System 3 - Mechanical Extraction Ventilation

Mechanical Extraction Ventilation

System 3 - Mechanical Extract Ventilation (MEV) System

System 3

Under Part F of the Building Regulations, System 3 covers continuous mechanical extract ventilation (MEV). This can be either a whole house centralised MEV system, or localised decentralised MEV fan.

The centralised MEV system is typically located in a loft space or hallway cupboard. Multiple ducts run from the unit to the kitchen, bathroom, ensuites and other wet rooms of a property to simultaneously draw moisture laden air from these wet rooms to control humidity levels.

Decentralised MEV (dMEV) are individual room fans which operate continuously to draw moisture from either the bathroom, kitchen, utility room or other wet room. Continuous mechanical systems should include either manual or automatic controls (i.e. humidity sensors) to operate on between trickle and boost modes.

They should always provide the minimum whole building ventilation rate, which can be calculated in the following steps:

Step 1: Determine the whole dwelling ventilation rate in Table 5.1b:

Table 5.1b - Whole Building Ventilation Rates

Step 2: Calculate the whole building extract rate from Table 5.1a by taking the ‘minimum high rate’ and multiplying this by the number of individual ventilated rooms in the dwelling. E.g. if an individual dMEV fan is fitted in the kitchen, bathroom and utility room, the total minimum high rate would be 13l/s, plus 8l/s for the bathroom and 8l/s for the utility room = 29l/s.

Step 3: The required extract rates are as follows:

The maximum whole dwelling extract ventilation rate (i.e. the boost rate) should be at least the greater of Step 1 and Step 2. The minimum whole dwelling extract ventilation rate should be at least the whole dwelling rate as found in Step 1.

For any design permeability, controllable background ventilators having a minimum equivalent area of 2500 mm2 should be fitted in each room, except wet rooms, from which air is extracted. As an alternative, where the designed air permeability is leakier than (>) 5m³/(h.m²) at 50 Pa background Ventilators are not necessary, but see the cautionary advise in paragraph 5.10 (Part F).

System 3 - Ventilation Solutions:


The ECO dMEV has been designed and developed to offer the market a constant volume, continuously running decentralised extract fan to achieve the highest energy efficiency, the lowest noise and the lowest life-cycle costs.

MEV Spider System

MEV Spider

The MEV Spider is a low energy, continuous mechanical extract ventilation system designed with multiple extract points to simultaneously draw moisture-laden air out of the wet rooms, whilst minimising the migration of humidity to other rooms.


The OZEO is centrally located in either the loft space or a hallway cupboard with ducts running from the unit to the kitchen, bathroom, en-suite and other wet rooms.