Reading Results

What the numbers mean and how to use them.

Running a Calculation

Click Calculate after building your system. Results update instantly. The diagram highlights the critical path in a distinct color.

Per-Segment Results

Each segment shows:

Velocity. Air speed in fpm (or m/s). Useful for checking whether you're within acceptable velocity ranges for noise and erosion.
Velocity pressure (Pv). Kinetic energy of the air, in in. w.g. Used as the reference for fitting loss calculations.
Friction loss. Pressure drop from wall friction over the straight-run length.
Fitting losses. Each fitting's loss listed individually, with its Co value shown.
Segment total. Friction plus all fitting losses for the segment.

Per-Path Results

Each path shows the sum of all segment losses from fan to terminal. Paths are listed from highest to lowest total loss.

Critical Path

The critical path is the path with the highest total pressure loss. It controls fan selection. Your fan must overcome at least this much resistance to deliver the required airflow to every terminal.

The critical path is not always the longest path. A short run packed with elbows and tees can easily outweigh a long straight run. This is exactly why you need to calculate instead of guess.

Fan Static Pressure

The recommended fan total static pressure is:

Fan SP = Critical Path Loss × (1 + Safety Factor %)

For a dual-inlet fan (return and supply duct both attached), the critical supply path loss and the critical return path loss are added together, then the safety factor is applied to the total.

Safety Factor

The safety factor adds a buffer to account for:

Construction tolerances (joints, hangers, actual vs. nominal duct sizes)
Fittings not explicitly modeled
Filters and coils at end-of-life (higher resistance)
General uncertainty in Co values

A safety factor of 10-15% is typical for commercial systems. Residential or simple systems may use less; complex systems with many unmodeled components may warrant more.

A larger safety factor is not always better. Oversizing the fan wastes energy, increases noise, and forces the system to damper down, which adds back the resistance you were trying to avoid. Size the fan to the job.

Balancing

Paths shorter than the critical path will have excess pressure available. In a balanced system, that excess is absorbed by dampers or by intentionally undersizing branches. DuctStatic shows the pressure difference between each path and the critical path, so you know exactly how much balancing is needed at each branch.