• Top 10 mistakes

    Top 10 mistakes

    when specifying
    MVHR systems

Not too infrequent we come across poorly performing installations which cause problems for the occupants. We have listed below the 10 most common issues with the specification of MVHR systems.


  • paul heat recovery scotlandCalculating the air flow values based on air exchange rates (room volumes) only, is a too simplistic approach, which does not take the use and occupancy of buildings into account. Unfortunately this is fairly common in the industry. We recommend to use the time proven PH calculation method, which takes the overall and per room occupancy, the type of extract room and the room volumes (capped off at 2.5m) into account.
  • paul heat recovery scotlandPVCu ducting is the most common used ducting system for domestic installations as it is relatively cheap and easy to work with. Unfortunately it has some major downfalls, such as high resistance due to tight bends, which increase noise and power consumption. Rectangular PVC ducting is particularly bad for the air flow. It is also difficult to get permanently air tight. On top of all this PVCu has questionable hygienic properties as over time growth of mould can manifest inside (see image). We recommend to use semi-rigid ducting with anti-bacterial lining or metal ducting.
  • paul heat recovery scotlandIn order to keep the noise levels and energy use low, best practice it to keep the air velocities at or below 2m/s in all peripheral ducting. If the duct diameters are too small or there are too few semi-rigid ducts being used, the fans have to work much harder to overcome the increased resistance, which makes the whole system more noisy. We also recommend to use 78/90 mm semi-rigid ducting, instead of the usually used 63/75mm type, as it has more capacity.
  • paul heat recovery scotlandCompromising on silencers/ attenuators is one of the most noticeable mistakes. Appropriate machine noise silencers for the supply and extract should be standard. An exhaust silencer can be added for larger systems if outdoor noise is an issue. Branch ducting systems should also have cross-talk silencers for most supply rooms and those extract rooms where necessary.
  • paul heat recovery scotlandAlthough there is nothing wrong with installing roof cowls, we want to issue a word of caution before specifying these: Appropriate flashings need to be addressed, as well as drainage of possible water ingress. Sources of air contamination (e.g. SVPs, flues) need to be avoided and the intake cowl needs to be above the maximum snow line. Slate vents are typically not suitable as they have a too small free area and can be blocked by snow. We recommend to specify wall grilles as the preferred option.
  • paul heat recovery scotlandThe intake and exhaust both carry cold outside air. If these are placed within the thermal envelope the ducts need to have vapour proof insulation. Insulated flexible ducting is therefore not the right choice as they can attract condensation and get soaked. We recommend to use EPP foam ducting, ideally with 43mm insulation thickness to minimise heat losses. Alternatively closed cell insulation can be applied to the intake and exhaust ducting, e.g. Armaflex, class O.
  • paul heat recovery scotlandFinding an MVHR unit with a good specification is not that easy: Promoted heat recovery rates, even those in SAP ratings have to be taken with a pinch of salt, as there are ways to make a poorly performing system look good on paper. Besides the heat recovery rate and specific fan power you should look out for the insulation thickness of the unit, volume flow constant operation, active frost protection, quality of filters, noise levels and the controls of the system. Typically Passivhaus certification figures give a more accurate representation of the performance.
  • paul heat recovery scotlandPlease don't overlook to specify a waste water connection for the condensation run off. Instead of a normal water filled trap, a waterless (dry) trap should be used, which cannot dry out in summer. Some units with enthalpy heat exchanger have got no condensate pipe. Also please think of the appropriate cables for the controls and their position.
  • paul heat recovery scotlandWhole house ventilation lives by slow air movements through the dwelling. To enable this, small transfer openings need to be provided for, which are typically 10mm undercuts underneath the doors. The height of the gap needs to be specified. Some rooms do not need any gap at all. Such transfer gaps are also necessary for normal intermittent extract fans.
  • paul heat recovery scotlandThe right choice of room terminals, often referred to as room valves is important. E.g. in kitchens the extract terminal should always have an in-built filter, which prevents grease from entering the ductwork. We recommend to specify a terminal with visible filter, as hidden filters are often forgotten about (out of sight - out of mind). Wall supply valves in many cases need to be different from ceiling valves as they have a different air flow pattern. Please avoid the cheap PVC disc valves, which cannot be properly locked. Customers should be consulted about the style of room terminals - there is a number of good quality products available.