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Standalone fan power limits against Approved Document L

A toilet extract fan, an in-line duct fan and a commercial kitchen extract fan, each run through the same calculation the app uses, with the specific fan power limit it applies compared with Table 5.9 for new and existing buildings.

Result
6 of 6 checks pass
Cases
3 worked examples
Version / published
v1.0 · 2026-09-28
Engine fingerprint
e0fcb28b1fc0

Why we publish this

A standalone fan is a local or zonal unit, never a central system, so the calculation picks one of three Table 5.9 rows from the fan's application. Choosing the wrong row changes the limit by a factor of two or more, so each application is checked against the printed row for both new and existing buildings. The 2026 edition takes effect on 24 March 2027; Table 6.9 of the 2021 edition in force until then prints the same limits.

What is not compared, and why

  • Fan sound power (CIBSE Guide B4:2016 §4.3.2). Guide B4 sets out the method but prints no worked example to compare against.
  • Shaft and input power. P = Q × Δp ÷ η is a definition rather than a printed result; it is checked in the regression suite.
  • The ASHRAE setting's Fan Energy Index (ASHRAE 90.1-2025 §6.5.3 with AMCA 208). Neither document is among those this page quotes from.

How to read the tables

  • Expected is the value printed in the standard, or a hand calculation from printed values. The working is shown under each quantity.
  • Tolerance is exact for tabulated values and selected sizes, and ±2 % for calculated quantities such as the fan input power (every value on this page is read from the table).
  • Every check re-runs automatically whenever the calculation code changes, and this page is regenerated from those results. The engine fingerprint above identifies the calculation code the results came from.

Sources

  • Approved Document L, Volume 2: Buildings other than dwellings, 2026 edition

Printed page numbers refer to the editions above.

Worked examples

FAN-01

Toilet extract fan: local window, wall or roof unit

Inputs

  • Toilet extract, 15 l/s at 80 Pa
Checks for case FAN-01
QuantityExpectedSourceMEPCalcΔToleranceResult
SFP limit, new building
0.3W/(l/s)
Approved Document L, Volume 2: Buildings other than dwellings, 2026 edition
Table 5.9 (printed p54)
0.3W/(l/s)0.00 %exactPass
SFP limit, existing building
0.4W/(l/s)
Approved Document L, Volume 2: Buildings other than dwellings, 2026 edition
Table 5.9 (printed p54)
0.4W/(l/s)0.00 %exactPass

FAN-02

In-line duct fan: zonal extract with the fan remote from the zone

Inputs

  • In-line duct extract, 300 l/s at 250 Pa
Checks for case FAN-02
QuantityExpectedSourceMEPCalcΔToleranceResult
SFP limit, new building
0.5W/(l/s)
Approved Document L, Volume 2: Buildings other than dwellings, 2026 edition
Table 5.9 (printed p54)
0.5W/(l/s)0.00 %exactPass
SFP limit, existing building
0.5W/(l/s)
Approved Document L, Volume 2: Buildings other than dwellings, 2026 edition
Table 5.9 (printed p54)
0.5W/(l/s)0.00 %exactPass

FAN-03

Commercial kitchen extract: fan remote from the zone, with grease filter

Inputs

  • Commercial kitchen extract, 1500 l/s at 600 Pa
Checks for case FAN-03
QuantityExpectedSourceMEPCalcΔToleranceResult
SFP limit, new building
1W/(l/s)
Approved Document L, Volume 2: Buildings other than dwellings, 2026 edition
Table 5.9 (printed p54)
1W/(l/s)0.00 %exactPass
SFP limit, existing building
1W/(l/s)
Approved Document L, Volume 2: Buildings other than dwellings, 2026 edition
Table 5.9 (printed p54)
1W/(l/s)0.00 %exactPass