Inadequate cooling and heating problems are commonly blamed on the equipment itself when the actual problem is poor/restrictive ductwork. Proper ductwork design is critical for proper heating and cooling performance. Some tips for residential ductwork friction loss and pressure drop.

As our HVAC equipment becomes more sophisticated deficiencies in the ductwork system design become more critical. The new higher efficiency ECM motors work to try and provide the design CFM regardless of the ductwork’s design. Unlike the PCS motors that ran at a constant RPM, the newer ECM motors run at constant torque.

Constant torque allows these motors to maintain the torque delivered to the motor when external static pressure (ESP) is higher than recommended and/or changes during system operation. ESP (the resistance to the movement of air) is increased when ductwork is undersized, poorly constructed and/or full or dirt or debris

So… How many X-13 motors have you replaced in the past 3 or 4 years? Could it be that the ESP of the older ductwork is too high because of poor design and that these motors are running ‘peddle to the metal’ to try and compensate?

The ductwork must be designed so that the airflow resistance matches the air handler’s design and residential ductwork friction loss and pressure drop.

For example:

If a blower motor is designed to deliver 1,200 CFM at 0.20” of water column (IWC), then the ductwork must be designed and sized to produce 0.20 IWC with 1200 CFM of airflow

Manual D Residential Duct Systems is the Bible for ductwork.

Pressure Drop –

A pressure loss between any two points in a duct system, measured in IWC.

Friction Rate –

A pressure drop between two points in a duct system that are separated by a specific distance.


FR = (PD x 100) / TEL
FR – Friction Rate in IWC
PD – Pressure Drop
TEL – Total Effective Length

In our next post, we begin to apply this to a duct system and how it relates to our duct slide rule or ductulator.

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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.