What is a centralised MEV (cMEV) system?

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. This is System 3 under Approved Document F.

How is cMEV different from dMEV?

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 System 3 under ADF.

How do I size the main duct for cMEV?

The main duct must carry the total flow from all branches. Size for a maximum velocity of 4-5 m/s to keep noise low. Typically 150mm diameter handles up to 55 L/s, and 200mm handles up to 100 L/s. Branch ducts to individual rooms are usually 100mm diameter for rates up to 13 L/s.

What fan duty does a cMEV system need?

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.

What SFP limit applies to cMEV systems?

Approved Document F limits the Specific Fan Power (SFP) for continuous cMEV to 0.5 W/(L/s) and for intermittent systems to 1.0 W/(L/s). This means a system with 40 L/s total flow running continuously must use no more than 20W. Modern EC motor fans typically achieve 0.2-0.4 W/(L/s).

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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 boost/trickle rates per Approved Document F.

Bathrooms

8 L/s boost each

Shower Rooms

8 L/s boost each

WCs / Cloakrooms

6 L/s boost each

Kitchens

13 L/s boost each

Utility Rooms

8 L/s boost each

Fan Position

How We Calculate This

This calculator designs centralised mechanical extract ventilation (cMEV) systems per Approved Document F System 3 requirements.

Design process

Sum extract rates from all wet rooms per ADF Table 5.1a
Size main duct for total flow at 4-5 m/s
Size branch ducts for individual room rates
Calculate system pressure drop from friction, fittings, and terminals
Select fan for flow rate at system pressure + 20% margin

ADF extract rates (boost)

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

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.