Duct Static Pressure Calculations for HVAC Engineers

Accurate pressure drop calculations enable proper fan selection and energy-efficient duct sizing. Underprediction causes inadequate airflow; overprediction wastes fan energy and first cost.

Total Pressure Components

$$P_{total} = P_{static} + P_{velocity}$$

Velocity pressure:

$$P_v = \left(\frac{v}{4005}\right)^2 \text{ “w.g.}$$

Fan static pressure:

$$FSP = P_{s,discharge} - P_{s,suction}$$

Friction Pressure Loss

Darcy-Weisbach equation:

$$\Delta P_f = f \frac{L}{D} \frac{v^2}{(4005)^2 \times 12.96}$$

Where:

$f$ = friction factor (from Moody chart or Colebrook equation)
$L$ = duct length (ft)
$D$ = hydraulic diameter (ft)
$v$ = velocity (FPM)

For rectangular ducts, hydraulic diameter:

$$D_h = \frac{4ab}{2(a+b)} = \frac{2ab}{a+b}$$

Alternative: Use duct friction chart (equal friction method)

Select friction rate: 0.08-0.15 “w.g./100 ft
Find duct size for given CFM at selected friction rate

Fitting Losses

Local loss coefficient method:

$$\Delta P_{fitting} = C \times P_v$$

Where $C$ = loss coefficient from ASHRAE Duct Fitting Database

Common fittings:

Fitting	Loss Coefficient C
90° elbow, R/D = 1.5	0.15-0.25
90° elbow, mitered	0.8-1.3
Branch tee, straight	0.10-0.15
Branch tee, 90° branch	0.9-1.8
Damper, fully open	0.2-0.5
Filter (clean MERV 8)	0.3-0.5

Total system pressure drop:

$$\Delta P_{total} = \Delta P_{straight} + \sum \Delta P_{fittings} + \Delta P_{equipment}$$

System Effect Factors

Common causes of system effect:

Inlet/outlet obstructions near fan
Elbows within 3 duct diameters of fan
Inadequate fan inlet duct length
Non-uniform velocity profile at fan inlet

Impact: Can increase required fan pressure by 0.25-1.0 “w.g.

Prevention:

Straight duct runs: 2.5D upstream, 5D downstream of fan
Turning vanes in elbows near fan
Inlet/outlet bell mouths

Design Pressure Drops

Target static pressure:

Low-velocity systems: 0.5-1.5 “w.g.
Medium-velocity systems: 1.5-3.0 “w.g.
High-velocity systems: 3.0-6.0 “w.g.

Component breakdown:

Straight duct (40-50%)
Fittings (25-35%)
Coil/filter (15-25%)
Terminals/diffusers (5-15%)

Practical Applications

Equal friction design: Select 0.10 “w.g./100 ft, size all ducts at this rate
Available static method: Work backwards from fan capacity
System curve: Plot $\Delta P$ vs. CFM to find operating point

Related Technical Guides:

References:

ASHRAE Handbook of Fundamentals, Chapter 21: Duct Design
ASHRAE Duct Fitting Database
SMACNA HVAC Systems Duct Design, 4th Edition