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Ductwork Sizing & Design for Central Air and Heating Systems

By Edited Nov 13, 2013 0 0

If you think that you’re not getting enough out of your centralized air conditioning or heating system, then you should consider checking out the ductwork sizing of your home.

Ductwork sizing can help ensure efficiency of your home’s heating or cooling system. Ducts that are of proper size make sure that there is enough air that makes it to each room, maximizing the occupants’ comfort. Ductwork sizing also aids in improving energy efficiency, noise regulation and operating expenses.

What is ductwork sizing?

Before ductwork sizing is properly explained, one has to know about ducts. Ducts are passages that are especially built for gases to flow through. Their main function is to guide the gas from one point to another through forced or natural flow. Forced flow refers to air flow that is supervised and controlled by mechanical processes, while natural flow means that the gas in the ducts are allowed to flow freely.

Air conditioning systems are built with ducts that come with fans and blowers. These fans and blowers create forced pressure difference and provide air movement in the system, allowing the air to go through the ducts and circulate around the network.

In any structure or establishment containing centralized temperature regulators such as central furnaces or air conditioners, you will find systems of supply ducts that transport hot or cool air throughout the whole building. For ventilation, however, a second network of return ducts captures exhaust air and brings it back to the mechanical room.

Ductwork sizing is, in simplest terms, calculating the size of the ducts in the network throughout the building and anticipating how much air movement can go through these ducts. Ductwork sizing, in other words, is the correlation between the size of the ducts and the rate of air movement within a given space of time.

How does ductwork sizing work? And how is it determined?

Mechanical engineers are specifically trained to be adept in ductwork sizing. When determining the ductwork sizing, they must assess quite a variety of differing factors. One of which is the structure’s size, and the climate it stands it. These engineers then measure and calculate how much air goes in and out of the building naturally through holes or any other openings ni the structure. Knowing both these factors, the engineers then decide what the size of the centralized furnace or air conditioner is needed to keep the building’s inhabitants comfortable.

The initial air pressure that goes in the duct network can be determined by the engineer by using the size of the central unit, and then predicts how far the air must travel to and from all the rooms by calculating the distance, along with the air volume that is required in each section. Another thing that is considered in ductwork sizing is the air pressure that’s lost while the air goes through the ducts themselves, both because of natural air pressure decreasing from distance, or trivial leaks and damages in the ducts.

After all the calculations, designers can then choose between two basic systems to determine proper ductwork sizing. The systems that are popular to people all over the world are created by the American Society of Heating, refrigeration and Air Conditioning Engineers for ductwork sizing. The most well-known system from ASHRAE relies on equilibrium of friction in the network of ducts. In this system, ducts are sized to equalize pressure that’s lost during air supply and return, creating a well-balanced network of heating, cooling and ventilation.

Static regain measurement is the main concept that’s involved in this second ASHRAE technique used for ductwork sizing. This system is much more complicated than the first, but when done correctly, results in improved energy efficiency. For ductwork sizing with static regain, engineers make use of ducts that create equal static pressure at every supply and return outlet in the home, including all exhaust vents and supply grills. This is also based on several charts that are developed by ASHRAE that are used in connection with data taken from the building.

Benefits of ductwork sizing

If you want to add a centralized heating or cooling unit in your home, get an engineer to help you with ductwork sizing, as ducts that are sized just right are important for energy-efficient duct networks. When sized too small, ductwork will create insufficient air flow and air velocity that’s too high, generating noise. Proper-sized ductwork, though, ensures efficient cooling or heating systems that deliver proper air flow.


You might think that ductwork sizing is a lot of work, but all the effort is worth it. Ductwork sizing can do you a long-term favor in terms of heating and cooling.



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