A few years ago, I had my house retrofitted with a whole house ac
system. The evaporator/heater unit is located in the second floor of
my house. It is a 4 ton unit for a 1700 square foot house. The
system works well except for one annoying thing. Because the return
vent is located on the door where the evaporator/heater unit is
located, the second floor is the coolest spot in the house during the
ac season, and the hottest part of house during heating season. Of
course, the first floor is either warmer or cooler than the second
floor depending on the season.

It would seem that a second return on the first floor is the most
logical solution, but I am sure there are many complications. One of
the most obvious ones is that there is no easy way of getting return
"pipe" to the first floor... especially for the ac that requires a
large diameter pipe as compared to pipe for heating.

Are there any websites that may discuss the options for "return
systems" in a household? Thank you in advance for any thoughts you
may share