National Oilheat Research Alliance 13 When making bends or turns with flex duct, the “bend” or “radius” should not be less than one duct diameter. sides also have the highest resistance to air flow compared to the other commonly used materials. Flex duct runs should be kept short and as straight as possible. Friction losses must be considered on long runs of flexible duct. Even properly installed, flex ducts cause two to four times the friction loss of the same size round duct. Check with your local jurisdictions prior to installation, since they may have limitations on the length of the runs and required insulation values. Sealing Ductwork Unconditioned air leaking into the return air duct can negatively affect system operation and efficiency. The return ducts are under negative pressure, so if they are not sealed (Figure 13, below), they will suck air into any ducts in unconditioned space in the attic or basement. Conditioned air in the supply ducts is under pressure, and if the ducts are not sealed, it will leak into unconditioned space. This is a waste of energy, and most local codes require the sealing of all seams and connections in metal duct. Existing ductwork is not always accessible as it may be covered with sheetrock or hidden in a wall. In these cases, there are materials that can seal from the interior of the ducts. Fiberglass ductboard is sealed with aluminum tape. It is critical to use the tape approved by the ductboard manufacturer and it must be applied according to the manufacturer’s instructions. Flex duct only requires sealing at each connection. Equivalent Length The amount of pressure lost through a duct is directly related to its equivalent length. Every time the air changes direction, pressure is lost. To calculate the pressure loss in an elbow or turn, its equivalent length to straight duct must be known. The equivalent length of the commonly used fittings can be found on the back of most air duct calculators. (See Figure 14 on following page). A round elbow has an equivalent length of 10′. This means that the elbow is the same as a 10′ length of round pipe. The equivalencies must be taken into consideration when calculating the pressure loss of the system. Use the fittings with the least equivalent length to minimize pressure loss. Sealing ductwork Figure 13
NORA Advanced Air Flow
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