Got called out on a no a/c call. This is on a 15 ton split downflow system. A few years ago, a "well-known" contractor did this changeout: new a/c condensing unit (Tempstar with 2-7.5 ton comps), 3 new Tempstar gas furnaces multi-positional set in downflow (all 3 are trippled together, which is not recommended and not supported my Tempstar) with all 3 vents running down, over, then back up & tied back into what was a common vent supply (10" I think), sitting on top of the original Carrier coil. The new furnaces are still smaller than the old coil, so they blocked off about 18" on one end. If I hadn't of seen it myself, I wouldn't have believed it. Needless to say, one of the comps is shot.