TradeCalculator.co.uk
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.
Continuous 8 L/s (ADF Table 1.2) · intermittent 15 L/s (Table 1.1)
Continuous 8 L/s (ADF Table 1.2) · intermittent 15 L/s (Table 1.1)
6 L/s in both modes (ADF Tables 1.1 and 1.2)
Continuous 13 L/s (Table 1.2) · intermittent 30 L/s with hood, 60 without (Table 1.1)
Continuous 8 L/s (Table 1.2) · intermittent 30 L/s (Table 1.1)
Sets the whole-dwelling minimum, ADF Table 1.3 (13 + 6 × bedrooms; 1-bed = 19 L/s)
Domestic Building Services Compliance Guide 2013 (Part L): 0.7 continuous extract, 0.5 intermittent
Your cost per dMEV fan unit
Next up: related pages
How We Calculate This
This calculator sizes individual dMEV fans for each wet room in a dwelling per Approved Document F (2021 edition). Continuous dMEV (System 3) uses the Table 1.2 high rates; intermittent fans (System 1) use the higher Table 1.1 rates. It also surfaces the Table 1.3 whole-dwelling target rate and the Specific Fan Power limit.
Continuous dMEV high (boost) rates — ADF 2021 Table 1.2
- Kitchen: 13 L/s
- Bathroom: 8 L/s
- Shower room: 8 L/s
- Utility: 8 L/s
- WC: 6 L/s
Intermittent rates — ADF 2021 Table 1.1
- Kitchen: 30 L/s with a cooker hood extracting outside, 60 L/s with no hood or a recirculating hood
- Utility: 30 L/s
- Bathroom / shower: 15 L/s
- WC: 6 L/s
Whole-dwelling minimum — ADF 2021 Table 1.3
For continuous extract the sum of all the continuous rates at their low (background) rate must be at least 13 + 6 × bedrooms (1-bed 19, 2-bed 25, 3-bed 31, 4-bed 37, 5-bed 43 L/s), or 0.3 L/s per m² of internal floor area, whichever is greater. ADF fixes no per-room low rate, so the calculator shows this whole-dwelling figure as a design target — set each fan's continuous background rate so the dwelling total reaches it. The Table 1.2 high-rate sum shown above is the boost requirement, not the quantity Table 1.3 tests.
SFP compliance
The Specific Fan Power (SFP) must not exceed 0.7 W/(L/s) for continuous extract (continuous dMEV) or 0.5 W/(L/s) for an intermittent extract fan, per the Domestic Building Services Compliance Guide 2013 (the operative Approved Document L guidance). This caps the maximum power consumption of each fan based on its extract rate. Work out the figure for a candidate fan with our SFP calculator (Specific Fan Power).
Frequently Asked Questions
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.
Approved Document F 2021 Table 1.2 sets the minimum continuous-extract HIGH (boost) rate for each room: Kitchen 13 L/s, Bathroom 8 L/s, Shower 8 L/s, Utility 8 L/s, WC 6 L/s. A continuous dMEV fan runs at a lower background (trickle) rate and steps up to these high rates when the room is in use. ADF does not fix a per-room low rate — instead, the sum of all the continuous extract rates must be at least the whole-dwelling rate in Table 1.3 (13 + 6 × bedrooms; for example a 2-bed needs 25 L/s, a 3-bed 31 L/s; or 0.3 L/s per m² of internal floor area, whichever is greater). Note: 30 L/s for a kitchen and 15 L/s for a bathroom are the INTERMITTENT rates from Table 1.1, not the continuous dMEV rates.
Specific Fan Power (SFP) limits come from the Domestic Building Services Compliance Guide 2013 (the operative Part L document), not Approved Document F. The maximum SFP is 0.7 W/(L/s) for continuous extract — which is what continuous dMEV is — and 0.5 W/(L/s) for an intermittent extract fan. So an 8 L/s continuous bathroom fan must draw no more than 0.7 × 8 = 5.6 W, and a 13 L/s continuous kitchen fan no more than 0.7 × 13 = 9.1 W. Modern dMEV fans typically achieve 0.2–0.3 W/(L/s), comfortably inside the limit.
For continuous dMEV systems (System 3), the fans run all the time at the background rate and boost automatically via humidity sensors or manual switches, so no separate overrun timer is needed — the fan never stops. For an intermittent fan, Approved Document F 2021 para 1.51 requires controls that keep the fan running for at least 15 minutes after the room is vacated only where the room has no openable window (typically an internal bathroom or WC); a wet room with an openable window does not need the overrun.
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.
Was this calculator helpful?
Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.