Home ventilation systems
Home ventilation systems use fans to move air into the house and can provide an alternative to opening doors and windows provided the model you choose is well designed and installed.
Is a home ventilation system right for you?
Home ventilation systems are not a good solution for fixing cold and dampness problems in homes.
If you are trying to make your existing home warmer and drier, your money will be better spent on sorting out your insulation, installing an efficient heating system and tackling dampness problems at source.
Ventilating your home is also important but can be as simple and cheap as opening doors and windows regularly - read more about ventilating your home here.
However, if you have done all these things already, then a well designed and installed home ventilation system can offer you the convenience of providing very good ventilation by delivering required air replacement continuously and independently of weather conditions.
If dampness is your main concern, then address the source of the moisture problem first, before looking at ventilation systems.
Be aware that the performance of ventilation systems can vary widely depending on a range of factors - the type of system and how well it is installed, your type of house and the climate.
Ask your supplier for independent test performance reports for the system they are proposing. You should also get a ‘no questions asked' guarantee of performance that includes removal of the system if it doesn't work and repair of all damage to your home - e.g. holes in ceilings should be fixed to prevent undue air leakage.
Can a home ventilation system heat your house?
A ventilation system is not a heating system.
Efficient heating systems include heat pumps, wood burners, wood pellet burners and flued gas heaters. Learn more about these heating systems.
Despite claims from some suppliers that ventilation systems can deliver "free heat" from the roof cavity, there is a lack of independent evidence to prove that there are significant home heating benefits from using these systems, especially in the coldest months of the year when you most need the heat.
Monitoring of two houses in Dunedin carried out by the University of Otago and BRANZ Ltd indicates that on winter days when heat would be most needed there may be little "free heat" available in the roof cavity. [1]
The heat available in the roof cavity will vary widely from house to house depending on the climate, time of year, time of day, weather conditions and sun exposure of the roof surfaces. On clear nights, roof cavities can become colder than the outside air.
In summer, roof cavities quickly become excessively hot, and systems without a summer bypass will have to be turned off for extended periods in order to avoid overheating your house.
Contaminants from the roof space
Another thing to consider, is that roof spaces often contain vermin, mould, and dust. As suppliers have yet to prove that home ventilation system filters are effective at reducing these contaminants to safe levels, EECA recommends that the supply air of home ventilation systems be sourced from the outside, not from the roof cavity.
The New Zealand Building Code requires homes to have means of ventilation with outdoor air to maintain air purity. Ventilation systems that draw air from the roof space and not directly from outside do not comply with ventilation standard NZS4303:1990 "Ventilation for acceptable indoor air quality" and cannot be used to comply with the Building Code Acceptable Solution for ventilation.
Types of home ventilation systems
There are two types of home ventilation systems commonly available in New Zealand:
- Positive pressure roof cavity ventilation systems
- Balanced pressure heat recovery ventilation systems
Positive pressure/ roof cavity heat transfer ventilation systems
Positive pressure/roof cavity ventilation systems are the most common systems marketed.
They force filtered air from the roof cavity into the house through a single, or multiple, ceiling vents. This pushes air inside the house out through gaps around doors and windows and other leakage areas.
How well these systems are able to ventilate the whole house depends on the performance of the fans, distribution of the ceiling vents throughout the house and the building air tightness.
In draughty houses the ventilation system will be unable to force the air into each room of the house. If the roof cavity is not properly sealed from the inside of the house (for example if you have recessed downlights) the system will short-circuit, i.e. indoor air will migrate into the roof cavity and be pumped back into the house again.
Air quality
Roof cavities are often polluted with dust, mould and vermin. To reduce the amount of contaminants in the air supply, positive pressure/roof cavity ventilation systems are usually fitted with filters. The quality of the air entering the house is highly dependent on the filter type and cleanliness. As suppliers have yet to prove that home ventilation system filters are effective at reducing these contaminants to safe levels, EECA recommends that the supply air of home ventilation systems be sourced from the outside, not from the roof cavity.
Balanced pressure heat recovery ventilation systems
Balanced pressure heat recovery ventilation systems are particularly suitable for homes in colder areas of the country, if they are already well heated and if they are reasonably airtight.
These systems have two fans for two separate air streams. One fan supplies fresh outdoor air into the house through several ceiling vents, while the exhaust fan extracts an equal volume of air from inside the house and discharges it to the outside. The exhaust air stream transfers some of its heat energy into the incoming air in a heat exchange unit, usually located in the roof cavity.
Some products include additional features to utilise heat in the roof cavity when it is available on sunny winter days, or to avoid the incoming supply air being warmed up by the exhaust air during summer nights to assist with cooling.
To ventilate effectively, the air must be able to flow freely between the supply and exhaust vents inside the house, requiring gaps around or vents in internal doors. Care is also required to ensure that a ‘short circuit' route is not created between the supply and exhaust vents which would result in areas of the house being bypassed by the system.
In winter, the heat exchanger transfers a portion of the heat in the warm exhaust air to the cold supply air, thus reducing the heat loss associated with the ventilation. The overall effectiveness of the heat exchanger depends on having an airtight house, to ensure that uncontrolled ventilation losses are minimised so that almost all ventilation air passes through the heat exchanger.
More information
- See the Consumer NZ report on home ventilation systems
[1] Inga J. Smith, Bonar R. Carson, and Mark R. Bassett. 2008. Is there free heat in the roof spaces of New Zealand houses? Mechanical ventilation systems and heat transfer. Paper presented to 3rd International Solar Energy Society Conference - Asia Pacific Region (ISES-AP-08) November 2008
