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I'm going to try and clear up the confusion around MEV, cMEV and dMEV systems. I know that it's not clear, because it initially confused the hell out of me even though I'd been working in the ventilation space for years.

MEV - Stands for Mechanical Extract Ventilation

MEV is often used to describe whole house ventilation systems, which is somewhat ambiguous as most wall fans can be described as 'Mechanical Extract Ventilators'. Having said that, MEV in the industry is more commonly attributed to any extractor fan which has a 'continuous' (or trickle) running function rather than the more conventional operation of an extractor fan, which is to run intermittently whenever it is actuated by the light switch or automatic sensor.

The more accurate term for a 'whole house unit' is cMEV.
cMEV - Centralised Mechancial Extract Ventilation

cMEV is the accurate term and some manufacturers and suppliers do use this to describe and label their products but many do not.

dMEV - Decentralised Mechancial Extract Ventilation

dMEV refers to continuous running extractor fans that are installed into each individual room.

cMEV Units are also Commonly Referred to as:
Whole house extractor fans
Multi-room extractor fans
Central extract fans
Multi-vent extractor fans
Multi-point extractor fans
The cMEv whole house unit usually sits in either the loft or a cupboard with ducting connecting it to all the wet rooms in the dwelling:

Bathrooms
Shower rooms
Washrooms
Utility rooms
Kitchen
This is why many cMEV units come with up to 6 inlet spigots. The damp and smelly air is then exhausted via a single duct running from the unit to outside air via a grille or cowl.

Continuous (Trickle) Extraction vs Intermittent

Building regulations allow developers to use either intermittent (standard) or continuous (cMEV, dMEV) operation fans. 'Approved Document F' shows a table of extraction rates in 'System 3' that explains the difference in extraction rates required for both different size dwellings and also the two different types of extractors. Basically, intermittent fans need to be capable of moving a higher air volume than their continuous running counterparts.

Noise & Performance
Often, when you tell clients that their fancy new cMEV fan is going to be running all of the time, they worry about the noise levels. However, the decibel levels are much lower than a standard intermittent fan, simply because the speed of the impeller rotation is much much lower when in continuous mode.

"A typical intermittent fan might produce 29-34 decibels at 3m on full power compared to the cMEV fan's 9 decibels at 3m in continuous mode."

For some context, regular human breathing produces 10 decibels at 3m…..

cMEV Boosting & Automation
Although continuous extraction at low rates is very effective, we still require a performance 'boost' when a shower is filling the room up with steam or when we're cooking 'mackerel surprise' in our kitchen.

Whole house ventilation systems have a boost mode which can be operated manually (usually via the light switch in the room) or by some other automatic sensor. This sensor would most commonly be humidity level based but could also be a PIR occupancy sensor. It's quite common for cMEV units to come with a humidity option as standard.

Mounting & Siting
Lofts - Although available in a variety of shapes and sizes, modern cMEV units are designed with a low-profile so they can fit between joists or in cupboards. The older designed units, were sometimes 'suspended' in the loft by supporting wire, to completely remove any vibration noise transference. Most cMEV whole house units can be fitted either horizontally or vertically to fit the space you wish to use.

Cupboards - Loft mounting is not an option in some dwellings, in these cases, cMEV units are fitted into a convenient cupboard.