Wood returns in a garage are not allowed here due to the fire codes!

 

Is the AH sitting on top of the plenum? In that case the plenum needs to be strong enough to support the weight of the AH and sealed. Im not aware of any requirements specific for building / framing plenums. Here in SoCal we are no longer allowed to use building cavities to convey air. Everything is usually sheet metal, insulated and sealed.

 

We are assuming that this is an air handler only, with coil in the bottom.

Not a gas furnace?

 

Metal would be easier, and easier to seal and insulate. You could even have a shop make it for you.
Then you could box it out easily with a soffit-metal studs/drywall, or wood/drywall. I wouldn't bother f'ing around with ductboard

 

Just frame it with 1.5" metal studs and use sheet metal roll to skin it. Wrap with insulation and your done. I've seen many made by duct shops they use drive for support I've seen lightweight angle iron in them too

 

Here in FL. just 2x4"s, plywood top and sides, wrap inside with duct board. Seal joints with mastic or flip board so silver side is facing out, than tape seams. We try to reroute drain so it's not inside plenum. As far as how to frame. Just think about how you would build a square wood box. Have to secure to house wall also. Use 5/8" plywood for top. Leave enough space on top to cut out your R/A opening for bottom of AHU.

 

Yes, I am in Florida. The way the original one was done was a mess. I was going to frame it like a home on 3 sides (house side not framed) I was just wondering the best way to insulate it. I was thinking about making a duct plenum box properly fabricated and dropping it into the wood box. I would have to cut a hole in the bottom for the piping to come through. The piping comes through a chase underground so no easy way to run it a different way. For the top part, I was thinking about making it a 1/8" higher and then letting the plywood squeeze it down in order to create a tighter seal. Of course have to cut opening for the AH and the Return air path. Right now there is spray foam everywhere.

 

Just cut drain pipe and copper low and elbow out front or side of stand. You could complete framing and install fiberglass duct board thru RA grill opening or reach thru the top. Need to bury drain and copper under duct board bottom piece. Than close outside and top with plywood.

 

I have seen this done before and am not a fan. Assuming I understand you properly I am not sure what the advantage would be and only see disadvantages on the ones I have dealt with in the past such as:

1) You no longer can easily access the pipes below the duct board or underground,
2) If you need to get under the AH unit to see or clean the coil, the duct board is now in the air and not supported by the concrete slab and will bend under your weight and
3) If you need to replace one of the lines, you will have to just about disassemble the AH support box.

On the other hand what disadvantage is there having the lines come up and through the top of the Air Handler box except another hole you have to cover.

 

I was referring to the final inspection. Unless this is on your house or a friend with no permit, do as you like. Inspectors do not want combustibles that are toxic in R/A. As far as accessing lines or replacing. You would have to still have to tear up everything anyway. How often would you have to perform those tasks? Going thru the top of the stand is fine.