Distribution System Components
Distribution Systems have a number of important components:
The Air-handling Unit is a cabinet that includes or houses the central furnace, air conditioner, or heat pump and the plenum and blower assembly that forces air through the ductwork.
The Supply Ductwork carries air from the air handler to the rooms in a house. Typically each room has at least one supply duct and larger rooms may have several ducts.
The Return Ductwork carries air from the conditioned space back to the air handler. Most houses have only one (or two) main return ducts located in a central area.
Supply and Return Plenums are boxes made of duct board, metal, drywall or wood that distribute air to individual ducts (or registers).
The ductwork is a branching network of round or rectangular tubes generally constructed of sheet metal, fiberglass board, or flexible plastic and wire composite material that is located within the walls, floors, and ceilings. The three most common types of duct material used in home construction are: metal, fiberglass duct board, and Flex-duct.
Flex-duct is installed between the register and plenum box or plenum box and air handler, usually in a single, continuous piece. While flex-duct has fewer seams, the inner lining and outer-insulated covering can tear or be pinched closed. Also longer flex-duct runs can restrict the flow of air. Proper design and installation are very important.
Both Metal and Fiberglass Duct Board are rigid and installed in pieces. Fiberglass duct board (like Flex-duct) is made of an insulation material. Ducts are built of sections of the duct board. The seams in the duct board should be carefully sealed with Mastic or high quality duct tape.
Rectangular Metal Duct, especially the kind used for plenums and larger trunk runs, is often insulated on the inside with fiberglass duct liner. If it is not insulated on the inside, metal ducts should be insulated on the outside using a fiberglass batt with an attached metal foil vapor retarder. The insulation should be at least two inches thick, and the vapor barrier installed on the outside of the insulation facing away from the duct.
The seams in the insulation are usually stapled together around the duct and then taped. All of the seams should be sealed before insulation is installed. All return and supply ducts located outside of the conditioned space (attics, crawl spaces, or basements) should be sealed and insulated.
Ductwork joints join pieces of ductwork. Elbows are manufactured pieces of duct used for turns. Boots connect ductwork to registers. Registers and grilles are the coverings for duct openings into the conditioned space.