Proper duct sizing is crucial for effective heating and cooling in buildings. Mechanical engineers consider building size, climate, and air flow to determine the size of ducts needed. ASHRAE offers two methods for sizing ducts, friction parity and static recovery. Improper sizing can be corrected with air dampers or duct fans.
In facilities with a central furnace or air conditioner, a network of supply ducts carries hot or cold air throughout the building. A second set of return ducts captures the exhaust air and returns it to the mechanical room for ventilation. Duct sizing can have a significant impact on the effectiveness of a home’s heating and cooling system. Properly sized ducts ensure that enough air reaches each room to maximize comfort for the occupants. Duct sizing also affects noise levels, energy efficiency and operating costs.
Mechanical engineers must consider a number of different factors when determining duct sizing for a structure. First, consider the size of the building and the climatic zone in which it is located. They then measure how much air naturally enters or leaves the house through cracks or openings in the building. By combining these two factors, the engineer determines the size of furnace or air conditioner 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. Then calculate the distance the air must travel to and from each room, as well as the amount of air volume needed in each area. Engineers also need to consider how much air pressure is lost as the air travels through the ducts themselves, either due to distance from the fan or simple duct leaks.
Armed with this information, designers can choose one of two basic systems to determine proper duct sizing. People in 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 is based on achieving friction parity within the duct network. This means sizing the ducts to equalize the pressure lost during supply and return air, creating a balanced heating, cooling and ventilation system.
The second ASHRAE technique used for sizing pipelines involves measuring static recovery. This system is more complex, but also results in better energy efficiency when used correctly. To size static recovery ducting, engineers use ductwork that creates the same static pressure at each supply and return outlet in the home. This includes all exhaust vents and supply grills and is based on a series of graphs developed by ASHRAE which are used in conjunction with data taken from the house.
If duct sizing is done incorrectly, homeowners can make simple duct modifications to improve efficiency and comfort. An air damper installed inside 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 blower or duct blower can be installed inside the duct itself. Duct fans supplement the main furnace or air conditioning fan and help increase airflow to distant parts of the home.
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