How to choose between MEV and MVHR
This question cannot be solved in few words, as the choice depends on plenty of criteria.
Energy aspect: the performance of MVHR strongly depends on climate
On the strictly energetic aspect, the choice between a humidity sensitive MEV and mechanical ventilation with heat recovery (MVHR) essentially depends on the climate of the installation zone and the occupancy rate of the home. The two systems are based on two different approaches to energy performance optimisation.
MVHR allows the recovery of a substantial part of the heat extracted by the ventilation thanks to a heat exchanger, while humidity sensitive MEV (or, more generally, a demand-controlled system) automatically reduces the rate of air renewal in order to limit heat loss as soon as the home is less occupied (which is usually most of the time). When the climate is temperate, the MVHR and the humidity sensitive MEV are almost equal. The MVHR becomes more beneficial in energy terms when the outdoor air is very cold, as it is the case of the Nordic countries.
From the power consumption point of view, the humidity sensitive MEV is much more efficient than the MVHR, the latter losing out due to the use of two fans instead of one in the MEV and to the pressure drop generated by the heat exchanger as well as by the filters.
Indoor air quality: variable results depending on situation and occupancy, increased maintenance requirement for the MVHR
As far as indoor air quality is concerned, the two systems can be quite comparable regarding the evacuation of indoor pollutants since the MVHR as the humidity sensitive MEV bases its performance on the modulation of flow rates according to the needs of the rooms. Flow rates are on average lower but better adapted to the needs, occasionally even higher than those of the MVHR.
However, the MVHR offers better protection of the dwelling from external particles (PM2.5, PM10, etc.) due to filtration of the outdoor air that is generally not possible with the humidity sensitive MEV. However, this is a double-edged sword since the filtration limitation of the humidity sensitive MEV is also an advantage versus MVHR in that it does not require filter maintenance. If the latter is not cleaned or replaced regularly (usually every 6 months), its clogging causes increase in pressure losses, generating over-consumption of electricity and noise.
The humidity sensitive MEV also has the advantage of directly admitting the outdoor air through the air inlets and not via ducts as it is the case for MVHR, ducts whose cleanliness and airtightness must be perfectly ensured.
MVHR is ideal for isolating external noises, while humidity sensitive MEV is easier to optimise for quiet operation
As far as acoustics goes, while MVHR ensures strong protection against external noises, the humidity sensitive MEV offers very quiet operation in rooms especially with a supply of fresh air running at very low pressure compared to MVHR where the installation of the supply units needs to be particularly well designed and arranged to avoid any noise problem.
As far as installation is concerned, MEV requires much less work than MVHR with its double duct network. The purchase cost of the humidity-controlled MEV system is also significantly lower than that of a MVHR system, between 3 and 6 times cheaper.
In short, MVHR or humidity sensitive MEV: it all comes down to priorities!
In summary, the choice between MVHR or humidity sensitive MEV is not a hard and fast rule; elements such as the budget allocated to the project, the need to filter outdoor air or to reduce electricity consumption, in particular, may guide the designer towards one or the other of the two ventilation techniques, each with advantages and very specific qualities.