MVHR :: A Passivhaus perspective

Reading a Passivhaus article they have these as the key Mechanical Ventilation Heat Recovery (MVHR) reasons:

  • firstly its purpose is to provide sufficient fresh air,
  • secondly it is to avoid draughts and discomfort and
  • finally it is to reduce energy demand;
    – without heat recovery, ventilation leads to unnecessary energy demand and can cause thermal discomfort.

In buildings with MVHR, fresh air is drawn in through a heat exchanger, past the stale air being extracted from the building. The heat exchanger is designed so that the exhaust air warms the incoming outside air, before it finally leaves the building. Importantly the two air streams do not mix, thereby maintaining high standards of fresh air supply throughout the home.

In order to circulate the fresh air throughout the home, two low energy fans are used; one on the supply and one on the extract. The fans only consume a fraction of the energy that the system manages to ‘harvest’ from the stale air. Measurements have shown that they can save more than ten times the amount of energy that they use.

Opening doors and windows

  • You can open windows and doors. Yes, in winter, you’d impact the integrity and efficiency of the system if you leave doors and windows for long periods of time. But even in a non MVHR / Passivhaus building, this is unlikely to occur.
    And steam, smells etc. are being naturally extracted, so this “venting” requirement is no longer there.
    In warm weather, it’s good to open doors and windows of a MVHR / Passivhaus building. Especially at night to let in the cooler air, that you can day time keep trapped in the house at that temp.