Mechanical Ventilation with Heat Recovery (MVHR)


 MVHR is an alternative to natural ventilation. This system does not rely on the weather so it is more reliable. it uses ductwork to direct air around the dwelling. As this air is directed around the house it passes through a central unit, which heats the incoming fresh air using the outgoing warm stale air. 

An MVHR system works by extracting air from certain rooms and supplying fresh air to others. The air that is extracted is warm indoor air from wet rooms such as kitchens and bathrooms - this air then passes through a heat exchanger which gives up the warmth from that air to the incoming fresh outdoor air. The incoming and exhaust air masses remain separate throughout and the “prewarmed” fresh outdoor air is then supplied to bedrooms, living rooms, dining rooms etc.

The central unit is designed to allow heat to exchange between ducts without allowing stale and fresh air to mix.

Because there is ducting to each room, vents to the outside of the building are not necessary. this makes the building more airtight, which reduces energy costs. The air change per hour cab also be controlled. the elimination of vents in the windows, walls, doors, etc. also means that:

  • There is less risk of condensation and mould growth.
  • Cold draughts are eliminated.
  • Pollutants are effectively removed from rooms.
The control unit is usually placed in the attic of the dwelling. it must be easily accessible to perform maintenance, such as cleaning the filters.


Mould Growth 
Mould growth and rot are consequences of poor insulation and/or ventilation techniques that lead to condensation and a build up of moisture on and in fabrics. Mould grows on many surfaces, most commonly on plaster, timber and wallpaper. 

The growth of mould is usually associated with condensation, but it can be caused by other factors, such as rising damp as a result of insufficient damp proof membrane (DPM) or damp proof course (DPC) in the substructure. Mould also grows in cold areas where there is a high level of humidity on a regular basis. The undesirable properties of mould growth in the home include appearance, odour and health considerations.

Mould is made up of fungi, which need a number of conditions to survive. Spore are the first essential element of mould growth. Spores are found in high numbers in the outside air and are easily transported indoors. Fungi require organic matter to thrive. This organic matter is found in the building materials used to construct the dwelling. Although fungi require food oxygen and suitable temperatures to thrive, it is the presence of moisture that leads to rapid mould growth.

Mould can lead to significant damage to indoor furnishings as well as respiratory problems for the people living in a building. it can be removed by cleaning, but to prevent it returning the problem must be addressed at source, by preventing condensation and by installing the DPM/DPC correctly.




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