Alex from Wrightsoft is back to discuss duct design. He also explains a common mistake made when designing ducts using a Ductulator.

The Ductulator is a common entry point for technicians who get into duct design, so it sticks with technicians despite its flaws. It makes more sense to reverse the process by picking a friction rate and pressure drop as the first step when designing ducts. Figuring out how to overcome restriction is the key to commercial duct design, but it can also work well in residential duct design.

However, we can't pull our desired friction rate out of thin air. We have to consider the sources that contribute to the friction rate. We must also consider both velocity pressure (moving forward) and static pressure (pushing against the duct walls). The less restricted the air is, the more energy there will be to go forward; low static generally indicates greater velocity (more airflow). The best designers understand these principles, so many of the best designers come from the HVAC service industry.

You can adjust the airflow by balancing duct sizing and restriction, such as from filters. Equipment sizing and Manual S are surely important, but airflow and velocity have a lot more to do with duct design and how fan speed, duct size, restrictions, and air mixing work together to establish comfort. Poor duct design can produce results that resemble those of oversized equipment.

Alex and Bryan also discuss:

• Figuring out desired friction rate and static pressure
• Changing tonnage and its effects on duct design
• Load calculations
• ACCA Manual T
• Emergency/backup heat
• Manufacturer coils and pressure drop
• Rules of thumb
• Principles at work in HVAC service

 

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