Airflow & Ductwork

CFM per ton, blower speed, static pressure, filter pressure drop and duct sizing — how to get the airflow a system actually needs before you ever touch the charge.

6 guides

• CFM per Ton and Setting Blower Speed
  What CFM per ton means, the normal cooling and heating airflow targets, how to set blower speed on PSC and ECM blowers, and how to verify you actually got the airflow you dialed in.
• Duct Sizing the Practical Way (Friction Rate Method)
  The friction-rate (Manual D) method explained in plain field terms — how to find your available static pressure, turn it into a friction rate using total equivalent length, and pick duct sizes that actually hit the design CFM at the static the blower can deliver, plus the undersizing mistakes that wreck systems.
• Filter Pressure Drop and MERV Tradeoffs
  How filter MERV rating relates to pressure drop and airflow, why a high-MERV filter in too small a slot chokes the system, and how to get good filtration without starving the blower by using more filter face area.
• Measuring Total External Static Pressure
  How to measure total external static pressure (TESP) on a residential system with a manometer, where to drill the test ports, what readings are normal vs too high, and what high static is telling you about the duct system.
• Return-Air Undersizing Problems
  How undersized return air starves the blower, the symptoms it causes (high return static, low CFM, comfort and freezing problems), how to recognize it, and practical ways to add return capacity.
• The Fan Laws in Plain Terms
  The three fan laws explained for techs — how CFM, static pressure, and motor horsepower each change when you change blower speed, why small RPM increases cost big power, and how to apply them in the field.

← All HVAC topics