Duct Static Pressure
Duct static pressure is the total resistance — friction of ductwork plus losses through filters, coils, dampers and grilles — that the AHU fan must overcome to deliver design airflow; fan selection is airflow at this pressure.
Why it matters on an MEPF project
Every metre of duct, every elbow, every filter stage spends pressure; the fan is bought to pay that bill at design airflow. Under-estimate static and the far diffusers starve; over-estimate and the fan burns excess energy for the building's life. Since fan power scales steeply with pressure, tidy duct design (gentle fittings, sane velocities) is a permanent energy dividend — the intent behind ECBC-era fan-power discipline.
In service, static pressure is the diagnostic: rising pressure across filters means loading; falling delivered airflow with normal static means leakage or open bypass. A pair of gauge points across the AHU turns "AC is weak" complaints into physics.
How it's specified in practice
| Parameter | Typical / working position | Governing reference |
|---|---|---|
| Duct friction | Rises with velocity² — sizing velocity is the design lever | duct design practice (IS 655 ductwork) |
| Filters | Clean vs dirty pressure drop band; change-out at marked ΔP | filter datasheets |
| Coils/dampers/grilles | Fixed component losses summed into total static | component data |
| Fan selection | Design CMH at total static, on the stable part of the curve | fan curves |
Common mistakes
- Selecting the fan for airflow alone, at "typical" static nobody computed.
- Crushed flexible-duct ends adding invisible resistance at every terminal.
- No test points — the system can never be diagnosed, only argued about.
- Answering starving far zones by speeding the fan instead of fixing the network.
Related on this site
Frequently asked
Why do rooms near the AHU freeze while far rooms are warm?
The classic unbalanced network: near branches steal air at low resistance. Balancing dampers set during commissioning — and left set — distribute the pressure budget so each outlet gets design flow.
What eats static pressure fastest?
Dirty filters and high-velocity undersized duct. One is maintenance, the other is design — both show up as weak airflow and higher fan energy.
Standards referenced: IS 655 (ductwork) · ECBC 2017 (fan power intent)