Mechanical Ventilation Explained

Utilisation of passive design criteria is becoming increasingly more common as peoples awareness and understanding of it’s advantages rise. Whether it’s a certified Passive House (Passivhaus), a Passive Home without certification or a low energy building.

The MVHR/ HRV (Heat Recovery Ventilation) system is an essential part of a passive home/ house to provide sufficient ventilation without the heat losses that are associated with any other form of ventilation. An efficient HRV system will reduce the heating demand from 35kWh/(m2a) to 15kWh/(m2a) at equal ventilation rates – compared with natural ventilation.

From this it can be seen that the process of recovering heat from stale, used air and transferring it to fresh yet cold air, has a major influence on energy and emission savings.

How does it work:

1 – Wet rooms and kitchens are extracted on a continual, but slow basis.

2 – Before the extracted air is exhausted out of the building, the warmth in the air is transferred by means of a heat exchanger into the fresh air, which is introduced into the building.

3 – The pre-warmed fresh air is introduced into all habitable rooms on a continual basis. Thereby the need to completely heat the fresh air as it enters the building is eliminated.  Typically cold outside air at 0 ºC can be warmed to 18 ºC by warm extract air at 20 ºC when an efficient system is used.

HRV systems are primarily ventilation systems, which reuse and distribute warmth from internal + solar gains and space heating systems. In some cases (mostly certified Passive Houses) all space heating can be provided by an HRV system. Please check if this is possible with your Passive House Designer beforehand.

HRV systems operate in a completely different way than air conditioning systems.

Advantages of HRV systems compared to air conditioning:

– It is important to highlight that the stale exhaust air and clean fresh air do not mix in the heat exchanger. The stale and the clean air are channelled through closely spaced but separate narrow sleeves in the core of the heat exchanger. Thus no bacteria,fungus or other pollutants can be transferred from the extract air into the supply air.

– The warming of cold outside air is passive. Apart from minimal expenditure to drive the two fans and for frost protection in winter, the heat recovery process is passive. In other words, without energy consumption.

Disadvantages of HRV systems:

Normally no active cooling can be provided. A gentle cooling effect in the warm season is achieved by a potential summer bypass. Ideally the design of the building should prevent excessive solar gains in summer.