Before we can apply the formulas in Part-1 of our multi-part series, it is important to understand a few more concepts… blower performance, duct performance, and system operating point.

Duct Pressures

The air pressures in residential ducts are very small. Usually less than 0.025 psi (either positive or negative). Because the pressures are so low, it is not practical to measure them in psi, it much easier to measure using inches of water column (IWC). 27.7 inches of water column equals 1.0 psi.

The Performance of Blowers

The blower is in HVAC systems move the air through the duct systems. The amount of air (Cfm) delivered by the blower depends upon the resistance of the ductwork. This external resistance is known as External Static Pressure (ESP). The ESP of a duct system has a direct relation to the amount of air delivered by the blower. Most manufacturers summarize blower performance by using a graph or performance table. NOTE – as the resistance increases, airflow rate decreases.

Blower Curve

System Duct and Blower Performance Work Together

As air is moved through the ductwork, resistance is created by the friction within the duct systems. As more air is moved through the ducts, the resistance increases.

When a blower and a duct work system are operating together, there is only one possible operating point, know as the System Operating Point. The system operating point is where the fan curve and the duct curve meet.

System Operating Point

In this example, the system delivers 1000 Cfm to the home. If this does not meet the needs of the home then the blower must be adjusted or the duct system must be modified. Slight adjustments can by made by adjusting the blower speed. Duct performance is based on duct materials, layout, geometry, and fittings. The only way to change duct performance is by altering duct materials, layout, geometry, or fittings.

The best way to prevent airflow problems is by designing duct work and airflow systems properly.

So, What is Our Objective?

Our objective is to design a system that will work with the blower that is supplied by the equipment manufacturer. In other words, the ESP or airflow resistance of the duct system must match the external static pressure produced by the blower when it is delivering the desired Cfm to the home. This is where understanding friction rate, duct slide rule, and pressure drop is essential.

As we progress through this series, you will begin to understand the relationship between pressure drop and friction rate. You will also know how to properly use your duct slide rule. Understanding these concepts is essential to becoming a better technician.

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Ron Walker
Ron Walker

After retiring from the U.S. Marines and achieving his B.S. degree, Ron Walker entered the HVAC field. He has been an HVAC technician, service manager, and business owner. Working as a service manager, he spent many years training HVAC technicians to be more technically competent and really understand their trade. His passion for teaching and helping others resulted in the creation of HVAC Training Solutions, LLC.