While installing a new furnace, I noticed that the supply ducts are extremely dirty, as least as far as I can see. This is a down-draft furnace with floor registers. I know that the flex-duct was never properly attached to the registers...partly becaused I repaired some 12 years ago when a cheap duct-cleaning service mucked up a couple junctions...and I saw how all of them were attached.
So I'm thinking, rather than spending the money on a quality duct cleaning service, I should just replace the ducting and do it right. I'll do the work myself (pretty handy, former general contractor). Repair & clean or replace? What do you advise? Also, I'm wondering what is the working life of flex ducting? Mine is 25+ years old now.