Free Home HVAC Ductwork Calculations

Calculating Yourself

• Three methods are used to calculate duct size: velocity, equal friction and static pressure. Velocity is based on airflow from the unit to the outlets. Equal friction is based on maintaining equal pressure in ducts, considering loss of flow due to friction. Static pressure seeks to maintain the same pressure in all areas but is very complicated and is rarely used. All three methods are based on mathematical equations, which a homeowner can use himself. Several websites offer these equations.

Basic Measurements

• Any calculation begins with basic measurements, starting with the output or airflow from the unit in cubic feet per minute. The next figure is the distance all the ducts cover, starting with a main supply duct originating at the unit and including all branches to vents or outlets. A return system also must be figured, because "used" air has to be returned to the unit in the same volume as it was put out. The length of the ducts affects air flow and thus size.

Duct Types

• The type of duct also must be established, since it affects the size needed. Air flows best through metal ducts and most efficiently through round ducts, but round flexible duct is easiest to install and is most common. A round metal duct has the best air flow but is hard to route around obstacles. Rectangular ductboard generally has the poorest air flow, but sometimes must be used in wall cavities.

Calculating

• Once you obtain the basic measurements, it is time for the calculation. Many heating/cooling vendors offer calculation services, but usually connected to purchase of equipment. A number of firms and websites offer equations and explanations that a homeowner can use. The Engineering Toolbox has a variety of programs to calculate duct size for any type of installation. A website called Perfect Home provides Air Conditioner Contractors Association manuals for use in all duct design and size calculations.

Ductulator

• One common home calculation system is called the Ductulator. It is a pocket calculator designed specifically for sizing ductwork by either the velocity or equal friction methods. You can buy the Ductulator from the manufacturer, but some heating/cooling suppliers will provide one.