Many older homes have ductwork installed in or under the concrete slab foundation. When these ducts break or rust out, a homeowner is left with few options to deal with the problem. Installing new ducts through the attic is the best option. Running new ducts overhead is a permanent fix and much less invasive than trying to remove the old ducts by digging them out of the concrete and removing sections of walls if necessary. Ducts can also be coated with a fast hardening liquid PVC type of material in some cases if they are not too damaged. Only a professional duct company with the necessary equipment can do this. This is the cheapest option in most cases, but may not be a permanent fix.
Run the larger main supply duct across the attic joists to the farthest location from the furnace. Leave a few extra feet of duct at this end to be cut to size when installing the vent register.
Run the other end of the large main supply flexible duct to the furnace plenum. Attach to the plenum by slinging the end over the outlet on the plenum. Secure with plastic duct straps. Wrap this union with the aluminum duct tape.
Install T-unions along the length of the large main supply duct wherever necessary for connecting the smaller side ducts that will run to your new ceiling registers. Run the smaller flexible insulated ducts from these new ceiling registers over to the T-unions and slide over the ends of the outlets. Secure with plastic duct straps and seal with the aluminum duct tape.
Cut out the holes for the registers to fit into the ceiling snugly using the register as a template.
Pull the ends of the ducts down through the ceiling and attach to the vent registers. Push the registers back into the ceiling and screw their plates into the ceiling per manufacturer's instructions.
Wrap all the unions with duct insulation and secure them with more plastic duct straps. Attach the main and side supply ducts to the ceiling joists with the galvanized duct hangars and the screw gun.