[ddawg16]

I am doing a 2-story addition to my house....

I have one spot where I can bring in an HVAC duct to supply air to the addition....yes, I know some are going to say that a 12" duct is not enough air to heat an additional 1000 sq ft....yes, I know my 70K btu unit is marginal....but....I live in S California....40 deg is considered a cold snap.....and I'm insulating the hell out of the house.

With that out of the way....I need to make a plenum that gets the air from the 2nd floor to the 1st floor....it will have to drop down through a hole that is 12"x8 3/4" (space between two floor joists)...once down, I can then connect my HVAC flexible ducts to it..it's droping down above a bathroom...that has a dropped ceiling which will give me about 8" of space to run the ducts.

The Plenum is would need to look something like this....

I assume the sheet metal they sell in the duct dept at HD would work....I have access to a brake...(a real big brake).

Any reason I can't just make the pieces...bend about a 1/4" lip....attach together with screws...seal the seams with metal tape and just shove it up between the joists?

Any reason I can't screw it directly against the joists? Since this duct will be between the 1st and 2nd floors...no reason to insulate?

Oh....hot air only....even though my forced air unit has the evap in it....I doubt I'll ever put in AC....being 4 miles from the beach makes it hard to justify.

 

[beenthere]

Screwing it fast to the joist will give you lots of noise as it expands and contracts from getting hot and then cooling down.

You can bend your own Pittsburgh on the sheet metal if you want to. Screws could present problems with expansion too, depending how may you used. And it would take a fair amount of screws to put the duct together.

Is the addition open to the main section. or do you have another way of getting return back to the furnace.

 

[ddawg16]

Screwing it fast to the joist will give you lots of noise as it expands and contracts from getting hot and then cooling down.

You can bend your own Pittsburgh on the sheet metal if you want to. Screws could present problems with expansion too, depending how may you used. And it would take a fair amount of screws to put the duct together.

Is the addition open to the main section. or do you have another way of getting return back to the furnace.

Hadn't thought of that.....good reason for running this past you guys.

On the return air...down stais is wide open....the HVAC return is in the hallway....about as central as you can get.

For upstairs, I am 'planning' on running an insulated duct from a central part of the master bedroom/bathroom through a small attic space and dumping into the stairwell.

Now that you have me thinking about the Penlum.....I think I know an easier and better way....maybe just a pre-made T would work....

 

[beenthere]

Sometimes you have to take a step back and look at something from a different angle to come up with the easier way.

Good luck.