I've got a 11 year old house with dual 4 ton units (up/down config), and the upstairs has failed needing thousands in repairs. Thinking at that age I may do better to replace the units than repair, I've called in two people for estimates. I also have complaints that the upstairs has never kept up on hot days or kept the floor evenly cooled.

First guy quoted new everything, upgrading the upstairs from 4->5 ton. Of note he didn't recommend any more vents or returns (I had a return question, so I asked specifically).

Second guy is sending me two options in writing tomorrow, one with new everything, the other reusing the existing furnace, just new AC. Also a 4->5 ton upgrade. He noted there were not enough returns and quoted adding one, and noted the outside copper pipes were not big enough for a 5 ton high efficiency unit, so he quoted upgrading those.

The return thing got me looking, so I've found a lot of things online with "rules of thumb" for both return and supply sizing and it appears to me the downstairs (still running, keeps the floor comfortable) has returns matching the 100inch per ton return sizing (barely) and a similar number of outputs, but the upstairs is more like 320 sq in return, and 340 of supply (based on straight round pipe size, not accounting for runs/bends).

So I'm thinking the problem upstairs is not 4->5 ton, but rather lack of returns and supplies. Also, just upgrading without more returns and supplies seems like a bad idea if they are already marginal.

It seems to me both of these guys should have measured and counted all returns and supplies before quoting complete system swap outs. Am I dealing with ding-dongs or is that just not normal practice? Is it worth specifically asking for someone to do the calculation, and if so do I want someone independent to do it? How do I find such a person?