Clare Parry explains what mechanical ventilation is and why we might want to use it as our homes become better sealed.
Mechanical ventilation is not something many Australians would be familiar with in their homes, but it is something many would have experienced in other types of buildings, such as offices and hospitals. Mechanical systems are often regarded as unnecessary for dwellings, but, as our Star ratings encourage us
towards better-sealed dwellings, these systems become important. Here I hope to outline the reasons why a truly comfortable and efficient home would include mechanical ventilation with heat recovery (MVHR, also called heat recovery ventilation, or HRV systems).
Air infiltration in homes can account for a significant component of the total heating and air conditioning loads in a building (around a third to a half), and this load can be particularly significant in low-energy buildings where all other loads have been reduced.
At a time when housing sustainability is on many householders’ minds, the best way to increase energy efficiency is to take advantage of the basics of building physics by making improvements to the envelope. This involves using basic passive solar design principles in conjunction with insulation and building sealing.
In MVHR systems, heat is transferred between the air flows in and out to greatly reduce thermal flows while allowing high levels of ventilation.
One of the great things about sealing up a building is that you gain control over your internal environment—you can open the
place up when you want, but when ambient conditions are not suitable the home can be closed up and the internal conditions will be more stable. Ideally, a well-designed home in most parts of Australia would not require heating or cooling at all.
However, sealing a building does mean you need to consider how the building is then ventilated.
What we refer to as ’natural ventilation’, and what the majority of Australian homes rely on for fresh air, is a combination of open windows and imperfections in construction (gaps and holes). This method of ventilation is largely imperfect; good natural ventilation relies on natural variations in pressure and
temperature, and the best designs use crossflow and stack principles to induce air flow into and through a building.
Relying on natural ventilation to provide adequate conditions for good health, as well as comfort, is likely to be insufficient in a well-sealed home. This is because the amount of air infiltration relies on a number of factors, including the time windows are open, openable area and prevailing weather conditions.
Mechanical ventilation provides a way to address this, using fans to move air into and/or around a building. A number of studies2 have also shown that the use of MVHR can be more efficient, in terms of reduced energy use and the resultant carbon emissions, than relying on natural ventilation. As with any system, appropriate system selection and design is key.