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Choosing the best ductwork design can mean the difference between having a heating, ventilating and air-conditioning (HVAC) duct system that works well and one that is inefficient. Many factors go into designing ductwork, including first understanding the requirements of the home or facility. Others — such as duct size, pressure balance, and use of duct equipment such as fittings and terminal boxes — also affect performance. Poorly designed ductwork and improperly sealed ducts can lower the efficiency of any HVAC system by as much as 40 percent.
The right ductwork design can only be determined if the heating and cooling loads of the indoor space are understood. A scale drawing of the building’s air distribution system should be made to indicate where ducts and other equipment will be placed, with the lengths of all ducts clearly shown. Selection and sizing of HVAC equipment for homes is specified in Manual S from the Air Conditioning Contractors of America (ACCA). This guide outlines procedures for using information on blower data, air flow, and room size to determine proper duct size.
An important ductwork design consideration is the pressure balance of the duct layout. Actual air flow can exceed design flow if the fan pressure is higher than the pressure loss. In this case, it takes partial closure of the balancing dampers, which control the air flow, to get the air flow rate down to what the duct equipment was designed to handle. This makes the system noisy when operating and harder to control.
The ductwork design phase also involves determining the air volume requirements in each space. The height of each heating duct, for example, needs to be determined and, if energy efficiency is the goal, round ducts are the best choice. Also, roofs can restrict space for ducts, and shafts or vertical ducts might be needed, depending on the building. The choice of duct layout and duct supplies depends on the structure; for example, laboratories require exhaust systems that have high pressure and need filters to reduce leakage.
A lot of detail goes into ductwork design. It is usually necessary to consult guidelines from the ACCA or seek help from HVAC trade contractors to select the right duct supplies and ensure proper installation. In addition, the location of utilities such as plumbing, communications, and electrical lines, and space requirements for access, must be taken into account when considering ductwork design criteria. When configured properly, an HVAC duct system performs efficiently and quietly.
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