Airflow Fundamentals
CFM, static pressure and why airflow problems are behind a large share of service calls.
What it is
Airflow is the volume of air a system moves, measured in cubic feet per minute. Static pressure is the resistance the duct system puts up against that airflow, measured in inches of water column. Together they describe whether the blower can do its job.
Total external static pressure
The sum of the pressure drop the blower must overcome outside the equipment cabinet, measured as supply static plus return static.
Why it matters
Refrigerant and combustion both depend on air carrying heat away. Too little airflow causes frozen coils, high limit trips, cracked heat exchangers over time, comfort complaints and premature compressor failure. Too much airflow causes poor dehumidification and cold drafts.
- Cooling commonly targets roughly 350 to 400 CFM per ton, confirmed against the manufacturer data.
- Furnaces target a temperature rise inside the range printed on the rating plate.
- Most residential equipment is designed around 0.5 inches water column total external static.
How it works
A blower produces a fixed pressure and flow relationship described by its fan curve. A PSC motor loses airflow as static rises. A constant torque motor holds up better. An ECM constant airflow motor tries to hold the commanded CFM and simply draws more watts until it runs out of capability.
- 1Drill or use existing test ports upstream and downstream of the equipment, outside the cabinet.
- 2Measure supply static and return static with a manometer.
- 3Add the absolute values to get total external static pressure.
- 4Compare to the rating plate maximum, then use the blower table to find actual CFM.
What can go wrong
| Cause | Where it shows up | Typical symptom |
|---|---|---|
| Plugged filter | Return static high | Low airflow, iced coil in cooling |
| Undersized return duct | Return static high | Noisy returns, low CFM even with a clean filter |
| Dirty evaporator coil | Supply static high | Low airflow with a clean filter |
| Closed or blocked registers | Supply static high | Uneven room temperatures |
| Crushed flex duct | Supply static high | One branch starved |
Tools involved
- Manometer with static pressure tips
- Anemometer or flow hood
- Thermometers for temperature rise or split
- Manufacturer blower performance table
Symptoms
Practice & Mastery
- Questions1
- Airflow Fundamentals Troubleshooting Scenarios2
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