Sizing the ducts for your air conditioning system is a complicated process that uses one of three different methodologies. The primary goal for duct sizing is that they will provide the ideal space for cooled and heated air to travel around your property. The ducts need to be large enough to provide an ample airflow without overworking your system or costing you a fortune in energy bills.
Sizing Methods:
There are three different methods that are used for most applications of modern duct sizing. The Velocity Method uses the speed of airflow to determine the optimal duct size. This method uses a series of equations to calculate this figure including air speed, air flow rate and the cross sectional area of the duct. This process involves a large amount of maths, which is likely to be beyond most homeowners.
Generally, the velocity method is used by professional contractors to assess the appropriate size and layout of the ductwork based on whether it is a commercial, industrial or residential application and whether main or branch ducts are needed.
The other methods typically used by professional contractors are the Constant Pressure Loss & Static Pressure Recovery. The Constant Pressure Loss calculations tend to need more components, but it can provide a more accurate assessment of the system pressure loss based on the ductwork layout and the materials used. The Static Pressure Recovery method is more focused on ensuring the same levels of pressure are achieved at all the system vents and inlets. This is considered to be the most complicated method of sizing, and it is rarely used in residential installations.
Simplifying for Residential Applications:
These methods tend to involve a great deal of maths equations, and this level of precision is usually a little excessive for a typical residential application. They are more appropriate for commercial and industrial properties, where the ductwork needs to keep multiple floors and a large layout comfortable. In residential applications, contractors tend to simplify this process a little to determine optimal duct size. The contractor will consider the CFM or cubic feet per meter for each room. This requires that the entire house is measured and the load calculations of room load/house load x the equipment CFM are completed. The contractor will also need to consider the Friction Loss Rate, which is based on the Available Static Pressure times 100 divided by the Effective Length.
Finally, your contractor will use software or a chart to determine the size of the ductwork needed based on these two factors. The overall goal of these calculations is to ensure that your ductwork is large enough to distribute the cooled or heated air needed throughout the entire house. If the measurements or calculations are incorrect, it could result in improper air delivery and a system that struggles to keep you comfortable.
If you are concerned about duct sizing, it is important that you speak to a professional HVAC specialist. Although there are many box solutions available, you may regret saving a few dollars if your ductwork is too large or small to effectively get the job done.