Ventilation systems

If you’re sick of stale air and weeping windows, but don’t know where to start, we’re here to help. Follow these steps, in conjunction with using our database, to narrow your options and find the best ventilation system for your home.

1. Sort out heating and insulation

You should only opt for a ventilation system if you’ve installed floor and ceiling insulation (if possible) as well as a decent heat source. In living areas this should be a fixed, efficient source of warmth, such as a heat pump or wood burner. In smaller rooms, portable plug-in electric heaters are sufficient.

2. Tackle moisture at the source

Identify any sources of airborne moisture, such as bathrooms, then consider installing a shower dome or extractor fan. Also look at fitting a plastic moisture control sheet to stop rising damp, and check if water is pooling under the home from any blocked drains.

3. Time for a dedicated ventilation system?

If problems persist, you can opt for a mechanical ventilation system, but note these cost upwards of $2000 (installed). A dehumidifier is a cheaper stopgap solution, but will cost more to run than a ventilation system.

4. Positive or balanced pressure?

The first thing to consider when looking at ventilation systems is whether you should go for positive pressure or balanced pressure – don’t confuse either with heat transfer systems.

• Positive pressure (also known as forced air) ventilation systems work by blowing drier air into your house from the roof space above the ceiling or, in some types, from outside. They suit older houses with wooden joinery better than modern houses with sealed aluminium joinery (unless windows are opened or additional vents fitted). But blowing cold air in from the roof space in winter will require you to ramp up your heating.
• Balanced pressure (also known as heat exchanger) ventilation systems extract warm, damp air from living spaces and pass it through a heat-exchanger to heat up dry air, which the system brings in from outside. This can fully meet Building Code requirements. They work best in modern homes and don’t have the same unintended cooling effect as positive pressure systems, so choose a balanced pressure system if your home is well-sealed.
• Heat transfer systems aren’t ventilation systems, they simply haul warm air from one room to another (usually from a living area to bedrooms, etc.), but you can add a heat transfer system on to some ducted ventilation systems.

5. Reliability

Our 2017 reliability survey of 804 Consumer members with ventilation systems found DVS and HRV systems are very reliable. Nearly 90% of respondents who owned these brands said their system had never needed repair. The reliability of MoistureMASTER and SmartVent systems was below average at about 70%. These were the only 4 brands with sufficient respondents for us to comment on their reliability and satisfaction.

6. Filters

Ventilation systems filter the air coming from your roof space or the outdoors. There are a wide array of filter types, usually designated by a letter and a number (for example, G4). In general, “F” filters remove smaller particles than “G” filters, so will catch more dust and airborne bacteria. The higher the second number, the more effective the filter – for example, an F7 filter will catch more nasties than an F6 filter. Some manufacturers offer optional advanced filters: carbon filters can be good for removing odours from the air (for example, the smell of everyone blasting their wood burner on a cold winter day), while HEPA filters are claimed to be effective at removing allergens. We recommend going for the standard filter initially, and only adding a carbon or HEPA filter if you find you’re dealing with odours indoors or suffering from allergies.

7. Optional heaters / summer cooling

Some systems offer the ability to add heaters to ducting, which can be useful for positive pressure systems if you’re concerned about cool air from the roof space reducing indoor temperatures. Summer cooling options, where the ventilation system introduces cooler air to the home, are available but can be expensive.

The building code has minimum requirements for ventilation: all the air in your house should be changed every 3 hours. It says the air in kitchens should be changed every hour; in bathrooms every 2 hours.

To achieve this, the code suggests extractor fans in kitchens and bathroom – and open windows elsewhere – to bring in fresh outside air.

But you won’t want to leave windows open while you are away from home or overnight, for security reasons. An option is to have trickle vents inserted into your windows. These can have insect screens and shutters to control pests and the amount of air flow. Examples are at www.joinerydev.co.nz and www.woodman.co.nz.

Once you've taken steps to reduce moisture at its source, an automatic ventilation system is an effective way of reducing condensation. And it's much more convenient than having to open and shut windows.

There are 2 main types of system:

• Positive pressure/forced air ventilation systems work by blowing drier air into your house from the roof space above the ceiling or, in some types, from outside. They suit older houses with wooden joinery better than modern houses with sealed aluminium joinery – unless windows are opened or additional vents fitted.

• Balanced pressure/heat exchanger ventilation systems extract warm damp air from living spaces and pass it through a heat-exchanger to heat up dry air which the system brings in from outside. This can fully meet Building Code requirements. They work best in more airtight, modern homes.

Which system is best depends on the design of your house, its floor area, the location, how much sun the house gets, the type of roof ... even the local climate.

Before you install any system, do some homework. We've outlined below how the different designs work and some of their pros and cons which should help you decide whether your house is suitable for any of the systems available. Consider what you want to achieve against the types of system – and also look for any extra features you might need to meet your particular requirements.

For best results, a system should be designed specifically for your house and your needs. But this can be quite costly, especially if you choose a fully automated system with multiple outlets or other options.

We recommend getting quotes from several installers who are familiar with your local climate.

There is quite a lot of confusion over so called “heat recovery” systems.

The most effective type of “heat recovery” unit is the balanced pressure system (see above), which extracts warm damp air from living spaces and passes this through a heat-exchanger to heat dry air which the system brings in from outside. Inline heaters are not required.

This type of system conforms to the heat recovery definitions of the American Society of Heating, Refrigeration and Air-Conditioning engineers (ASHRAE). Local ventilation standards are based on ASHRAE standards.

For a number of years, the HRV company advertised products which claimed to recover heat from the roof space. In our opinion, these were positive pressure systems. HRV was not alone in this approach; other makers used similar advertising.

Positive pressure systems can undoubtedly extract warm air from the roof space when it's available (on sunny summer days, for example).

But often when warmth is most needed, on cold grey winter days, the roof space is also very cold. A study from the University of Otago shows that in winter the air in the roof space is usually colder than inside the house, and on winter nights can be colder than the outside air.

The HRV company now sells both positive pressure and balanced pressure systems.