My 50 year old house brings the return air from the four corners of the first floor. The air travels between the joists which are capped (on the bottom) with sheet metal. These two channels are then joined at either end of a large sheet metal run (about 2 ft square) that runs perpendicular to the joists and then joins up the furnace.

The vents themselves are backed by the masonry exterior wall and are a constant source of cold air. I'd like to replace the return lines with insulated flexible pipe since they run through the unheated crawl space. How big do the return lines need to be (I have central heat and air, the AC unit is two and a half tons), the house is about 1800 sq feet.

As an aside, this return system has stayed remarkably clean. I pulled away the sheet metal to try and insulate the return vents from underneath and there was only a slight layer of dust in the box formed from the joists and the sheet metal cover.

Thanks,
Mike

I would recommend that you add insulation on the outside of the return run. Better home stores sell a type of insulation made for this application. It is aluminim foil on one side a yellow semi-rigid fiberglass on the inner side. You should also closely inspect these returns for gaps and leaks. Caulk or use high quality tape made for air ducts. This is not the standard duct tape you will find in most store. DUCT TAPE is now a brand of tape and it is not really made for air ducts.

I agree with ballengerb1, insulate the out side of the duct. Do not use flex duct on the return air the core will come apart and collapse over time. I would not use flex duct on return or supply.

There is one radically different solution. Leave the ducts be. Seal up the crawl space good, caulking between the plate, band joist, and floor above. Seal the walls with masonry paint. Cover the floor with plastic, Then run fiberglass batts down the wall and out about a foot. That is what I did. You could also pay to have foam applied. I have nice warm floors including the hardwood floor in our family room and never any frozen pipes. My crawl space stay about 60 degrees year around and dry. I don't even have moisture condensing on the pipes in the summer.

I agree with labman If that is the rout you want to take. But look at the cost and the time of payback. If you have good windows,well insulated attic and high efficient furnace or heat pump, I would do that to. If you don't you will never see the gain for the money.