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What is External Static Pressure in HVAC & Duct, How to Calculate it & External Pressure Calculation Sheets.

 What is External Static Pressure?

Perhaps one of the most frequently ignored factors in setting up a duct system is the External Static Pressure (ESP) is the measurement of all the resistance in the duct system that the fan has to work against. Examples are filters, grills, A/C coils and the ductwork. It is the sum of the suction pressure (negative) and discharge pressure (positive) created by the equipment blower.

Blowers move air throughout the system and are designed to overcome restrictions in the system external to the equipment such as system components and ductwork. They are rated for a given CFM at maximum External Static Pressure on high speed.

 “If you know the External Static Pressure (ESP) you can determine the CFM.”

External Static pressure is measured using a manometer and is expressed in inches of water column (i.e., #” WC). The air flow rate or CFM of a motor is directly related to the external static pressure. The higher the ESP, the lower the CFM. The lower the ESP, the higher the CFM. High ESP readings indicate that there is excessive resistance in the system. This may be caused by dirty filters, a dirty evaporator coil, closed dampers, restricted supply or return grills or undersized duct. If you know the ESP you can determine the CFM.

Also download: Ductmate for Duct Sizing and HVAC Duct Design

Importance of External Static Pressure.

  • Proper static pressure will allow your system to deliver heated & cooled air optimally, which translates into lower utility costs, less failure of expensive parts and greater comfort in your home.
  • High static pressure is the leading cause of compressor failure.
  • High static pressure can reduce your cooling efficiency (SEER) by 50%
  • High static pressure can reduce your capacity by 40%-50%

     What are the steps and how to estimate / calculate the External Static Pressure of a fan?

    The below calculation sheet is very simple and all engineers can make use of it in order to estimate / calculate the External static pressure (ESP) of any fan in any HVAC equipment.

    It is worth mentioning here also that ESP has to be calculated for different scenarios to ensure that the calculated ESP is indicating the maximum ESP drop in the system which will be for the Index run noting that the index run is not mandatory to be the furthest duct run. The index run is the duct run with the maximum external static pressure drop.


     1. Write down the "DESIGN AIR FLOW RATE" required for the fan. In case of heating or air conditioning equipment, the design air flow rate is what you get from the cooling load program. 

     2. Go to the friction loss chart in any HVAC standard (CARRIER, etc.) or use a duct Calculator to get the "FRICTION FACTOR" based on the required air velocity and duct size according to the area of application (low pressure system, Medium pressure system, etc.). 

     3. Measure the duct length from the design drawings and check different scenarios as explained above since the index run is the duct run with the maximum external static pressure drop not the furthest duct run. 

     4. Use the below sheet (very simple excel sheet) to add the duct run, friction factor, pressure drop in all accessories and repeat the same for the return air duct (or Fresh air duct whichever is greater). 

     5. Add whatever required safety factor to secure the unit selection.

So, External Static Pressure drop value can be defined accordingly.

Here are External Static Pressure calculation sheets for HVAC Design engineers designing duct system for chilled air supply/return duct, exhaust duct and fresh air duct system.

These excel worksheet's includes the following conversion and useful sheet which are mentioned below

  • How to find Duct Diameter and Friction Loss? 
  • How to find Duct Diameter and Velocity? 
  • How to find Duct Velocity and Friction Loss? 
  • How to find Air Mixture? 
  • How to find Equivalent Rectangular Duct 
  • How to find Equivalent Round Duct 
  • How to find Dynamics Loss Coefficient?

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