Older homes with forced air systems never had central air in mind, only the mechanics of heating. This house had exactly what you need for heating, with all of the return air vents in the flooring on the first floor. Hot air rises, so pull it down.
The third floor had the two lovely rooms in the roof line, but it had no HVAC or electrical services. With a favorable layout of the furnace and a useless closet, we were able to do what can be a real challenge in older homes – bring air returns and supplies to the third floor. Some space was lost, but the gain is huge.
Meet the dynamic duo:

We covered up the existing floor air return and reworked it to accommodate servicing the upper floors.
The second floor closet was only ten inches deep – rather useless. I was happy to lose it, cover it over, and ultimately make a large walk in closet just in front of it.
One of the two supply lines to the third floor was easily hidden in an awkward space in the kitchen (southwest corner), created by a chimney that I framed in. The creation of this space also allowed me to run electrical and plumbing upstairs. The other supply line, in the northeast corner, needed to be covered by a “bump-out.”