I have two systems in my home that I'll be replacing in the next couple of weeks. These pictures are of the downstairs unit, located in a utility room off the garage. This furnace and evaporator coil are the originals installed in the mid-70's, and they've earned the right to retire.
Today, I pulled the front panels (scraps of that lovely mid-70's wood paneling) off the return-air box the furnace is sitting on so I could have a look inside. It was just as nasty as I thought it might be after 37 years. The return duct is a 14" round pipe in the adjacent crawl space that connects to a 20X20 filter grille about ten feet away. I see no caulking or other sealing agents. What I do see is mold and mildew general filth--and a return duct that's a little smaller than it ought to be for a 2 1/2 ton AC and 80,000 btu furnace that's somewhat blocked by the undersized hole they made in the wall.
I've never seen an installation quite like this. Is this sort of return-air box common for that period? The plywood top is both sagging and de-laminating, and the whole deal looks like dumpster bait to me.
If you were doing this job, how would you approach this? Keep the bones and replace the sides and top, scrap it and build a new one, or scrap it and just bring the return air duct through the wall and into the side of the furnace? The supply duct that goes into the crawl space could be raised by 6 or 8 inches to make more room for the return if necessary. The 3 sales guys who've bid on this so far didn't seem overly concerned, but then again they weren't the guys who'd have to do the install.