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Why MEV & d-MEV are superior to PIV

By Kiran Sagoo on Fri 11 November 2022

Why MEV & d-MEV are superior to PIV

With the Covid-19 pandemic came a heightened awareness of just how important ventilation is to our daily lives. Schools, in particular, were highlighted as a major focus for the quality of their ventilation and were urged to open windows, even during winter – all to reduce the spread of the virus. 
Yet even without considering virus transmission, badly ventilated spaces can cause or contribute to other health problems, including asthma, allergies, and a weakened immune system. In addition, if no action is taken to fix or counteract poor ventilation, mould can grow - causing more health issues and can also have a negative impact on concentration levels. 
Many businesses, institutions, and private homeowners are examining how they can improve ventilation. However, before investing in ventilation, they should consider how different systems work and what the best option is for their needs. 

 

What is PIV? 

Positive input ventilation (PIV) is usually located in a loft and works by pumping and circulating air from the outside or the loft into the house and pushing stale air out of gaps and cracks in the fabric of the building – this is done by the pressure that comes from the fan inside the PIV unit.  

 

Why is PIV an outdated and ineffective technology? 

PIV systems were developed in the 1970s and are now considered outdated and much less efficient than modern ventilation methods. Simply put, PIV has been surpassed by newer systems, which deliver much better results. 

PIV systems’ reliance upon passive natural leakage from the building is one reason it’s considered outdated – especially as homes are now becoming more airtight. More modern systems actively extract stale air out of the building and use smarter technology. Also, if the householder opens any window this immediately changes the airflow in the dwelling as to how it was designed.   

 

What is d-MEV and MEV? 

We now have centralised Mechanical Extract ventilation (MEV), as well as decentralised Mechanical Extract Ventilation (d-MEV).  

Both extract moist, stale air from wet rooms (bathrooms, kitchens, utility rooms and en-suites). They extract air continuously from the inside and release the air to the outside with the use of ducting. 

The key difference between MEV and d-MEV is that MEV systems have a central unit, usually positioned in a roof space or cupboard which connects all extractor vents together. However, d-MEV is installed in each room with ducting that links directly to the outside (not connected to a central unit).  

 

Why are d-MEV and MEV superior to PIV? 

When putting PIV up against MEV/d-MEV, it’s clear MEV/d-MEV is a better solution to ventilation: 

  • PIV needs natural ventilation leakage, which is unlikely to exist in airtight modern homes 
  • PIV is no longer recommended as a preferred method in government regulations Part F document whereas MEV/d-MEV is 
  • Many manufacturers say that you need extractor fans alongside PIV 
  • D-MEV is installed directly in wet rooms, which helps prevent the moisture from spreading into other rooms and offers humidity control at the source (not the case with PIV) 
  • Some PIV systems shut off at 27 degrees, after which the building goes completely unventilated. And with climate change pushing global temperatures up, this is an obvious limitation 


Although MEV/d-MEV are great solutions to ventilation in wet rooms, many are now adapting to Mechanical Ventilation with Heat Recovery (MVHR) which goes further than MEV and d-MEV by making good use of the stale air being extracted. Since this waste air is warm, MVHR systems use it to heat the fresh air coming into a room. At a time when energy prices are soaring, this kind of cost-effective system is invaluable. Of course, it’s also much better for the environment due to the units being energy efficient. 

Home ventilation costs vary according to the solution you want to opt for. The size of your house, the age of the house, its structure, and many other variables are considered. If you are interested in knowing more about what we can offer, contact us and one of our experts will be happy to discuss your requirements.