This blog follows on from our previous blog on Ventilation and Indoor Air Quality.  If you have not already read Part 1 we suggest you do so, but here is a bit of background for those who have already read it.



Recent NZ based research asked if new houses in New Zealand that rely on natural forces for ventilation are healthy places to live?

Historically NZ houses have provided poor indoor living conditions – Has it improved? A survey of fifteen free-running houses (windows/draughts/natural forces used for ventilation) built after the year 2000 in the three climate zones of the New Zealand Building Code confirms the results of simulations, that ‘indoor air quality and thermal comfort are generally poor’.

In other words, the reliance on natural forces (free flow ventilation) for the provision of healthy homes in New Zealand climates is not enough. The research confirmed that ventilation was crucial for the quality of the indoor environment.

Source:  Healthy and affordable housing in New Zealand: the role of ventilation,,Rosemeier, Kara, University of Auckland

“Without mechanical help the wind is the main driver for air exchange, but wind is not steady…… you don’t know where is what at any given time so whether opening a window will affect how many cubic metres of air exchange. Without constant pressure you don’t know what’s happening really.”


Source: Rosemeier, Kara,houseplanninghelp.com/hph096-is-natural-ventilation-fit-for-purpose-with-kara-rosemeier-from-passive-house-academy-new-zealand)


While there are various approaches that would improve indoor air quality, dilution is considered the best solution.

When you bring in about 30m³ of fresh air per person every hour then you can be reasonably certain that your contaminants are diluted to a level where they are not harmful.”

Source: Rosemeier, Karahttp://www.houseplanninghelp.com/hph096-is-natural-ventilation-fit-for-purpose-with-kara-rosemeier-from-passive-house-academy-new-zealand.


Intrinsic Link to Energy Consumption

Whilst ventilation has a critical role in the provision of good indoor air quality, it is intrinsically linked to the energy consumption of houses. While a higher rate of fresh air supply will dilute airborne pollutants, without effective heat recovery, it will result in excessive energy needs for the provision of thermally comfortable indoor spaces..

An Efficient Heat Recovery System is Best

A viable ventilation strategy must ensure the comfort of building occupants and healthy indoor conditions, whilst minimising energy consumption.  In many New Zealand homes, achieving a thermally comfortable indoor situation is likely to impact heavily on energy consumption, if the energy efficiency of house is not improved.  Therefore, well designed houses, with sufficient insulation levels and no, or minimal, thermal breaks, must be considered to provide an environment conducive to reducing heat loss through the building envelope.

The research found that with well-designed forced ventilation systems, and an airtight building envelope, lower air change rates were sufficient to dilute indoor air contaminants to levels below concern.  Plus, an efficient heat recovery ventilation system was found to be the most advantageous option. It was also found to be ‘economically viable under plausible assumptions for economic boundary conditions’. 

Source:  Healthy and affordable housing in New Zealand: the role of ventilation, Rosemeier, Kara, University of Auckland


The installing of a forced air system however does add to the cost of housing, and the quality and purpose of various types of ventilation systems varies.  Full consideration of the cost versus benefit is required at the design stage, along with a well thought out house design, capable of minimising costs and maximising comfort.


Extracts from Some Interesting European Reports on Air Quality, and Good Arguments for Adequate Ventilation!

In 2000, the World Health Organization (WHO) identified a right to healthy indoor air, based on universal human rights and ethical principles (WHO, 2000b). Despite this, very few legally binding threshold values for indoor air pollutants in residential buildings exist internationally or nationally.

The WHO considered air temperatures below 16°C at relative humidities above 65% as increasing the risk of respiratory, cardiovascular and allergic diseases (WHO Regional Office for Europe, 1987).  Their report highlights the existence of these conditions in several bedrooms in the areas (in Europe) under study.

To reduce dust mite populations it is recommended that the water vapour content in air is kept below 7 g/kg air for “some time” in winter (refer and (European Collaborative Action Indoor Air Quality and its Impact on Man, 1992)). Their report suggested that spore sampling undertaken in those houses surveyed had the threshold exceeded for at least 50% of monitored week in either the bedroom or living room.

A further Blog will cover in detail the MHRV Ventilation Systems which continuously provide the fresh filtered air, warmed by the stale air being removed.

In the meantime, if you would like to find out more about about building for health, comfort and energy efficiency, grab our FREE guide here.


Ventilation systems

Mechanical Heat Recovery Ventilation (MHRV)


MHRV is a system that uses the heat in stale exhaust air to preheat incoming fresh air (with the aid of a fan).   

This reduces the energy required to bring outside air up to ambient room temperature.

Diagram: passivehouse.com.au


Like the human respiratory system, this exchange of air is performed in a single area of the home, your ventilator core (the lung).

Outgoing air and incoming air never mix in the heat recovery process, they simply pass in separate channels in the ventilator core, allowing an exchange of heat through conduction.

Stale air containing any contaminants, spores or allergens is forced outside and fresh filtered air is allowed in, improving air quality and a healthier living environment.

Forced ventilation breaks the reliance on wind and occupant behaviour to provide good indoor air quality. A fan becomes the main driver for ventilation.

In a typical situation fresh air is supplied to bedrooms and living areas, and exhaust air extracted in kitchen, toilets and bathrooms.  Most sensible heat can be recovered from outgoing air, and transferred onto supply air, when an efficient heat exchanger is used. Heat is conducted through a thin plate that keeps the airways separate.


Efficiencies of MHRV

The ‘efficiency rate’ of an MHRV unit determines how much energy will be saved by using that particular device. Although it requires the operation of a fan (hence the ‘mechanical’ word in its title) on a continual basis, the energy recovered from the inside air is many times that of the energy required for the fan.

Typical efficiencies range from 55% to 75%, but some extremely efficient European models are rated much higher.  Whilst the European units are manufactured overseas, they are easily available through NZ distributors.

Allergen Control

Most allergen causes are removed with the installation of the highly efficient heat recovery unit.  This unit cleans and circulates the air in a room providing fresh pollen-free and dust-free air once every three hours and does not release ozone back into the air from the filtration process.   This is great news for everyone, especially for those with allergies or respiratory.  A German Zehnder system, for example, can remove 98% of pollen from your air and the filters at the entry of the controlled air system block particles up to 3 times thinner than a human hair.

Don’t confuse HRV, the system, with PPV systems

Whilst each has their own uses and benefits, 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.    

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 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

However, the downside 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 temperature.  In order 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.  In order 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.

The two systems really have two separate purposes, PPV is specifically for ventilation where condensation is an issue, while HRV (the system) is focused more on controlling your environment with regard to heating, cooling and air filtration.  If you are looking at a applying these key principles into the design of your home, we will advise on the best options and ensure you get the real deal.  Importantly we will look at the whole package, including house design and its thermal envelope.

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 be energy efficient. As an example: a mechanical ventilation system can double as a comfortable and economical heating system, only when heating requirements are kept to a minimum. Proper ventilation concepts for each New Zealand climate zone need to be considered alongside the house design in terms of the thermal performance of the building envelope.

For more information on MHRV and passive house you can download our free brochure on ‘Building For Health, Comfort and Energy Efficiency’ here.  You can also contact us to discuss your project and these key principles in more detail.