PIV Positive Input Ventilation

Over the last few years I have come across many poorly installed or laid out PIV units in damp mouldy properties. So I tend not to recommend them. Not because the theory doesn’t have merit, just the installation and maintenance.


If you live in a polluted zone, or suffer from hay fever, then you should consider the merits of filtered air.

If you have a heavily occupied or polluted space, such as an office or factory, then the push – pull ventilation of PIV working together with continuous flow is essential.


Many consider PIV to be:

  • Expensive compared to their sister product, the continuous flow fan (which is effectively a “negative output”, with no filtration),
  • Cause heat loss in the centre of a property,
  • Unattractive, at least in the case of horizontal units, typically installed in flats.
  • Hard to install or for an owner to use to provide balanced ventilation across a property,
  • Hard, if not impossible for occupiers to control, there will be times when there is greater need against less need, when thermal loss could be saved.


  • There is relatively little thermal gain from PIV units in a flat, a sun heated loft may have thermal gain, I am checking the claimed 3°C thermal gain.
  • Have the following as a minimum:
    • A good extractor fan in any internal wet room, that is bathroom, kitchen, toilet, possibly utility room for drying clothes,
    • Trickle vents is all rooms, evenly distributed, always open,
    • Similarly even gap under the door.
  • Check inflow and outflow air speeds, about 5 l/s per person and are identical (or make sure installer does, under your supervision),
  • Check temperature or incoming air is similar to outside air temperature (using a laser thermometer, check annually in winter), if not then it suggests leakage,
  • Use a vacuum cleaner to clear dust/pollutants from filter at least once a year.
  • Monitor internal surface relative humidity with a datalogger, see https://surveyor.tips/datalogger

This post is to be continued.

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