System Effect: Why Axial Fans Underperform Once They Are Installed
By the NextAir Systems engineering team. Last reviewed October 2026.
A fan is tested on a rig with long, straight, smooth ducts on both sides. Nobody installs a fan like that. In a factory the fan sits next to a wall, the outlet discharges into a bend, a damper is bolted on the inlet because that was where there was room, and the delivered airflow comes up short of the datasheet. The commissioning engineer blames the fan, the fan supplier blames the duct, and the real cause has a name: system effect.
System effect is well understood, and it is quantifiable. We explain it, show how a modest loss at the fan moves the operating point in a worked example, and list the installation habits that cause most of it.
The short version
- AMCA Publication 201 explains that a fan’s rating comes from laboratory testing, and that its performance in a real system can fall short because of how the system connects to it. The allowances are called system effect factors (SEF), and the designer must include them.[1]
- In our worked example, an unplanned 80 Pa of system effect loss at the design flow cuts the airflow from 14.0 to 13.3 m³/s, a shortfall of 4.7%.
- Fans of different types, and even the same type from different manufacturers, do not necessarily react to the system in the same way.[1]
- DOE guidance says the duct system curve must include all system effect factors before the fan is sized.[2]
What system effect is
A fan’s published curve assumes the air arrives at the inlet evenly and leaves the outlet in a smooth, developed flow. If the air arrives swirling, or lopsided, or the outlet discharges into a bend before the velocity has settled, the fan cannot develop its rated pressure at the rated flow. The shortfall is a loss that is attributed to the system and not the fan, and it is not in the duct pressure drop calculation. That is why it is so often missed.
AMCA Publication 201 sets out the principle. Fans are rated from tests carried out under standard conditions, and the publication reviews the more important reasons performance decreases when the fan is installed in an actual system. It gives allowances, called system effect factors, that the design engineer must take into account to estimate fan and system performance reasonably.[1] It also warns that different types of fan, and the same type from different manufacturers, react differently to the same connections.[1] We do not reproduce its factors here, because they depend on fan type and geometry, and you should read the publication or ask the manufacturer for the value for your arrangement.
How a small loss moves the operating point
To see the size of the effect we use a deliberately simple model. A fan curve falls from a shutoff pressure of 800 Pa to zero at 20 m³/s, following P = 800 × (1 − (Q ÷ 20)²). A duct system is designed so that the fan operates at 14.0 m³/s, where the fan develops 408 Pa and the system resistance is 2.08 Pa per (m³/s)². Now add a system effect loss, which behaves like extra resistance and scales with the square of flow, of 40, 80 or 120 Pa at the design flow.
| System effect loss at design flow (Pa) | Actual airflow (m³/s) | Actual airflow (m³/h) | Shortfall against design | Fan pressure at the new point (Pa) |
|---|---|---|---|---|
| 0 | 14.0 | 50,400 | 0.0% | 408 |
| 40 | 13.7 | 49,185 | 2.4% | 427 |
| 80 | 13.3 | 48,055 | 4.7% | 444 |
| 120 | 13.1 | 46,998 | 6.7% | 459 |
Worked example. With no system effect the fan delivers 14.0 m³/s. An 80 Pa loss, which is only about 20% of the 408 Pa the fan develops, drops delivery to 13.3 m³/s, a shortfall of 4.7%. The designer sees a fan rated for the pressure and the airflow, and the site sees 48,055 m³/h. Fan curves of real axial fans are not simple parabolas, and stall regions complicate the picture. A flatter curve would lose more airflow for the same pressure loss, because a flat curve gives up little pressure as flow rises, so treat the figures as an illustration of the mechanism, and probably a mild one. The mechanism itself is general: every pascal lost at the fan comes out of the airflow.
The shortfall is worse than it looks, because delivered airflow drives everything the fan was installed for. A factory that needed to remove a given heat load, as in the heat removal calculation, will run hotter by the proportion of the shortfall.
The usual causes
- Inlet obstructions. A wall, a beam or a guard too close to the inlet starves the fan and distorts the flow.
- Elbows near the outlet. Air discharging straight into a bend carries an uneven velocity profile into it. The bend loses more and the fan’s performance suffers.
- Dampers and louvres at the fan, especially with blades oriented to throw the flow to one side.
- Sudden transitions. An abrupt expansion or contraction at the outlet wastes the velocity pressure instead of recovering it.
- Inlet swirl. Air entering from a plenum at an angle can set up rotation that either helps or hurts the fan, depending on its direction.
- Parallel fans interfering. Fans too close together affect one another, and the effect is tied to how parallel fans interact.
How to avoid it
- Ask the fan manufacturer for the system effect factor for your arrangement, and add it to the duct pressure drop before sizing.
- Allow straight duct on both sides of the fan where you can, and discharge into a diffuser, not a bend.
- Keep the inlet clear, at least a fraction of the fan diameter from walls and obstructions, as the manufacturer advises.
- Specify the guarantee point and the test conditions, as in fan performance testing, so everyone agrees what was promised.
- Measure the installed airflow at commissioning. A traverse in a straight section of the duct, or a calibrated inlet nozzle, tells you what the fan is actually delivering.
The sizing method, including where the margins come from, is in how to size an axial fan, and reading the curve for stall is in how to read an axial fan curve.
Where this fits with NextAir
NextAir’s axial fans range up to 1,600 mm, 2,400 Pa and 185,000 m³/h, and we state the installation assumptions on the quotation so system effect is addressed at the start. Send your layout drawing to our axial fan manufacturer team and we will flag the arrangements that cost you airflow. Our axial fan guide covers the range, and factory ventilation is the most common application.
Sources
- AMCA / ANSI Webstore. AMCA 201-23 Fans and Systems.
- Plant Engineering (US DOE Motor System BestPractices material). Building energy efficiency into fan systems.
Figures and tables on this page are calculated or compiled by NextAir Systems and you are welcome to reuse them with a link back to this article.
Frequently Asked Questions
What is system effect in fan installations?
A loss of fan performance in an installed system, compared with its laboratory rating, caused by how the ducts, bends and obstructions connect to the fan. It is accounted for with system effect factors.
Where do I find system effect factors?
In AMCA Publication 201, Fans and Systems, and from the fan manufacturer, who can give the value for your fan type and arrangement.
Why does my fan deliver less air than the datasheet says?
The most common causes are system effect from poor inlet or outlet conditions, higher duct resistance than designed, and operation near stall. Measure the airflow and compare it with the system curve.
Does system effect depend on the manufacturer?
Yes. AMCA 201 notes that fans of the same type from different manufacturers do not necessarily react to the system in the same way.
How do I check installed airflow?
Do a duct traverse in a straight section, or use a calibrated inlet nozzle or flow station, and compare the result with the design airflow.
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