Airflow
Airflow, CFM, and Static Pressure Quick Check
Use this when you have hot and cold rooms, noisy returns, frozen coils, or a unit that feels starved for air.
Rules of thumb that hold up in the field
- Measure total external static pressure across the equipment, not random cabinet points.
- Check filter, blower wheel, evap coil, return grille coverage, and dampers before touching refrigerant.
- Compare measured airflow to blower table or ECM target, then decide whether restriction or setup is the real miss.
Quick reference
| Item | Field target | If it misses |
|---|---|---|
| Cooling airflow | ~400 CFM/ton | Low airflow raises coil split and can ice the coil |
| Total external static | Stay within manufacturer max | High static points to restriction, undersized duct, or wrong fan setup |
| Return vs supply pressure split | Balanced enough to explain the total | One side dominating helps you find whether the choke point is supply or return |
Manufacturer fan tables win over generic rules every time.
Fast diagnosis pattern
- High return drop with smaller supply drop usually means filter, return drop, or return grille restriction.
- High supply drop with normal return drop usually means coil, supply trunk, balancing dampers, or closed registers.
- Normal static with comfort complaints pushes you toward zoning, balancing, insulation, or room-by-room delivery issues.