What is Specific Fan Power (SFP)?

Specific Fan Power (SFP) is the total electrical power consumed by a ventilation fan system divided by the airflow rate it delivers, measured in watts per litre per second — W/(L/s). It is the primary metric for assessing ventilation energy efficiency under UK Building Regulations. Lower SFP means more efficient use of electricity to move air.

What are the Part F SFP limits?

Approved Document F (2021) specifies maximum SFP values: 0.5 W/(L/s) for continuous extract ventilation (MEV/dMEV), 1.0 W/(L/s) for intermittent extract fans, 1.5 W/(L/s) for balanced supply and extract systems (MVHR), and 1.8 W/(L/s) for commercial ventilation systems. These limits apply at the design flow rate.

How do I calculate SFP for a balanced MVHR system?

For balanced systems (MVHR), add the power of the supply fan and extract fan together, then divide by the maximum airflow of either the supply or extract side (whichever is greater, as they should be balanced). SFP = (supply fan W + extract fan W) / max(supply L/s, extract L/s). The limit is 1.5 W/(L/s).

What happens if my fan exceeds the SFP limit?

The ventilation system will not comply with Building Regulations and will fail commissioning. Options include: selecting a more efficient fan motor (EC motors achieve lower SFP than AC motors), reducing duct system resistance (larger ducts, fewer bends), or changing the fan type. SFP is checked during commissioning and must be documented.

How do I reduce SFP in an existing system?

Replace AC motors with EC (electronically commutated) motors — this alone can halve the SFP. Reduce system resistance by replacing flexible duct with rigid, removing unnecessary bends, increasing duct diameters, or cleaning dirty filters. Ensure dampers are fully open and not restricting flow unnecessarily. Re-commission after changes.

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SFP Calculator — Specific Fan Power

Calculate Specific Fan Power (SFP) and check compliance with Approved Document F limits. SFP = total fan watts divided by flow rate in L/s.

System Type

Determines the SFP compliance limit

Fan Power (Extract / Main)

Electrical input power at design flow

Flow Rate (Extract / Main)

How We Calculate This

Specific Fan Power (SFP) is calculated by dividing the total fan input power in watts by the system airflow rate in litres per second. This metric is mandated by Approved Document F to ensure ventilation systems are energy efficient.

Formula

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

ADF limits (2021)

Continuous extract (MEV): 0.5 W/(L/s)
Intermittent extract: 1.0 W/(L/s)
Balanced MVHR: 1.5 W/(L/s)
Commercial: 1.8 W/(L/s)

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.