AirSelect3D Blog
Specific Fan Power (SFP) Explained — EN 16798, SFP_int and Why Your AHU Quote Depends on It
Specific fan power is the number that decides whether your air handling unit is efficient, compliant and quotable — yet it is routinely computed too late in the selection process. Here is the working engineer's version.
The definition
SFP is the electrical power absorbed by the fans divided by the airflow they move:
SFP = P_electrical / q_v — expressed in W/(m³/s)
For a complete AHU, SFP_int (internal SFP) counts only the pressure the fans spend inside the unit — filters, coils, heat exchanger, casing — excluding the external ductwork the building designer owns. This is the figure ErP regulates and EN 16798 classifies.
What drives SFP up
Every component you add spends pascals, and pascals cost watts:
- Filters — a fouled ePM1 filter stage can add 150–250 Pa. Two-stage filtration doubles it.
- Heat recovery — plate exchangers typically cost 100–250 Pa per side; rotary wheels less, at the price of leakage classes.
- Coils — each row adds air-side dP; a 4-row heating coil at 2 m/s face velocity sits around 30–40 Pa.
- Face velocity — dP grows roughly with the square of velocity. A casing sized at 1.6 m/s instead of 2.4 m/s can halve the component pressure drops.
- Fan efficiency — the same duty on a fan running at 45 % total efficiency instead of 60 % costs a third more SFP.
Typical classes
EN 16798-3 groups SFP into classes (values for the internal share vary by configuration; the pattern is what matters):
| SFP class | W/(m³/s) | Reads as |
|---|---|---|
| SFP 1 | ≤ 500 | Excellent — generous cross-sections, efficient fans |
| SFP 2 | 500–750 | Good, competitive |
| SFP 3 | 750–1250 | Average, check ErP margin |
| SFP 4+ | > 1250 | Hard to justify in 2026 |
A double-deck plate-HRS unit at 5 000 m³/h with sensible face velocities and EC plug fans lands around 300–400 W/(m³/s) — comfortably SFP 1. The same duty forced into a casing one size smaller can double that.
The design loop that keeps you compliant
- Size the casing from face velocity, not catalogue habit. Target ~1.8–2.0 m/s across coils; the sweet spot balances footprint against dP.
- Pick components with live dP feedback. Each pick should immediately update the unit total — supply and extract sides separately.
- Watch SFP_int continuously. If the readout only appears in the final report, you will iterate by reprinting PDFs.
- Check both seasonal Eurovent ratios — fan power interacts with heat-recovery bypass in summer. See our Eurovent ECP-05-2026 guide.
Where the tooling matters
In AirSelect3D the SFP_int readout sits on the results rail next to the 3D model and recalculates on every drop, resize or product swap — sourced from the actual manufacturer fan engines (Ziehl-Abegg, eBM Papst), not curve fits. A configuration that drifts over the ErP threshold flags itself in red before the selection is ever saved, and the EN 16798 derivation prints on the dossier with full engine-version traceability.
Design your next AHU in 3D — in five minutes.
AirSelect3D runs certified manufacturer engines (Camfil, Ziehl-Abegg, eBM Papst, Friterm, Hoval) and ships an ErP-compliant Eurovent dossier with every selection.
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