Metal Duck Work Under Home, Duck work is directly beneath floor.
Best way to insulate areas, joints, elbows where heat can escape in these sections that are joined from furnace to room vents. Best way to insulate and prevent this heat loss, causing more gas to heat furnace, heating duck work, loosing heat, while trying to heat 2500 sq. ft. home.
Best way to prevent the duck work having these weak areas, stop from loosing heat, lower gas bill?
There are caulk products for sealing duct joints available at home stores. There are also insulation products for insulating them, usually rigid insulation boards. If the space below is a basement, you may not want to fix leaks or insualte, since that heats the basement. If a crawlspace, the ducts should be insulated.
I'm assuming this is metal duct work, not flexible tubes.
Seal air leaks at the seams using metallic duct tape (NOT the gray vinyl tape often called duct tape... it's good for almost everything in the world EXCEPT ducts).
If you really want to insulate the ducts, rectangular ducts are usually insulated using a rigid insulation board that has a foil outer surface, made for this purpose. There are fiberglass insulation wraps that can be used on round ductwork.