Mike:

M My house is 55-years old and all the outlet and return vents are
M positioned near floor level in the walls. Central A/C and a furnace
M share the ductwork, and the A/C doesn't work well in the upstairs
M bedrooms. I'd like to have additional outlet vents in those rooms
M positioned near the ceiling so that I could open them when using A/C
M (furnace vents closed) and close them when using the furnace (furnace
M vents open). It looks to me like a reasonable way to add an outlet in
M my house would be to open up the wall above the existing ductwork,
M extend the ductwork to near the ceiling, install a vent there and
M close up the wall. Has anyone tried this either via contracting or as
M a DIY project? Any thoughts or recommendations?

Seems like it should work though can't be sure for the reasons the
others gave (Manual J, etc.)

The office my father works in has the heat and a/c through ceiling
vents, the returns are in the walls. Close the upper one or lower one
depening on the season. Seems to work fine, though was designed this
way.

One thing you might want to consider before adding vents is to change
your vent covers where the heated/cooled air exhausts into the room.
Possibly spraying the cool air up (towards the ceiling) may help.

-
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