• Top 10 Mistakes

    Top 10 Mistakes

    when designing MVHR Systems

Apart from the Passivhaus Designer Course, very little training is available in the UK on how to design MVHR systems. Architects and installers usually rely on the MVHR suppliers to design their ductwork and components. It is not surprising that the quality of the designs varies and some common mistakes appear frequently. Please see below the top 10 mistakes when designing MVHR systems:

  • paul heat recovery scotlandWe are co-operating with a number of architects who are thinking of services and duct routes right from the initial design stage. When it comes to the design of a MVHR system, this can have a major impact on how complex the ducting system is and even if one or two ventilation units are needed.
  • paul heat recovery scotlandStructural beams are often an obstacle in the service zone. It is advantageous to co-ordinate the structural design right from the start with all larger services, such as ventilation ducting. Sometimes it is necessary that services penetrate through beams.
  • paul heat recovery scotlandMVHR units need to be easily accessible for filter changes and maintenance. If they are positioned in loft spaces with no access path, it can become an adventure for occupants as they balance over joists to avoid falling through the ceiling. Also the specified service space around the MVHR unit needs to be maintained.
  • paul heat recovery scotlandSome of the ducting components need more space than others, e.g. air manifolds and silencers. The MVHR unit needs to be positioned in such spaces that these items fit into the ceiling or in other service areas. It is therefore important to incorporate the joist layout into the final design drawing.
  • paul heat recovery scotlandMVHR design drawings with too little detail leave an open door for misinterpretation on site. Duct diameters, silencers, type of terminals and other components should always be shown and narrated, as well as their position within the structure.
  • paul heat recovery scotlandIf supply and extract ducts are positioned in spaces that are cold, the efficiency of the system will suffer. It is recommended to keep these fully within the thermal envelope, e.g. under 100% of the loft insulation. Otherwise sufficient duct insulation needs to be applied; we recommend 100mm of ductwrap or similar. Intake ducts also need insulated if situated in cold loft or eave spaces as these can heat up with solar gains and eliminate the cooling effect of the summer bypass.
  • paul heat recovery scotlandIntake and exhaust ducts within the thermal envelope should be insulated with vapour proof material and kept as short as possible. The recommendation for Passivhaus buildings is max 1.5m each. Directional combination grilles help to keep these duct lengths short.
  • paul heat recovery scotlandThe position of the external intake and exhaust terminals can vary. Although a distance of 2m is mentioned in the BRE Digest 398 document, even at 500mm distance horizontally very little cross-contamination occurs in most cases. It should however, be avoided to place the intake close above the exhaust as the exhaust air is slightly warmer than the surrounding atmosphere (especially in frosty weather) and will rise into the intake path.
  • paul heat recovery scotlandThe external intake of the MVHR system should not be situated in elevations which are exposed to strong wind or pollution, e.g. gas flues or chimneys. For roof cowls the prevailing wind direction should also be regarded to avoid contamination from flues or the MVHR exhaust.
  • paul heat recovery scotlandWe often find that the extract terminal in wet rooms is positioned close to the door and not near the shower or bath. Please always consider the ventilation path to avoid short-circuiting of extract ventilation.