Home ventilation systems

Home ventilation systems use fans to move air into your house and can provide continuous ventilation regardless of the weather and without the need to open doors and windows. This helps to maintain air quality and remove moisture from everyday living activities. 

Ventilation systems by themselves are not an effective way to heat your home or fix dampness problems at their source.

Is a home ventilation system right for you?

A well designed and installed home ventilation system can offer you the convenience of good ventilation by delivering required air replacement continuously and independently of weather conditions. Ventilation, whether provided by a ventilation system or by opening doors and windows, is necessary to maintain air quality and remove the moisture we create in our everyday activities.

If dampness is your main concern, then address the source of the moisture problem first, before looking at ventilation systems. Ventilation systems by themselves are not an effective way to heat your home. If you are trying to make your existing home warmer, your money will be better spent on insulation and installing an effective heating system.

Performance

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 modifications of your home.

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 type available in New Zealand. They force filtered air from the roof space 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 struggle 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 older recessed downlights) the system can short-circuit, i.e. indoor air will migrate into the roof cavity and be pumped back into the house again.

Roof spaces are often polluted with dust, mould and vermin. To keep the air supply clean, 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 how often it is cleaned. 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.

Research recommends that you should not install this type of system for heating purposes

University of Otago research shows that the heat available from moving roof space air into your home (as the most common type of ventilation system does) does not provide significant benefits compared with what you need to properly heat a home in winter.

The research also found that pumping air from the roof space into the living area would often push internal temperatures away from the desired level, rather than toward it.

In summer, roof cavities quickly become excessively hot, and systems that pump roof space air into your house without a summer bypass will have to be turned off for extended periods in order to avoid overheating your house.

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. Some of the heat from the exhaust air is transferred to 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 two factors:

  • having an airtight house, to ensure that uncontrolled ventilation losses are minimised so that almost all ventilation air passes through the heat exchanger
  • having a temperature difference between the inside and outside air. The larger the difference the better the heat exchanger will work. In many areas of New Zealand the temperature difference will not be enough for the heat exchanger to make much difference.

Optional electrical heating unit add-ons

Some ventilation system suppliers offer the option of an electrical heating unit add-on, to provide some pre-heating of air coming from the roof cavity when it is cold. These are known as electric in-line duct heaters.

Most electric in-line duct heaters don't have sufficient capacity to meet a home's heating needs.

Like any other type of electrical heaters (except heat pumps), electrical in-line duct heaters are a relatively expensive and inefficient way to heat a home, particularly if you already have a more effective heater (like heat pump, wood or wood pellet burner, or flued gas heater).

Electric in-line duct heaters lose some heat through the ducts, so it's actually more efficient to use a heater directly in the room you want to heat.

 

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