MVHR – in 5 minutes

MVHR – in 5 minutes

Mechanical Ventilation and Heat Recovery (typically abbreviated to MVHR) – quite a mouthful to say, but what is it? And why is it becoming more relevant in today’s buildings? 

Why do we need ventilation? 

Imagine making a cup of tea in an airtight room. The air you are breathing slowly turns from being nice and fresh to being laden with moisture from the kettle’s steam, the oxygen in the room slowly depletes as you breath, even odours and volatile organic compounds (VOC’s) found in everyday items begin to build up…

Without ventilation there is no way of replacing this stale air with fresh air. Put simply, ventilation provides the fresh air that is vital for providing oxygen, removing smells and VOCs, and minimising moisture. 

Why not just open a window? 

In two words: heat loss. Heat loss is one of the primary contributing factors to energy use in buildings – which themselves make up a significant portion of our total energy use and therefore contribution to climate change. Heat loss primarily occurs though;

  • intended ventilation such as opening windows and mechanical ventilation 
  • air infiltration e.g. unintended air leaking through the building envelope 

Whilst historic buildings were typically so ‘leaky’ there was not necessarily the need for additional ventilation, this also meant a vast amount more heat loss through draughts. Modern building methods are resulting in increased levels of airtightness. This is great for reducing heat loss, but means considered ventilation is even more important to ensure good air quality. This has lead to a building mantra of ‘build airtight, ventilate right’.

What does MVHR do? 

In a snapshot MVHR does what it says on the tin i.e. it mechanically ventilates and it recovers heat. MVHR mechanically ventilates your home using ducts to extract air, typically from serving rooms such as bathrooms and kitchens, and supply air to the ‘habitable’ spaces such as living rooms and bedrooms. This ensures that ‘waste air’ is taken out the home and fresh air is brought in to replace it. 

It recovers the heat from this waste air through a specially designed unit so that heat from waste air is recycled into the incoming fresh air thereby both preventing wasted energy by recycling the heat within the waste air and providing habitable rooms with fresh, nicely pre-warmed air. 

An additional benefit is that MVHR units filter all the incoming air which is ideal for allergies and asthma so you can ensure your fresh air really is just that – something which cannot be said for opening a window, especially within urban areas. 

Things to consider

At Grainge Architects, we have a great understanding of how to design a building that incorporates MVHR to great effect. Having said that, a ventilation specialist will always be involved in the specification, design and commissioning of MVHR units and this would often be done by a project’s mechanical and electrical consultant. Key elements to consider to maximise the benefits of MVHR are: 

  • Air tightness: the greater the airtightness, the lower the heat loss  
  • Location of MVHR unit: well-located for efficient duct runs and on an external wall will maximise a unit’s efficiency 
  • Duct runs within floor buildups: consideration is needed for how the ducts are accommodated either underneath or between structural joists. There are also options on the type of duct to use (circular vs rectangular, rigid vs semi-rigid, plastic vs metal etc). 

Summary: Part of a passive approach for healthy, comfortable, AND low energy buildings. 

A well-designed MVHR system improves air quality, thermal comfort, and heating efficiency. Whilst originally most often used in commercial settings or high-performing, low energy houses (including certified passive houses), MVHR is becoming increasing common place as we strive to create comfortable, healthy buildings, that address energy use as part of a considered response to climate change. 

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