Relative Humidity

24th May 2019

The Brink Flair 325 MVHR unit comes with an optional Enthalpy Heat Exchanger and at CVC Systems we often get asked whether an Enthalpy Heat Exchanger is needed. We also often get asked whether the air in a Passivhaus is dry as a result of the ventilation. The answer is ‘yes’ the air is drier in a Passivhaus with MVHR, in comparison to one with poor ventilation. This however is generally regarded as a good thing – excess moisture can lead to the growth of mould, provides a good environment for bacteria, cockroaches and dust mites. It can also harm the health of the occupants and cause damage to the structure of the building and furniture.

On the other hand, if the relative humidity internally is too low it can lead to dry skin, itchy eyes, dry nose and throat, creating a very uncomfortable environment for the occupants. It is suggested that the ideal level of relative humidity in a home is between 40% and 60%.

The major sources of moisture inside the home are the occupants, cooking, showering and plants. When an MVHR (without an enthalpy heat exchanger) is used, that moisture is lost to the outside, all that is transferred to the incoming air is the heat.

The standard plate heat exchanger also removes moisture from the incoming air, as the cooler incoming air is warmed in the heat exchanger by the warmer extract air from the house, which has the result of reducing the relative humidity of the air entering the house. Normally, the warm moist extracted air is cooled by the incoming air which results in the moisture condensing in the heat exchanger where it is discharged via a condensate drain.

The Flair 325 comes with an optional Enthalpy Heat Exchanger. This works by allowing water vapour from the extracted internal air to transfer to the incoming air, raising the relative humidity. The quantity of moisture that is transferred depends on the relative humidity of the indoor and outdoor air and may run to about 60%. This is particularly useful with cold, dry alpine environments, with low levels of relative humidity. The enthalpy heat exchanger also recovers both thermal and latent energy from the extracted internal air; energy which would ordinarily have been lost to the atmosphere.

The average relative humidity in the UK is between lows of 70% and highs of 90% which is considered high, suggesting humidity recovery is not that big an issue in the UK.