Nice forum here. Thanks in advance for any ideas. Here is my story...

We have a 100 year old 2 story Denver Square with a full basement and one room added on the back. 900 sq ft per floor, plus the 300 sq ft single floor addition off the back.

When we moved in we hoped we could add central AC to the forced hot air heating system when the furnace went. Well, the furnace went one cold winter's day, and it was not until then that we noticed that there were no return ducts from the 2nd floor. The heating system has a supply vent to every room, and 3 return vents in the floor of the first floor. We looked and looked for options in closets, etc, but couldn't find a reasonable solution to get return air from the second floor to the basement furnace. It seems to work fine for heating.

Now we are adding flooring to our attic for storage and thought it might be a good time to update the 100 year old knob and tube wiring and look into central AC "while we're at it". We've had several options offered:

#1 put a big old AC unit in the basement with the current furnace.
#2 put separate AC in the attic for the 2nd floor with new ducting and put AC in the basement system. And
#3 put AC in the attic with new vents for the upstairs, and let gravity take it downstairs and possibly use the fan in the basement system to circulate it throughout the 1st floor. (A la the heating system).

Since we live in Denver, we don't use the AC a lot, and certainly don't need a lot of humidity control, but are interested in quiet. One guy recommended hanging the attic unit from the roof trusses to make it quieter (is that a good idea?).

Because our house is close to the neighbors, we can't put the condensor on the side of the house and we are thinking of putting it on the roof of the first floor addition - thoughts?

Thanks again for any help. Tom