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What is MVHR heat recovery ventilation?

Definition of MVHR

MVHR (Mechanical Ventilation with Heat Recovery) provides fresh filtered air into a building whilst retaining most of the energy that has already been used in heating the building. Heat Recovery Ventilation is the solution to the ventilation needs of energy efficient buildings. Mechanical ventilation with heat recovery (MVHR), heat recovery ventilation (HRV) or Comfort ventilation are all names for the same thing. A heat recovery ventilation system properly fitted into a house provides a constant supply of fresh filtered air, maintaining the air quality whilst being practically imperceptible.

MVHR works quite simply by extracting the air from the polluted sources e.g. kitchen, bathroom, toilets and utility rooms and supplying air to the ‘living’ rooms e.g. bedrooms, living rooms, studies etc. The extracted air is taken through a central heat exchanger and the heat recovered into the supply air. This works both ways, if the air temperature inside the building is colder than the outside air temperature then the coolth is maintained in the building.

At what level of airtightness is MVHR worthwhile?

Although MVHR can be installed in any building, there is a rule of thumb that its use is not justified unless the air permeability of the thermal envelope is at or below 3 air changes per hour when tested at 50 Pascal (equivalent approximately to 3 m3/m2.h @ 50 Pa for average dwellings).

Recent research by the Passivhaus Trust has shown that MVHR ventilation could be used in less airtight homes (eg even a fairly draughty home of 9 @50Pa) as it would still have lower carbon emissions rate than natural ventilation. However, in very draughty homes (eg higher/ worse than 9 @50Pa) the efficacy and efficiency of an MVHR will decrease.

If there is no temperature difference between inside and outside then there is no energy to be recovered. When you have a significant temperature difference between inside and outside then the thermal stacking effect (or chimney, hot air rising effect) becomes the driver that will cause your building to ventilate itself through the leakiness of the envelope. If it is windy then you will have wind as the driver ventilating your house through the envelope leakiness. By using intermittent extract at point of need, e.g. shower, kitchen, you will simply reverse some of the leakiness.

Tackling condensation

If your building has poor insulation/thermal bridging and or other damp ingress due to for example lack of proper damp proofing, then it can be tempting to look for technological solutions. In such a situation you will have problems with condensation and probably mildew where the condensation is forming. Yes heat recovery ventilation can help reduce these symptoms; however, our advice would be to sort out the problem not cure the symptom.

The importance of good MVHR design

If you are intending to fit MVHR heat recovery ventilation into a new build or refurbishment then it should be considered from early in the planning stage. For any MVHR heat recovery ventilation system to give its best performance the devil is in the detail. System design will make the difference between a highly efficient system working un-noticed in the background and one that is constantly in the awareness of the inhabitants and wasting both heat and electrical energy. These systems necessarily require ducts to be run through the building and the routing is important both from the point of view of the building and the efficiency of the ventilation system. Indeed, layout of rooms can be influenced by ventilation needs.

The installation of MVHR in a building will also affect the heating system design. One of the effects of heat recovery ventilation is to equalise the temperature throughout the building. Therefore it is important to prioritise the heat supply to the rooms that should be warmer. If the building is of sufficiently high standard then it might be only necessary to put heat directly into certain rooms.

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