Very common in NZ in existing homes, positive pressure ventilation systems work by taking air from the roof space and filtering and pumping it into the main living areas via ducting and vents or self-contained ceiling vents that have their own built-in fans.
PROS
- Lower initial costs – they’re relatively cheap to buy and don’t tend to need lots of ductwork.
- Easy installation – everything can be mounted in the roof space with a power source and a duct through the hallway ceiling or other central location. You can also have a second duct to bring air in from outside rather than taking it from your roof space.
- Cheap to run – run by small fans, they don’t add much to the power bill.
CONS
- Additional heating costs – positive pressure systems are cheap to run but they only ventilate, not transfer or recover heat. Also, because they are adding cool air into the home your heating system will need to take up the slack.
- Doesn’t handle humidity - in locations with naturally high humidity they end up simply drawing in air that is already full of moisture.
- Questionable air quality - air from the roof space is not fresh and likely full of particles of the dust, dirt, mould and insect and rodent droppings that might have built up over the years. Even though this system includes a filter, it’s simply not possible to ensure none of this gets through.
- Non-compliant – following on from the above, for new builds, ventilation systems that draw air from the roof space no longer comply with current ventilation standards. New build ventilation standards state air must be drawn from outside. New PPV systems will offer this.
- It needs a summer bypass – your roof space will become too hot in summer so you need to switch it off so it doesn’t push that air into your home and overheat it.
- Potentially increased moisture – according to the Energywise site, research has found that systems not controlled by humidity sensors can increase moisture levels in your house!
Balanced pressure heat recovery systems
These systems work by pulling in fresh air while pushing out the old. It’s also fitted with a heat exchanger that recovers heat energy from the outgoing air and uses it to warm up the incoming air.
PROS
- Healthy breathing environment – the systems extracts pollutants from your home without letting in pollens and dust from outside.
- Helps with heat recovery - if you have a heat source the system will distribute fresh heated air around your house instead of just having the heat in one room.
- Compliant – balanced pressure heat recover systems are in keeping with current ventilation building code standards. In fact, in some areas there are council bylaws that mean you must have mechanical ventilation if your home is near an airport, motorway or train line.
- Better security – you don’t have to leave windows and doors open to air out the house.
- Appropriate for all climates – some systems aren’t good for really cold or really humid environments. A balanced system will work in all of them.
CONS
- The colder the better - the larger the difference in temperature between indoor and out, the more heat the heat exchanger will recover. So, while appropriate for all climates, this kind of system performs at its peak in winter months in colder climates. Ask an expert about what’s best for your home. In this web forum a homeowner in Wellington discusses whether their issues could be addressed with a much more economical solution than installing a total ventilation system.
- Expensive – these systems are costly to install and more expensive to run than positive pressure systems.
- May not remove moisture from incoming air – make sure it has a heat exchanger or it is not a true heat recovery system. Also, if your home is already damp it won’t dry it out.
- Can’t DIY – positive pressure systems are uncomplicated and can be installed yourself with the right tools and knowledge. Balanced pressure systems must be professionally designed and installed.
Extractor fans
Did you know the average home produces up to 8L of moisture every day simply through activities like cooking, showering and even breathing! That’s why some areas of the home need an extra helping hand. Here are some tips for extractor fans (and range hoods) in the kitchen, bathroom and laundry:
- Vent to the outside. DO NOT vent into your ceiling space.
- Consider the size needed for the type of room. Too small and they’ll be noisy and useless.
- Consider where in the room they should go - place them close to the moisture source.
- Turn them on BEFORE you start showering, cooking, drying clothes etc. Leave them on until most of the moisture has cleared.
- Shutting the door and leaving a window open a crack helps air flow in, improving the extractor fan's effectiveness.
- Clean them regularly to maintain their performance.
Remember, a ventilation system is just that – for moving air in, out and around your home. If your main goal is to heat your home, they work well in conjunction with a heat source but shouldn’t be relied on for heat alone. For more information on ventilation systems, including the difference between HRV, ERV and HVAC, read our article from November 2017.
