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Study Guide: Airflow Diagnostics: Static Pressure, CFM, and Restriction Clues
Source: https://www.fatskills.com/nate/chapter/airflow-diagnostics-static-pressure-cfm-and-restriction-clues

Airflow Diagnostics: Static Pressure, CFM, and Restriction Clues

By Fatskills Exam Guides Team — the exam nerds behind 28,500+ quizzes and 2.1M practice questions across 500+ global exams.

⏱️ ~2 min read

The short field answer:
Static pressure is system resistance. CFM is delivered airflow. If static goes up, delivered airflow often drops unless the blower can compensate. Dirty filters, crushed ducts, closed dampers, plugged coils, and undersized returns all show up as airflow problems before they show up as comfort complaints. (Trane)

Clue What it usually means
High total external static pressure Restriction in filter, coil, supply, return, or dampers
Weak delivered air at registers Low CFM, duct loss, blower issue, or high static
ECM blower ramps hard then faults/cuts back Restriction is severe enough that the motor hits protection limits
Furnace temperature rise high / limit trips Airflow is likely too low
Cooling coil icing with weak airflow Evaporator load/airflow problem often comes before charge blame

Trane’s fan literature ties airflow directly to total static pressure and fan curves. Lennox documents a real-world fault where the blower cannot provide requested CFM because static is too high and specifically tells the tech to check the filter and duct system. Goodman’s furnace literature also reminds techs that around 400 CFM per ton is a common cooling target and that temperature rise must stay within the rating-plate range. (Trane)

The practical rule:
Do not make final charging decisions on a system with bad airflow. Airflow problems distort the very readings you planned to use. In real troubleshooting, static pressure is often the thing that saves you from replacing good parts.


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