HVAC Air Handling Size

Heat/Loss Gain

• The Air Conditioning Contractors Association provides manuals to assist in calculations. Manual J provides formulas to calculate the heat loss/gain ratio for each space in the house. Distance from the unit will affect this ratio; air flow changes with friction of air flowing through ducts. This will determine the air flow needed for each room, the number and location of supply vents and the size of return vents to carry air back to the unit.

Unit Size

• Once air flow requirements are established for each room, ACCA Manual S will supply the requirements for heating and cooling units to meet those calculations. This will include not only the thermal output of the unit, measured in British Thermal Units for heating and tons for cooling, but also the size of blower needed to force the air through ducts.

Duct System

• The next step is designing a duct system to distribute the air. Duct work should be based on a trunk and branch scheme, with a large central duct feeding air from the unit to smaller branch ducts to each vent, located on outside walls, usually near a window. A return system will collect "used" air and return it to the unit, typically through large openings in a wall in a hallway or other central location, with a filter to clean dust and debris before the air re-enters the unit.

Air Flow Influences

• Air flow is measured in cubic feet per minute, starting with the output from the unit. The type of duct will affect air flow calculations. Air flows more efficiently through a round duct than through a square or rectangular one. It moves more smoothly through metal than through fiberglass or other material. The most efficient air flow is through a round metal duct, but the most efficient duct to install is a fiberglass flexible duct.

Calculation Methods

• Three methods can be used to determine the size you need: velocity, based on the velocity of flow through duct work; equal friction, which tries to maintain constant pressure against the loss from friction of air flowing through ducts; and static pressure, a complicated method to try to maintain the same pressure at every outlet. These equations are so complex the best way to use them is with a device such as the Ductulator or a program from The Engineering Toolbox or Elite Software, which take the basic figures and calculate air flow and duct size requirements.