Airflow | United States
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.
Built for United States field work where techs bounce between split systems, furnaces, heat pumps, package units, and mixed local code adoption.
Ticket note prompts
- Record return, supply, and total external static with the filter condition and blower setup noted.
- Document whether the restriction was at the return, supply, coil, filter, or zoning hardware before recommending duct changes.
- Note any rooms, branches, or dampers that still need balancing after the equipment-side fix.
Comeback prevention
- Verify final airflow against the fan table or ECM target instead of assuming the new pressure is good enough.
- Recheck coil split and blower amperage after the airflow correction so the service record matches the final operating state.
- Flag undersized return paths or closed zone dampers in the customer notes when they are still limiting the system.
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.