There are a couple key pieces of information needed to design a duct system, the first being the External Static Pressure (ESP) you will design around. I assume by now, you are aware that your available static pressure is the ESP minus your Device Pressure Losses (DPL). The key is to know: What External Static Pressure do you start with?

For Permanent Split-Capacitor (PSC) motors, the typical maximum static pressure is .50” w.c. Ideally, you would not like to design around the maximum for anything. If your install crew does not install the duct system exactly how you laid it out, there is no way for you to turn up the volume of air and increase the static to overcome the restrictions. But, if you designed around a medium speed and static, you will be able to overcome these slight deficiencies and restrictions.

For Electronically Commutated Motors (ECM), the variable speed either constant torque or volume models are able to deliver the needed CFM at a much higher static, efficiently. These systems can range in static pressure from .30” - .80”, all operating at similar CFM. So, what External Static Pressure do you choose for design, and set up after installation?

This is actually simpler than you think. The answer is really dependent on what you will have for Device Pressure Losses, or not have. In other words, what will you be adding to the system that is not shipped in the box from the unit with the blower? For Air Handlers with a packaged coil, or Furnaces without an A/C Add-on, a lower External Static Pressure should be used to avoid excessive air velocity and a noisy system.

Here is a simple guide that should help you identify a great starting point for your duct design, based on my experience and years of duct diagnostics.

When designed correctly, and installed as planned, take a guess what the measured external static pressure should be?