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What Are the Different Methods for HVAC Duct Sizing?

A vent cover leading to ductwork.
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  • Written By: S. Lilley
  • Edited By: Angela B.
  • Last Modified Date: 23 August 2014
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Heating, ventilating and air conditioning (HVAC) ducts are large tubes or pipes that run throughout the infrastructure of a home or building. These ducts are connected to a main heating or cooling unit and carry air throughout the building. To maintain proper air flow, which allows all areas of a building to be heated or cooled equally, the size of these ducts must be determined in one of a few very specific ways. These ways include the Manual J method, a method that involves knowing a room's desired temperature, and a method that involves using a ductulator to perform calculations.

The existing layout of a home or building plays a big part in proper HVAC duct sizing. The size of each room that will need a duct must be taken into consideration, along with the way the ducts need to be installed in walls. It is always recommended to install ducts that are as short as possible, but building designs don't always allow for this to happen. Figuring out duct sizes — the unit's physical capacity — involves taking into account just how long the duct has to be given design restrictions.

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The first method of HVAC duct sizing is what is called a Manual J. This method, which is usually performed by a professional contractor, involves using a computer program and the technical specifications of the heating or cooling unit in question to determine the proper HVAC duct sizing requirements for a building. This software is designed to be as technically accurate as possible and takes into consideration things such as existing airflow, the load size of the heating or cooling unit, and the overall design temperature of the place in question. The Manual J method was developed by the Air Conditioner Contractors of America (ACCA).

The second method of HVAC duct sizing is more likely to be used by homeowners trying to handle the sizing themselves. It involves finding information such as the ideal temperature in each room, the cubic feet per minute of each room and the average British thermal units (Btus) of the HVAC system to determine the proper HVAC duct size. The design of the ducts also plays a part in determining what size they should be.

A third method for HVAC duct sizing involves using a device called a ductulator. This is a device that allows the user to input certain calculations about the areas where HVAC ducts will be installed, such as air volume and air velocity, and uses that information to output proper supply and return duct sizes. Instructions on how to perform these calculations on a room are often included with the ductulator device.

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