Passive House MVHR

All of the rooms within the thermal envelope (inside the continuous air-tight layer) are ventilated via the use of a mechanical ventilation system with heat recovery. The requirement for performance of an MVHR system, within a Passive House, is a minimum heat recovery efficiency of 75%, with an energy consumption of no more than 45Wh/(m³) (sub-criteria).

The ventilation system provides a steady stream of fresh warm air to the building which is an absolutely vital requirement as the building envelope renders the building almost completely airtight. The average air change rate per person in a Passive House is 20-30 m³ h־¹ (sub-criteria) with higher air change rates in rooms with excess moisture, or odours such as kitchens and bathrooms. The air flow rate of the ventilation system must be balanced which is most commonly achieved by dividing the house into supply air rooms (such as bedrooms and living/dining rooms) where fresh air is provided; overflow areas (such as hallways and stairwells/landings) where the air is drawn across a space; and exhaust air rooms (such as kitchen, bathrooms and laundry rooms) where the stale air leaves the building.

The heat recovery part of the ventilation system recycles the heat from the used stale air and uses this to pre-heat the fresh clean air being pumped in to the building (with a minimum efficiency of 75%). This means that in order to reach the Passive House internal design temperature of 20ºC (sub-criteria) the building only requires an additional 4-5˚C from space heating, as the other 15-16˚C will be recovered from the exhaust air, and channelled back into the building through the supply air.

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