I have a two story house with only 1 AC unit. The indoor unit is in a closet upstairs, with a return plenum directly below it at floor level. The return plenum is open to the second floor via a return grille in the wall, and one of the side walls is the back wall of my daughter's closet. The thermostat is located on the first floor.

My issue is that the downstairs stays pretty cold in the winter and the upstairs can get really hot. It seems like the return is feeding mostly hot air from the upstairs into the unit and cycling air upstairs. There's not much pull from downstairs through the stairwell. With the downstairs cold, the thermostat keeps the heater running.

I would like to add a return downstairs by making a path from the plenum through my daughter's closet wall, down through the floor, and to the ceiling below. I figure I can open a hole in the wall between the plenum and the closet and one in the floor, then build a cabinet over the floor hike and up against the wall to make a plenum between them. Then I seal up the joist space above the first floor between the hole and where I want the return.

Is there any reason I can't do this? Any requirements for fireproofing? Should I run metal duct from the plenum to the return and seal the floor opening with fire caulk?

That's a new one on me. Must be some history to how that design evolved - must have been an architect? haha

If allowed to stiffarm the CODE questions (a real HVAC will give better answer later) then I say A-OK but would be a home run only if the new 1st Fl RA was near that one 1st Fl thermostat (or moved thermo to new RA). Give some thoughts to sealing off that 2nd Fl RA as well.

If you had the money, its food for thought to have a mini-split system come out to give you options for 1st Fl and relocate 1st Fl thermo to 2nd Fl, then seal off old 1st Fl registers.

Not to mention the consequences of stealing space from a females closet.....

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Bump

Following. I have the exact same setup in my house, and also have issues with temperature differential between first and second floor.

With no replies as yet, may I weigh in again?

In ALL 2-story designs past, the HVAC setups have been
1. "Best" scheme System A for 1st floor, System B for 2nd floor, each with its segregated RAs & thermostats, ideally separated to avoid one air flow from skewing past the others thermo, setting it off prematurely
2. "Saving money" scheme has only one System, then to install a duct damper that would "throw" air to the A thermo 1st floor or B thermo on 2nd floor. The thermos worked best if near their own floor's RA ... segregated, etc. Repeat: thermos near RAs both 1st & 2nd floors, duct damper throws air to thermo "calls"/demands as much as 80-20% from one system.

#2 may be the only way to deal with the design sold you without adding another system or at least a unit* to a problem/needy room or two.

*when you see a window unit hanging out a building known to have central HVAC to all rooms (even on campuses) then that room has special needs "central" can't solve itself.

Yeah, I get how the systems work. No, the system was not sold to me, it's how the house was designed and built. Adding in a VAV and second thermostat is a very expensive proposition. Adding in a return would be much easier. My only question is on the code requirements.

OK
Any RA chase/duct/etc must be sealed & R-6 to 8 rated
so if gyp.bd/sealant/mastic chase interior seems impossible, then dropping an insulated flex-duct might work, even though not a desirable SF for air flow desired.

In-the-day Dallas, etc. allowed open framing chases as RA in every room, so long as exterior gyp.bd was sealed.
Today, variances to many IRC (CODE) items have been granted/grandfathered by City to antique houses not on any historic register BUT these items were left "status quo" and your project would be judged as that or not.

FYI the CODE book on interpretations such as this is larger than the CODE itself.

My bet: dropping a R-6+ flex duct would be OK