Central Air OK Downstairs Upstairs still warm?

HI: Can someone tell me Why my old Central Air Condition seems to cool downstairs very cool but upstairs does not get very cool at all! When the heat is on it does upstairs very well. The Vents are not clogged I checked! Someone told me to leave it on for a couple of days and THAT would make a difference. Seems like a long time to get up stairs! Any help would be appreciated. Thank YOU TOM

 

well, naturally the cold air is heavier and the downstairs would stay cooler. I've had a similar problem with mine. I have the evaporator coils in my warm air furnace and it takes care of the downstair pretty good, although if i had an airhandler on the second floor it would be better, but back to the problem. I installed a duct booster in the basement on the duct that goes upstairs and i also ran another flex duct for another return for the upstairs only and it helped out the flow of cold air up there. It really did the trick for me. Hope I've helped you.
Have a great day.
Joe