Fan efficiency grade and the Fan Energy Index answer different questions, which is why one replaced the other.
FEG asks how good the fan is at its best point. FEI asks how good your selection is at the point you will actually run it.
Fan efficiency grade rates peak total efficiency of the bare fan and ignores the motor and drive. FEI compares actual electrical input power at your duty point against a reference fan, so it includes the whole assembly and penalises oversizing. FEI of 1.00 matches the baseline, and 1.25 means 20 percent less power.
How the old fan efficiency grade worked
A fan efficiency grade gave a number based on peak total efficiency, in steps such as FEG 67, 71 and 75.
A fan met the requirement if its peak efficiency reached the threshold, and codes then added a rule that the selection must sit within a percentage of that peak.
That second rule was the weak point, because it was frequently ignored. So an efficient fan could be installed at a terrible duty point and still count as compliant.
How FEI fixes it
FEI drops the fan efficiency grade idea entirely and compares power at your duty point instead.
| FEG | FEI | |
|---|---|---|
| What is rated | The fan alone | Fan, motor and drive together |
| Where it is measured | At peak efficiency | At your duty point |
| Penalises oversizing | Only indirectly | Directly and immediately |
| Compares fan types | Poorly | Fairly |
| Typical threshold | FEG 67 or better | FEI ≥ 1.00 |
| Expresses | A grade | A ratio |
Because the whole electrical assembly counts, an EC fan gains ground that a bare impeller efficiency figure would never show.
Reading an FEI value
- Below 1.00 fails most current requirements.
- 1.00 to 1.10 is compliant but unremarkable.
- 1.10 to 1.30 indicates a sound, well-sized selection.
- 1.30 to 1.60 is strong, usually EC with a backward-curved wheel.
- Above 1.60 is achievable on large clean-air duties.
- Finally, the value changes with duty point, so always state the point.
That last item catches people out. The same fan can score 1.35 at one duty and 0.95 at another.
Why oversizing hurts so much
This is the behaviour FEI was designed to expose.
A fan chosen two sizes too large runs far left on its curve, where efficiency is poor and the reference fan would do the job with much less power. So the ratio falls even though the fan itself is well made.
Under FEI, correct sizing is worth more than premium hardware. A modest fan used well beats an excellent fan used badly, every time.
Improving your score
- Size the fan so the duty point sits near peak efficiency.
- Reduce system resistance, since every pascal removed raises the score.
- Choose a backward-curved wheel where the envelope allows it.
- Use an EC motor, because drive losses count against you.
- Fix inlet conditions, as system effect costs real watts.
- Finally, control on demand rather than throttling with dampers.
Our fan efficiency guide covers the index itself, and the regulation guide covers what each market requires.
What to ask suppliers
Three questions separate real data from a generic table.
Ask for FEI at your stated duty point rather than a best case. Ask what test standard produced the underlying data. Finally, ask whether the figure includes the motor and drive, since that is the whole basis of the index.
Any manufacturer with proper test data answers all three in minutes. For the formal definition and reference fan values, see AMCA.
One more habit is worth adopting. Put the duty point and the required index straight into the enquiry, rather than asking for a fan and checking compliance afterwards. Suppliers then propose selections that already meet the requirement, which saves a round of revisions.
FEG and FEI FAQ
Is FEG still used?
It appears in older specifications and some regional codes. New work increasingly uses FEI, since it closes the loophole that let a good fan be installed at a bad duty point.
Can two fans have equal efficiency but different FEI?
Yes, easily. If one sits closer to its peak at your duty point, its FEI is higher even though the catalogue efficiency figures match.
Does FEI apply to small fans?
Usually above a power threshold, with exemptions for embedded and safety fans. Check the version of the code applying at the project location.
How do I raise FEI without changing the fan?
Lower the system resistance. Bigger ducts, cleaner filters, radius elbows and better inlet conditions all move the duty point and raise the score.
Fan efficiency grade judged the product. FEI judges the decision. That shift makes correct sizing the cheapest compliance measure available.
