SFP Calculator — Specific Fan Power

Specific Fan Power (SFP) is the standard measure of how efficiently a fan or whole ventilation system uses electricity to move air. You calculate fan SFP by dividing the total fan input power in watts by the airflow it delivers in litres per second, giving a figure in W/(L/s). The lower the SFP, the less electricity the fan draws for the air it shifts. For dwellings the SFP limits are set by the Domestic Building Services Compliance Guide (published under Approved Document L, the energy-efficiency part of the Building Regulations) to keep ventilation systems efficient as homes get more airtight.

Formula

SFP = Total fan power (W) / Flow rate (L/s)

For a balanced system (MVHR) you add the supply and extract fan powers together and divide by the larger of the supply or extract design flow rate, because the two sides are balanced. The result is the same unit, W/(L/s).

Part F sets the air, Part L sets the fan power

These are two different things. Approved Document F sets the minimum amount of ventilation a dwelling needs, in litres per second. Approved Document L (via the Domestic Building Services Compliance Guide) limits how much electrical power the fans may use to deliver it, in watts per litre per second. SFP is a Part L energy metric — Approved Document F itself does not quote SFP figures.

Dwelling SFP limits (Domestic Building Services Compliance Guide, Part L)

• Intermittent extract fans: less than 0.5 W/(L/s)
• Continuous supply: 0.5 W/(L/s)
• Continuous extract (MEV/dMEV): not worse than 0.7 W/(L/s)
• Balanced MVHR (supply + extract with heat recovery): 1.5 W/(L/s)

SFP is verified at the design flow rate during commissioning and recorded for Building Control. It is also an input to the SAP/EPC calculation for the dwelling, where a published (or default) SFP is multiplied by an in-use factor that allows for real-world ductwork and fan settings.

How to lower an over-the-limit SFP

Specifying an EC (electronically commutated) motor instead of an AC motor is the single biggest lever and can roughly halve the SFP. Beyond that, cut system resistance: use rigid duct rather than flexible, size ducts generously, minimise bends, keep filters clean and make sure dampers are not throttling the flow.

Non-domestic (commercial) limits

Commercial systems use Table 6.9 of Approved Document L Volume 2, 2021 edition (incorporating 2023 amendments). For new buildings the maximum SFP at design flow is 2.0 W/(L/s) for a central balanced system with heating and cooling, 1.9 for heating only, and 1.5 for all other central balanced systems; local supply or extract units (such as toilet extract) are limited to 0.3 W/(L/s). Existing buildings get slightly higher allowances. Non-domestic SFP must be calculated to BS EN 16798-3 at the full design load (BS 8850 for fan coil units) — confirm the applicable band for your project.