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Ventilation systems

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Stop mould and “crying windows”.

If your home suffers from streaming windows, mouldy curtains and a damp unhealthy feeling, you need to improve your ventilation. We explain how to stop the sources of moisture and what to look for in a ventilation system.

Our database lets you compare the specifications and price of most ventilation systems on the market but we don’t currently test ventilation systems.

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Controlling condensation

Winter condensation is a widespread problem.

In winter we spend more time indoors, creating moisture from cooking, cleaning, washing and even breathing. When we're out of the house we leave it closed up for security.

Insulation adds to the problem. We trap the heat of living areas by keeping doors shut and using heavy curtains and carpets.

This all comes at a price. Warm air holds water better than cold air. Because it's sealed in, the moisture builds up then condenses on cold surfaces such as windows and walls.

The solution is simple – better ventilation. Making it happen is less simple.

If you're living in a draughty old Victorian villa, you shouldn't have too much of a problem with ventilation. But modern houses are much more airtight, so natural ventilation is minimal.

Extra heating is part of the solution, combined with water extraction near the sources. Rangehoods intercept steam from the kitchen; extractor fans are effective at drying out bathrooms. You could also consider a dehumidifier. While these can help control condensation, they’re expensive to run (up to $2.50 a day), often noisy, and must be run constantly. With a dehumidifier you are controlling the symptoms and not dealing with the problem. While not the ideal solution, dehumidifiers have their place.

An automatic ventilation system is a better way of controlling condensation. Whichever way you attack the problem, remember it's even more effective if the amount of water released into the air is reduced.

Sources of moisture

Activity Litres
Cooking 3.0 / day
Clothes washing 0.5 / day
Showers and baths 1.5 / day (per person)
Dishes 1.0 / day
Clothes drying (unvented) 5.0 / load
Gas heater (unflued) Up to 1.0 / hour
Breathing, active 0.2 / hour (per person)
Breathing, asleep 0.02 / hour (per person)
Perspiration 0.03 / hour (per person)
Pot plants As much as you give them

Stopping the sources of condensation

Simple ways to reduce the amount of water released into your home.

  • Fit extractor fans over the cooktop or stove, and in the bathroom. They must be ducted to the outside.
  • Always use close-fitting lids on pots when cooking.
  • Vent the clothes dryer to the outside.
  • Close doors when cooking, showering or using the dryer, to limit the spread of moist air.
  • Avoid using unflued gas heating.
  • Limit the number of pot plants in the house.
  • Check that the ground under the house is dry. If it's wet, cover with polythene (if this is feasible), taping the joints, and ensuring a tight fit around piles. Check that drainage systems are diverting water away.
  • Fix any leaks in the roof or around windows.
  • Remove open vented downlights or replace them with new downlights that don’t leak your warm damp air into the ceiling.

Automatic ventilation systems

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.

“Heat recovery” units

Ventilation system ducting.
Ventilation system ducting.

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.


We received information on 804 ventilation systems in our 2017 appliance reliability survey.

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