On Mon, 22 Jun 2015 14:15:00 -0700 (PDT), stryped
wrote:

My house was built around 1998 and has the original split system with a carrier 3 ton condenser outside and the air handle with propane furnace in the crawl space.

Because of its age, and the outside condenser making noises, I had someone quote a new system. The quote started at 8,000 for an Amana basic system and went to 15,000 for a Bryant with all the bells and whistles. Ouch......

To make a long story short, the person quoting found some things that probably need to be addressed. One, my filter grill is in a wall in the house but there is no "box" behind the grill attached to the return air vent. There is just a hole in the floor with the return air flex attached.

The installer made a note that under the house, the previous installer rather than attach a return plenum just ran the return line directly off the side of the air handler.

Lastly, there is a very small "plenum" just covering the A coil. from this is about 3 feel of flex duct that attached to the hard metal main 18 inch duct the supplies the branches that go to individual rooms.

So, I would like to build a filter housing, return air plenum, and maybe build a supply plenum myself 1st. Then maybe decide if I want to tackle replacing the current condenser/air handler.

My question is, how would you go about this size wise and mainly, is it necessary to use galvanized sheet metal or better to use regular metal then paint it? I can mig weld seems if necessary, but obviously that destroys the galvanized coating. What gauge would you use?

Thanks for your help!
I can't tell you how to properly size the ductwork but surely there
are places on the web with that info. But when it comes to galvanized
sheet or painted sheet I would go with galvanized. I wouldn't weld
anything. If you need to make custom duct I would suggest soldering. I
did this in my house when I had to modify the installed ducting. I
just plain couldn't find any stock ducting for a couple places. So I
bought ducting that was as close as possible to the desired sizes and
shapes, some flat sheets of galvanized steel, and a roll of the same
stuff. I bent the pieces to size and I was going to use screws to
hold it all together and that aluminum tape to seal the seams. The
stock ducting was made so that the seams were sealed by crimping and I
couldn't do that. And even though that aluminum tape is made for this
type of work, and seems to stick forever (unlike DUCT tape) I decided
to try soldering. The ductwork was amazingly easy to solder. So I
soldered my custom pieces and only used the aluminum tape to seal the
seam where my custom work met the original work.
Eric