Relacionar Columnas Air Distribution Systems - section 3Versión en línea NCCER Air Distribution Systems - section 3 trade terms por Valerie Jimenez 1 Reducing-Trunk duct system 2 Extended-Plenum duct system 3 Radial system 4 R-value 5 Perimeter-loop system Each perimeter outlet is fed directly from the supply air plenum. The duct work can be installed in a concrete slab, crawl space or attic. Used in structures built on concrete slabs, it has a continuous duct embedded in the slab. Provides the best performance when properly sized, the pressure drop is maintained from one end to the other due to decrease in trunk size. A number that is used to indicate the ability of insulation to resist the flow of heat. The higher the R-value the better the insulating ability. Uses round or rectangular trunks for the main supply and return, the ducts remain the same size and can be installed in a crawl space or attic.