The term HRV stands for Heat Recovery Ventilation System, and should not be confused with company names or other systems that are not true heat recovery systems. Many ventilation systems are better described as, positive pressure ventilation (PPV) systems. Whilst both improve the health of a home, their purpose, scope and how they work are very different.
How Does PPV Work?
PPV’s work by drawing the air “stored” in the ceiling and pushing it through ducts into the various rooms in a home. On sunny days, the air inside a roof space is generally drier and warmer than the air circulating inside the house, but there is a limited supply of this air in the ceiling space. By pushing this warm air into your home, the system simultaneously forces out existing damp air, thereby reducing condensation.
The slight positive pressure sustained within the house helps to guard against infiltration of environmental pollutants. Massey University research (Fortes, Phipps, & Fleming, 2003) found a slight reduction of relative humidity in some houses where these systems were used and resulting positive impacts on mould occurrence.
diagram: Moisturemaster.co.nz
Cause & Effect of PPV: Ventilation Management
A potential downside of PPV is that moist interior air is typically forced out through cracks and gaps in a non-airtight building envelope, where it may contribute to moisture build-up in building cavities. To try and balance the pressure, positive pressure systems are sometimes combined with an intermittently running extraction fan, or passive vents in kitchen or bathrooms. Without a successful balance, room air may also get pushed into the attic, where it is then “recycled”.
A drier home is a warmer home, but as the available warm air in the roof space is pushed into the house, it is replaced with cooler air from outside, which in turn, is pushed into your home, once again reducing the internal temperature. For the PPV or forced-air system to continue to provide adequate warm air into your home, it needs to be heated which requires further heating and energy costs. University of Otago research (Fitzgerald et al., 2011) raised serious doubts regarding the suitability of positive pressure ventilation systems that utilise roof air for the provision of free heat.
The air in any roof space isn’t fresh or healthy as it is generally full of dust mites and spores. To clean this air, the system needs to filter it prior to it being introduced to your home, this may not be to the same extent as those introducing fresh air directly from outside. Regardless, the filter needs to be extremely good.
Two Quite Different Purposes
1) PPV is specifically for ventilation to manage condensation and moisture in a home.
2) HRV (the system) is focused on controlling your environment – providing a comfortable healthy the living environment with minimal energy consumption.
In Conclusion
While ventilation systems have a crucial role in the provision of acceptable indoor air quality, it needs to be part of a comprehensive design standard to reach their full potential and simultaneously be energy efficient.
If building new, consideration of the design, and thermal/airtight envelope will provide an optimal environment for MHRV efficiency, making this a sensible choice to enhance all aspects of interior comfort and wellbeing.
However, in an existing home with low levels of air tightness and moisture issues then PPV could be the solution.
For more information on MHRV and passive house you can download our free brochure ‘Homeowners Guide to Creating an Energy Efficient or Passive Home’. You can also contact us to discuss your project and these key principles in more detail.
