My parents had a new AC system installed last year, but they kept the old duct work, which is probably 30-35 years old.
The system is cooling very slowly (three hours to drop from 80 degrees to 77 deg). The air temperature at a ceiling vent was about 70 deg in a room that was about 77 deg.
I've looked at the duct work in the attic and found a few places where the rectangular fiberboard duct with aluminum foil on the exterior had lost its foil. It may have been damaged by the installer, but I think the foil may have deteriorated in those places. In those places I could feel air passing through the fiberboard.
In other places, the foil appears to have separated from the fiberboard so that the foil puffs out but doesn't appear to leak.
There are a few joints that have come partially untaped.
At least one slip in joint (where the rectangular duct meets the air handler and on the duct side of an anti-vibration flex joint) is leaking quite a bit (possibly because the fiberboard duct expands outward when the system is pressurized).
The smaller round ducts (foil covering something) that feed the ceiling vents seem to be mostly intact, but I haven't inspected them closely or looked to see what's going on inside the foil. I didn't think to look, but I think they are flexible with fiberglas insulation under the foil and not steel-walled.
Does duct work like this have a limited lifespan?
How should we decide whether to replace it or just repair it?