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What would happen if I had 2 HVAC units share a common return air duct?
I am working on the deisn for a two story single family residence that would have two airconditioning units, one for the first floor and one for the second floor. The first floor unit has a return air chase that extends from the first floor all the way up through the second floor. The second floor would have its own separate return air chase, except that the available location on the floor plan would not work in the attic; the attic space above this return air chase would not be enough to fit the air handlers.
I've often wondered what the effects would be if both units pulled air from a single return air duct that would split off in the attic to both units. When both units are running, then both units would be pulling air from both first and second floor, but what would happen if one unit were turned off? The other unit would still pull air from both floors (zones), but would it also back flow air through the unit that is shut down?
Are there any other issues to be concerned about? Would this arrangement compromise the efficiency of both the units?
I look forward to a reply, in the mean time, I will continue exploring other locations in the plan for the second floor return air chase, or I will have to consider using a closet unit on the second floor.