The air distribution system can make a big difference in both the cost and the effectiveness of geothermal heating and cooling. It also has an important effect on personal comfort and health.
- The air handling component is either a separate cabinet or is part of the cabinet that houses the geothermal heat pump, and includes the plenum (see below) and blower assembly that forces air through the ductwork.
- The supply ductwork carries air from the air handles to the rooms. Typically, each room has at least one supply duct and larger rooms may have several.
- The return ductwork moves air from the room back to the air handler. Most buildings have one or more main return ducts located in a central area.
Types of duct material
The most common types of duct material are metal, fiberglass and flex-duct, a flexible plastic and wire composite material. Both metal and fiberglass duct board are rigid and installed in pieces; flex-duct is installed in a single, continuous piece.
Rectangular metal duct, especially the kind used for plenums and larger runs, is often insulated on the inside with fiberglass duct liner.
If not insulated on the inside, metal ducts should be insulated on the outside using a fiberglass batt with an attached foil vapor barrier. The insulation should be at least two inches thick, and the vapor barrier should be 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 the heated space - in attics, crawl spaces or basements, for example - should be sealed and insulated.
Fiberglass duct board, like flex-duct, is made of an insulation material. The seams in the duct board should be carefully sealed with mastic or high-quality duct tape.
Flex-duct is installed between the register and the 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. In addition, longer flex-duct runs can restrict the flow of air; proper design and installation is important.