One way to size and design HVAC duct systems is to use the velocity method, which means that the duct size and design is determined based on the airflow speed of the system being used. This method takes into account the size of the home and then selects the size and number of ducts based on several factors. These factors include the air speed itself (velocity), along with the airflow rate (movement of air within a cubic meter of space) and the cross-sectional area of the ductwork.
The path of the airflow through the ventilation system is another important consideration when deciding where to run the ducts and how many vents you should have. Generally speaking, the more twists and turns in the ductwork, the more resistance this provides to the air. With increased resistance, there is need for a more powerful air-conditioning system that can push the air through the ductwork out into the rest of the home.
Duct size is more specifically dependent upon the supply air requirement for each room of the home. This is based on a calculation that determines the amount of heat loss or heat gain for each. This information is then used in conjunction with the static pressure of the blower. These figures are used to determine the proper length for the duct runs, fittings and devices.
The duct size and design of an HVAC system cannot be arbitrarily imposed upon a home or building based just on calculations. Practical considerations have to be made. One of the primary considerations is the available space for the ducts in the attic and basement space of the home. Adequate space has to be available so that ducts are not crammed into spaces where they don't really fit. This can result in damage to the ducts or even risk of contact with electrical or plumbing fixtures. The air-distribution system should also have adequate space to avoid such cramming effects.