I have a 1940 brick rambler in Virginia of 1000 sq ft with a 500 sq ft addition that has no central cooling. It has a boiler/radiators in the basement for heating. I have a 2.5 ton condenser and air handler that I want to hook up to ducts to be installed in the attic. I found a similar house which is to be demolished nearby with ductwork being sold cheap. The duct sizes seem to match fairly well with what I need.
I have not done a real manual J, but this house has no wall insulation. I have tried to tighten it up with new windows, ceiling insulation and have installed an attic vent with air channels from soffit holes to keep the attic air moving.
Does anyone have any experience with using older ducts? They look like what you get now, but maybe there is something I am missing.