I have a 22 yr old house with gas forced air furnance, ac unit was added about 3 yrs after I bought the house, the furnance is in the basement and ducts were designed for heating - floor vents as well as air return ducts mounted low, so AC is really efficient. What I do is close most of the 1st floor vents and air returns, rely on the 2nd story vents and also leave the filter slot on the furnance open to draw in the cold air that collects in the basement, this seems to work circulate cold air better.
It's a rheem furnace, the heat cycle has a scavaging function that works well, but the ac function runs the blower for a minute or two before shutting down, I've taken to manually operating the fan from the thermostat to suck in the cold basement air . Is there anyway to lenghten the time the fan runs when in AC mode ?
As an fyi a relative who worked in HVAC recommended that I not open the filter slot on the furnance in the basement, but I think this has helped, at one time I contemplated putting a real cold iar return in the basement and might do so.