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How Is Ductwork Sizing Determined?

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  • Written By: B. Turner
  • Edited By: W. Everett
  • Last Modified Date: 28 August 2014
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In structures with a central furnace or air conditioner, a network of supply ducts transports hot or cool air throughout the building. A second set of return ducts captures exhaust air and returns it to the mechanical room for ventilation purposes. Ductwork sizing can have a significant impact on the effectiveness of a home's heating and cooling system. Properly-sized ducts ensure sufficient air reaches each room to maximize comfort for occupants. Ductwork sizing also effects noise levels, energy efficiency, and operating expenses.

Mechanical engineers must evaluate a number of different factors when determining ductwork sizing for a structure. First, they consider the size of the building and the climate zone where it's located. They then measure how much air enters or exits the home naturally through gaps or openings in the building. By combining these two factors, the engineer determines what size furnace or air conditioner is needed to keep the home comfortable.

Based on the size of the central unit, the engineer can determine the initial air pressure within the duct network. He then calculates the distance the air must travel to and from each room, as well as the amount of air volume needed in each area. Engineers must also consider how much air pressure is lost as the air travels through the ducts themselves, either due to distance from the blower or to simple leaks in the duct.

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Armed with this information, designers can choose one of two basic systems to determine proper duct sizing. People throughout much of the world rely on systems created by the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) to size ductwork. The most widely used ASHRAE system relies on achieving equal friction within the duct network. This means sizing ducts to equalize pressure lost during air supply and air return, which creates a balanced system of heating, cooling and ventilation.

The second ASHRAE technique used for ductwork sizing involves measuring static regain. This system is more complex, but also results in improved energy efficiency when used correctly. To size ducts with static regain, engineers use ducts that create equal static pressure at every supply and return outlet in the home. This includes all exhaust vents and supply grills, and is based on a series of charts developed by ASHRAE that are used in conjunction with data taken from the home.

If ductwork sizing is performed incorrectly, homeowners can make simple modifications to the ducts to improve efficiency and comfort. An air damper installed within a duct allows users to regulate airflow, or reduce air pressure in one area to bring more air to other parts of the home. If air is moving too slowly through a duct, a fan or duct blower can be installed within the duct itself. Duct blowers supplement the main furnace or air conditioning blower, and help increase airflow to distant parts of the home.

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