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cMEV System Calculator — Centralised Mechanical Extract
Design centralised mechanical extract ventilation systems. Calculate fan duty from total branch resistance, duct sizing, and continuous high extract rates to Approved Document F Vol 1 (2021).
8 L/s continuous high rate (ADF 2021 Table 1.2)
8 L/s continuous high rate, as a bathroom (ADF 2021 Table 1.2)
6 L/s continuous high rate (ADF 2021 Table 1.2, sanitary accommodation)
13 L/s continuous high rate (ADF 2021 Table 1.2)
8 L/s continuous high rate (ADF 2021 Table 1.2)
Sets the busiest-branch velocity (kitchen 13 L/s through 100mm ≈ 1.7 m/s). Keep ≤3 m/s for quiet running.
Continuous extract (cMEV) ceiling 0.7 W/(l·s) - Domestic Building Services Compliance Guide Table 34 / AD L. Modern EC fans reach ~0.3–0.5.
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How We Calculate This
This calculator designs centralised mechanical extract ventilation (cMEV) systems to the continuous mechanical extract ventilation requirements of Approved Document F Vol 1 (2021), paras 1.60-1.66 (the system the pre-2021 edition labelled "System 3").
Design process
- Sum the continuous extract high rate of every wet room per ADF 2021 Table 1.2
- Check the dwelling total is at least the Table 1.3 whole-dwelling rate (or 0.3 L/s per m²)
- Size the main duct for total flow at a low velocity (~3 m/s for quiet running, up to 4-5 m/s max)
- Size branch ducts for individual room rates
- Calculate system pressure drop from friction, fittings, and terminals
- Select the fan for the flow rate at system pressure + 20% margin
ADF 2021 Table 1.2 continuous high rates
- Kitchen: 13 L/s
- Bathroom / Shower: 8 L/s
- WC / sanitary: 6 L/s
- Utility: 8 L/s
These are the minimum continuous mechanical extract high rates for a continuous (cMEV) system. The higher intermittent rates in Table 1.1 (kitchen 30, bathroom 15) apply to intermittent fans, not to a continuous centralised system.
Frequently Asked Questions
A centralised Mechanical Extract Ventilation system uses a single fan unit (typically in the loft or a cupboard) connected to multiple wet rooms via ductwork. Extract air is drawn from kitchens, bathrooms, and WCs through individual branch ducts to the central fan, which discharges to outside. Approved Document F Vol 1 (2021) calls this "continuous mechanical extract ventilation" (paras 1.60-1.66); it is still widely known by its pre-2021 shorthand "System 3".
cMEV uses one central fan with ductwork to each room, while dMEV uses individual fans in each wet room. cMEV is more efficient and easier to balance for larger properties (3+ wet rooms) but requires more ductwork. dMEV is simpler to install in smaller dwellings or retrofit situations. Both are continuous mechanical extract systems under Approved Document F Vol 1 (2021) — Para 1.60 explicitly allows a central extract system, individual room extract fans, or a combination.
The main duct must carry the total flow from all branches. Keep the velocity low to limit noise — many designers cap domestic extract ducts at around 3 m/s, at which a 150mm duct carries roughly 55 L/s and a 200mm duct roughly 100 L/s. If you allow up to 4-5 m/s, the same ducts carry about 71-88 L/s (150mm) and 126-157 L/s (200mm), so choose the velocity limit to suit the noise target. Branch ducts to individual rooms are usually 100mm diameter for rates up to 13 L/s.
The fan must overcome the total system pressure drop: duct friction (straight runs), fitting losses (bends, branches, reducers), and terminal losses (extract valves). A typical domestic cMEV system has 80-150 Pa total resistance. The fan should be selected with a 20% margin above the calculated system resistance.
Specific Fan Power (SFP) is an energy-efficiency requirement set by the Domestic Building Services Compliance Guide (under Approved Document L), not by Part F. Its Table 34 limits SFP to 0.7 W/(L/s) for continuous extract (cMEV/MEV), 0.5 W/(L/s) for intermittent extract fans, 0.5 W/(L/s) for continuous supply, and 1.5 W/(L/s) for continuous supply-and-extract with heat recovery (MVHR). For a continuous system at 21 L/s the fan should therefore draw no more than about 15W. Modern EC-motor fans typically achieve 0.2-0.5 W/(L/s).
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.