Increase my home air conditioner effeciency?

Since cool air is heavier than warm air and the basement is typically much cooler than the rest of the house (for many reasons), it was suggested to me that I could block off the HVAC air intakes with a piece of cardboard in the upstairs section of the house while creating an opening in the intake vent in the basement near my furnace so that cooler air is entering the duct work (and requiring less energy to be cooled) and then pumped throughout the house.

Would this work? Have any major drawbacks?
Thanks

Answers:
I dont think thats a good idea you need the returns up high to remove the heat. having adequate return is just as important as the supply. i think if you did what you just explained i think it would be hotter on the 2nd floor.

Other answers:
Blocking off any air return will create mold!
Blocking off any air return will create mold!
Just yesterday, I was speaking with a guy that works for a heating/cooling agency. He suggested that I check the vents on my AC intake (mine is outside) and spray them off, making sure there are no leaves or dirt on it. Since again, mine is outside, he said that it will not harm my AC unit.

I wouldn't mess with the intake vent if I were you, just make sure that you have doors closed to any rooms you don't need cooled and use fans to better circulate the air.
The only concern is air circulation. By closing all the upstairs intake vents and only drawing from the basement, you could create some problems with proper circulation in some areas of the house. A compromise could be to not completely block all the upstairs vents, but to partially block them with the addition of the basement vent intake. If you find some hot spots, you can further open intakes in those areas.
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