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Ventilation with heat recovery for efficiency

Ventilation units with heat recovery are key in terms of energy savings, as they ensure that the warmth carried by the exhaust air is not wasted, but first transferred to the incoming fresh air without the two air streams ever physically mixing. In extremely hot conditions, heat exchangers can also work in reverse so that the heat carried by the incoming air is transferred to the exhaust air and thus pre-cooled before entering the rooms. These systems should also be equipped with automatically controlled bypasses, thus allowing the incoming air to bypass heat exchange, for example, during the night at times when days are warm and nights are cool.

A Passive House can only function with a highly efficient heat recovery, as ventilation systems without heat recovery waste far more energy per year than a Passive House uses for heat (at the same rate of air exchange, a ventilation unit without heat recovery may lose about 24kWh/(m²yr) whereas a Passive House’s maximum space heating demand is only 15kWh/(m²yr).

The ventilation systems used in Passive Houses must thus have heat recovery efficiencies of at least 75% while the electricity consumption for such systems should not exceed 0.45 Wh/m³ of the transport air volume. Additionally, the acoustic load of the ventilation systems for use in Passive Houses should not exceed 25dB. Pipes and values should be planned accordingly, making use of silencers.

Ventilation with heat recovery for comfort

A ventilation system with heat recovery ensures that plentiful, nearly room temperature fresh air enters the building in a controlled manner. Draughts are eliminated and residents need not actively air out the rooms. It is important that the fresh air entering the building not exceed 30m³ per hour per person, so as to avoid overly dry air. Such a ventilation system should not be confused with air conditioning systems; humidifying the air within the ventilation system is to be avoided for reasons of hygiene.

The ventilation systems used in Passive Houses provide unparalleled indoor air quality through the use of a high quality, F7 filter at the suction point (the unit must also be equipped with a drain). During heat recovery, the exhaust air must not mix with the supply air. Due to reasons of hygiene, a humidifier within the ventilation unit is not possible. It is important to remember that Passive Houses utilise ventilation systems, NOT air necessarily conditioning systems.


Protection against mould

In order to avoid the build up of moisture and mould, continual aeration with a mechanical ventilation system, good thermal protection and a thermal bridge free structure, all hallmarks of the Passive House Standard, are a must. Window and door frames must be well insulated. Triple low-e glazed window panes with noble gas filling should be used, although double glazing may be sufficient in hotter climes. Thermically separated, non-aluminium spacers at the glass edge seal are also important.

Contact Details

Passive Building Structures,
Dernawilt Rd, Roslea,
Co. Fermanagh BT92 7FH,
Northern Ireland.

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Website: www.passivebuildingstructures.com

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