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!