Understanding Duct Systems For most homes, there is no component with more duct is more expensive than the individual compo- potential to effect efficiency and comfort than the nents in rigid metal ducting, but labor time for duct system. Although there are ductless heating and assembly is significantly reduced. While flex duct is cooling systems, such as windows units, space an attractive product to the novice or do-it-your- #3 heaters, or packaged terminal systems, which are typ- selfer, differences in performance between the two ically used in motels rooms, the vast majority of types of ducting require some special knowledge of homes in the southeast utilize a forced air duct sys- installation to achieve good results. Because it has a tem. Like a $2000 stereo playing through $50 speak- much rougher inner surface, the diameter of flexible ers, a poor duct system will result in disappointing duct required will differ from rigid metal piping. S h e e t performance. A highly efficient, very expensive heat- Additionally, flex duct cannot support its own ing and cooling unit will deliver unsatisfactory com- weight, so it must be carefully suspended with prop- fort and higher than expected energy usage if the er materials. ductwork is improperly designed or installed. Duct Leakage The Construction of Ductwork Duct leakage is a major efficiency loss and occurs in A forced-air duct system consists of three major almost every duct system. Obviously, a leak in a sup- components; an air-handler, which contains a blower ply duct allows air that has been conditioned to be and is connected to a coil or furnace, supply ducts, forced outside the conditioned space. But a house is which distribute conditioned air throughout the not a closed system. Air leaking out of the home house, and return ducts, which bring air from the from the ductwork will be replaced by outdoor air. home to the air-handler to be reconditioned. In the When this infiltration comes in the form of humid, southeast, duct systems are predominantly construct- 150° attic air during the summer or 20° air on a cold F a c t ed of rigid galvanized metal or flexible ducting or winter night, heating or cooling loads increase dra- flex-duct. A very small number of systems are built matically, raising operating costs and lowering com- with rigid fiberglass ductboard. The most common fort. In fact, in many homes, duct leakage may result method of duct design in modern homes is a combi- in a 25% loss of system efficiency. nation of a rigid metal trunkline with flexible duct branch "run-outs" to each supply register. To mini- mize noise, flexduct is often used for return ducts. Galvanized metal ducts are put together on-site with a combination of sheet metal and prefabricated pip- ing, elbows, and other fittings. While the individual E n e r g y materials are relatively inexpensive, considerable -75 CFM +325 CFM -400 CFM labor is required to join these components together, House Pressure seal the air leakage with mastic, and insulate all of the exposed surfaces. S R Air Handler Flexible ductwork is a prefabricated material with all (400 CFM Airflow) components except the support built into a com- +25 CFM plete, flexible unit. Sold in varying lengths and diam- +50 CFM eters, each section of flexduct contains a plastic inner Figure 1: In this example, a small and outer liner, which seals the conditioned air inside amount of duct leakage results in and moisture outside, fiberglass insulation, and a spi- negative pressure on the home, ral steel wire which provides structural support. Flex bringing in 75 cfm of hot, humid air. T r i - C o u n t y E M C 1 - 8 6 6 - 2 5 4 - 8 1 0 0 - (4 7 8 ) 9 8 6 - 8 1 0 0 w w w . t r i - c o u n t y e m c . c o m Duct tape is not a suitable means of sealing leakage. Flex duct run-outs should not exceed 12 feet in Because the glue will eventually dry and seperate, cloth length. or even foil tape does not provide a permanent seal. Leaks around access doors in the air-handler should Mastic is the answer. An acrylic based product, mastic be sealed with removable foil duct tape. will strongly adhere to galvanized metal, plastic Return filters should be placed in filter grills if pos- flexduct linings, wood, and almost any other surface. sible. Locating the filter in the attic or crawlspace Retaining its flexibility over time, it will not crack even results in poor maintenance practices for many after years of constant expansion and contraction. homeowners. All condensate drains should have a trap to prevent #3 unconditioned air from entering the air-handler or Basic Design Guidelines conditioned air from being forced down the drain. To maintain efficient performance with minimum air- flow noise, ducting should be correctly sized with an emphasis placed on smooth and direct airflow paths. Flex Duct S h e e t Although maintaining the correct amount of airflow, Use wide straps to support flex duct which is measured in cubic feet per minute (cfm), is Mastic boot spaced at 5 foot intervals to floor Strap inner liner and crucial, higher than normal airflow velocity will make a outer insulation home feel drafty and uncomfortable and will also ele- vate airflow noise levels. The following basic rules should be followed to ensure proper duct design and installation: Mastic Mastic before Run lines straight boot seams attaching flex using metal elbows at duct bends and corners All ductwork must be designed following the Air- Never puncture inner Conditioning Contractors of America (ACCA) liner. If repair is needed; install a coupling and Manual D guidelines, which are considered the seal properly industry standard for duct design. All ductwork must be sealed with an approved Figure 2: Guidelines for the proper F a c t mastic. No duct tape of any kind is permissable. installation of flexible ductwork. A trunkline must terminate in an endcap. No branch run-outs should be attached within 24" of the endcap. Sealing Ductwork All ductwork, including supply boots and any other exposed metal or wood framing used in duct con- Mastic or caulk struction, must be insulated to a minimum of R-6 with a vapor barrier. Joints between insulation seg- Mastic Mastic ments must be sealed to prevent moisture from Collar with strap; condensing within the insulation. mastic on take-off Mastic Branch run-outs from the trunkline should be made E n e r g y at a 45° angle. To reduce the velocity of the return airflow, any sys- Mastic to seal tem over 2.5 tons must have more than one return. refrigerant and The face velocity of airflow at the return/filter grill condensate line Removable foil tape should not exceed 2.0 cfm/in2. This means there Mastic should be roughly 1.4 ft2 of free air return for every ton (12,000 BTUH) of capacity. Flexible duct must be suspended as straight as pos- sible with 2" mesh supports no more than 5' apart. Hanging wire must not be used. Flexible ducting Mastic or caulk should never be laid on ceiling trusses or allowed to make contact with the ground. An elbow must be used on any angled connection Figure 3: All ductwork should be of flexible ducting to a ceiling or floor boot. properly sealed with mastic in all of the Supply trunklines should not exceed 25 feet without areas shown here. a reduction in diameter.