AirSelect3D Blog
Specific Fan Power (SFP) Explained — EN 16798, SFP_int and Why Your AHU Quote Depends on It
SFP (specific fan power) is the electrical power absorbed by an AHU's fans divided by the airflow they move, expressed in W/(m³/s). SFP_int — the internal share, spent on filters, coils, heat exchanger and casing — is the figure EN 16798 classifies and EU ErP regulation caps for non-residential ventilation units. A well-sized double-deck plate-HRS unit at 5 000 m³/h with EC plug fans typically lands at 300–400 W/(m³/s), comfortably SFP 1. Yet the number 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.
SFP_int vs SFP_v — which one is on your datasheet?
Datasheets and consultants' schedules quote two SFP flavours, and mixing them up is the most common reason two documents "disagree" about the same unit. SFP_int covers the unit's internal pressure losses only — it is the manufacturer's number, and the one ErP caps. SFP_v (sometimes written SFP_total or system SFP) adds the external static pressure of the ductwork, so it describes the installed system, not the box — it is the building designer's number, always higher, and the one EN 16798-3's ventilation-system classes refer to. When a consultant's schedule says "SFP ≤ 1 500" and your dossier says "SFP_int = 620", both can be right at once: they are measuring different spans of the same air path. Always state which one you are quoting, and at which external pressure.
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 (typical range across manufacturer datasheets; see ISO 16890 filter classes). 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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