Manual D - Maximum External Static Pressure 

As a retired HVAC distributor training manager I taught manual J and manual D for years. I thank you for your excellent videos that are so important for techs to understand! Sadly understanding air flow is still the “ step child” of our industry. Keep up the good and important work.

Help! Im an HVAC tech in the field trying to become better. I don't understand airflow very well. But from what I have gleaned is this: 1. With most systems, try not to add (deduct) more than .5inwc of static pressure to the system rating. 2. Ductwork size is designed with velocity as its priority -- i.e V=Q/A 3. PSC motors have trouble overcoming more than .5 of additional added Static pressure? -- What is Friction rate? How does it relate to the static pressure? How should I design ductwork in a retrofit situation. Is it possible to predict the static pressure of a system before its installed?