I have an outside unit, but, years ago, when I had a regular furnace and it was in the basement, there was a switch on the thermostat to operate as fan only and it did bring the colder basement air up to the main level.

The fan switch on my thermostat has a "ON" position but then the fan ("blower") would be running at 100% without the condenser/AC unit on. I tried it a couple of times but not very effective since there is not a large/sufficient-sized return vent in the basement.

I thought of placing a floor fan at the base of the stairs in the basement facing upstairs and turning it ON for several hours and doing the same with another floor fan on the first floor facing upstairs to the second floor.

Basement air is not good quality air for the rest of the home unless you have made efforts to making it good quality air. If you have made that investment, a direct recirculating duct from the top floor to the bottom floor can be installed. You will need a route for that duct from top to bottom. You cannot use the existing ductwork that is attached to your blower.

Charles Ross and DeWayne,

The basement is finished with a bathroom and four separate rooms (e.g., laundry room, utility room, open space/room, room with a bed). A radon remover equipment is present and working properly. I have tested for radon every three to five years and have results within EPA guidelines. No mold present via regular testing.

I may try this after speaking my AC tech for input: https://youtu.be/g93ZpHteQ_0

If the air handler is located in a mechanical room or furnace room, cutting a return air opening in the return plenum may violate your state's building code. The 2018 International Residential Code (IRC,) which forms the core of most state's building codes, states the following:

M1602.2 4. Return air shall not be taken from a closet, bathroom, toilet room, kitchen, garage, mechanical room, boiler room, furnace room or unconditioned attic.

Be aware that an opening that close to the blower will be loud; it could bother occupants of the basement-level bedroom.

I was not aware of this code. Thanks Charles for letting me know.

Looks like this may be an option: https://youtu.be/F1FzBjUVAO0

It's not ideal to have a single air handler serving multiple levels, but absent an automated zone-control system, you may find that balancing dampers improve comfort on each level. I have balancing dampers on my first floor HVAC system which I adjust seasonally to direct more/less air to the master suite. A purely mechanical system is typically preferable to anything electrical in my experience.

Is it possible to retrofit a balancing damper in a 40 year duct network?

You can purchase balancing dampers for round or rectangular duct work. They're pretty inexpensive and easy to install.