Hi!! My third floor has little way for air to flow into the rest of the house.

In the summer after closing all vents on the first floor, I run the Heatpump fan constantly. It moves the air in the house around, and upstairs is more pleasant.

I'm thinking about putting a fan in the 2nd floor big return duct. It's a short distance from the big return duct on the 3rd floor, one right turn. I could run it even on a timer at the plug to accomplish the same thing as the Heat Pump fan of getting the air in 3rd floor to move to the 2nd, and take the stuffiness out of it.

The HP is very old, but is running fine. No expensive bills. Not running unusually long. Freon's not leaking. BUT the fan is ever so slightly off balance (because I messed with it 20 years ago), so it slightly shakes my bed. Which is fine during the day but annoying at night.

Fans sitting on the stairs are cost inefficient, and ugly and angle doesn't work.

After reading around, I'm puzzled. My questions are :

1) Can a fan pulling around of the return duct move air between the two rooms? Not to make the HP's a/c flow better, but to simply move the air.

2) Who makes a fan that's cheap that can be added on? No bells, I don't need to time it with my heatpump.

3) Is there anything bad to a HP to run it's fan constantly?