MVHR systems inherently dehumidify. Unlike active dehumidifiers this process is passive and costs only a fraction of the cost.
So, drying clothes inside, cooking and bathing is not a problem with a correctly specified MVHR.
However, in some cases MVHR can overshoot the mark and dry out the dwelling too much in winter. This is expecially the case in very large dwellings, where a lot of ventilation is needed. For this reason PAUL has invented a different type of heat exchange core, that prevents such over-dehumidification risk.
As an alternative to the standard heat exchanger, most of our heat recovery units can also be equipped with a highly efficient, Enthalpy Heat Exchanger (ERV), which are moisture-transmitting counter flow heat exchangers.
In living spaces with low humidity, it provides a convenient humidity, due to the additional moisture recovery from the extract air. The warm and cold air flows are separated by a selective polymer membrane film (dpoint technologies). The hygiene is ensured at any time due to the anti-microbial properties of the membrane (Microban®), only heat and water vapour are transferred. The transfer of gases, pollutants and smells is excluded.
Please avoid cheaper cellulose based enthalpy heat exchangers, which have only a limited life-span.
Up to 75% moisture recovery ensures a convenient room climate at any time, without the air overly drying out in the cold season.
This is especially important for larger dwellings with low occupancy levels, as the internal moisture generation is in dis-balance with the ventilation requirement of a large building envelope.
Another advantage of ERV units is that low humidity levels in the heating season can be avoided, which sometimes occur with HRV systems in larger dwellings with few occupants. Low humidity levels can affect our health and well-being. They also greatly increase the life-span of corona viruses.
The ideal relative indoor humidity ranges between 40 and 60% rh. It is ok, if the humidity increases in summer up to 75%. Please be aware that the indoor temperature will affect the relative humidity. The colder a room, the higher its rh value.
The heat recovery rate of an ERV core is typically a bit less compared to a HRV core, but the increased humidity levels indoor mean a latent heat gain due to the reduced evaporation losses. This closes the performance gap in terms of heat recovery between the two systems.
A new ERV membrane developed in 2026 offers very similar heat recovery rates than the HRV core.
By the way, we don’t recommend using cellulose based ERV cores, as they will degrade over time. High performance polymer based materials will not show any degradation in foreseeable time.
The Zehnder ComfoAir Q, ComfoAir 160, 810 and 200 MVHR units can also be provided with an enthalpy core. For this option, the MVHR will not have any defrost pre-heater as the risk of condensation freezing in the heat exchanger is minimised and it is inherently frost protected down to about minus 6 deg. C, or lower, depending on the humidity level of the extract air.
If the outdoor temperatures drop lower than that, the system will go into frost protection mode to prevent frost in the HRV core.
Following MVHR units come with ERV cores by default: ComfoAir 70, ComfoSpot 50.
Although the ERV technology is more needed in Europe with dryer climate and prolonged deep frost in winter, we find there is a need for these here in the UK, too. We specify ERV cores for larger dwellings with comparably low occupancy, in order to prevent low humidity levels in winter. So far this has been a very successful approach.
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