What is dMEV ventilation?

Decentralised Mechanical Extract Ventilation (dMEV) uses individual low-power fans in each wet room (kitchen, bathroom, WC, utility) running continuously at a low trickle rate and boosting when the room is in use. This is System 3 in Approved Document F. Each fan is independent — there is no central fan unit or ductwork between rooms.

What extract rates does ADF require for dMEV?

Approved Document F Table 5.1b specifies: Kitchen 13 L/s, Bathroom 8 L/s, WC 6 L/s, Utility 8 L/s. For continuous dMEV, fans run at trickle rate continuously and boost to these rates when the room is in use. The trickle rate should provide at least the whole dwelling ventilation rate when all fans are summed.

What SFP limit applies to dMEV fans?

Approved Document F limits the Specific Fan Power (SFP) for continuous dMEV fans to 0.5 W/(L/s). For intermittent fans, the limit is 1.0 W/(L/s). This means a fan extracting 8 L/s continuously must consume no more than 4W. Modern dMEV fans typically achieve 0.2-0.3 W/(L/s).

Do dMEV fans need overrun timers?

For continuous dMEV systems (System 3), the fans run all the time at trickle rate and boost automatically via humidity sensors or manual switches. No separate overrun timer is needed as the fan never stops. For intermittent systems, a 15-minute overrun timer is required in bathrooms and WCs per ADF.

How does dMEV compare to cMEV (centralised extract)?

dMEV uses individual fans per room (simpler installation, no ductwork between rooms) while cMEV uses a single central fan with ductwork to each wet room. dMEV is easier to install in existing properties but harder to balance. cMEV is more efficient for larger properties with many wet rooms. Both are System 3 under ADF.

TradeCalculator

dMEV Fan Calculator — Decentralised Mechanical Extract

Size decentralised mechanical extract ventilation (dMEV) fans per Approved Document F. Calculate fan requirements for each wet room with SFP compliance check.

Bathrooms

8 L/s boost per ADF

Shower Rooms

8 L/s boost per ADF

WCs / Cloakrooms

6 L/s boost per ADF

Kitchens

13 L/s boost per ADF

Utility Rooms

8 L/s boost per ADF

Fan Operation Mode

Continuous = System 3 per ADF

How We Calculate This

This calculator sizes individual dMEV fans for each wet room in a dwelling per Approved Document F (2021 edition) Table 5.1b requirements.

ADF Table 5.1b extract rates

Kitchen: 13 L/s boost
Bathroom: 8 L/s boost
Shower room: 8 L/s boost
WC: 6 L/s boost
Utility: 8 L/s boost

SFP compliance

The Specific Fan Power (SFP) must not exceed 0.5 W/(L/s) for continuous operation or 1.0 W/(L/s) for intermittent operation. This limits the maximum power consumption of each fan based on its flow rate.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.