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
One of 68 working definitions in the MEPF & solar engineering glossary. The buying side of the same decisions is in the buyer's guides; the numbers are in the calculators.
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)