How does positive pressure ventilation work?
BRANZ puts it simply: these systems bring air inside to create a positive internal pressure, and the air leaks out through gaps and open doors and windows. For that to work, the fan has to supply air faster than the house leaks it. BRANZ gives typical continuous fresh air flow for air supply systems as around 3 to 20 litres per second.
Consumer NZ says positive pressure systems blow drier air in from the roof space above the ceiling or, in some types, from outside, through a filter on the way in.
Roof space air or outside air?
This matters more than the badge on the box. BRANZ is blunt: 'Air from the roof space is not fresh outdoor air.' In its view, ducted systems that take warm air from the roof space don't meet Building Code clause G4 Ventilation, because they don't bring outdoor air inside.
Winter is the other catch. BRANZ says roof space air can be colder than room air at night and on overcast days, so these systems need to be able to slow down or stop. It also cites EECA and University of Otago research which calculated that, most of the time, pumping roof space air into a house gave no heating or cooling benefit.
A system that draws from outside answers the fresh air question, but outside air is cold in winter too. BRANZ says incoming fresh air needs heating once it's below about 13 degrees outside, or 6 degrees below the room temperature you want.
Which houses does it suit?
Consumer NZ says positive pressure suits older houses with wooden joinery better than modern houses with sealed aluminium joinery. BRANZ makes a point that pulls the other way: the more airtight the home, the more effective the system.
That tension is exactly why a home ventilation specialist should see your house before recommending one. Ask where the pushed-out air will leave, and why your house suits this type over balanced heat recovery. It's also no fix for a wet subfloor, a roof leak or a bathroom without an extractor fan: sort those first.
What a positive pressure install involves
A look at the house
The specialist checks roof space access, insulation and room for the fan unit, then your joinery, draughts and where the damp shows.
Air source and filter
Roof space, outside, or a unit that can switch between them, plus a filter grade to suit any allergies in the household.
Wiring
Fixed wiring for the fan or controller is prescribed electrical work. The registered electrical worker who does it owes you a Certificate of Compliance within 20 days.
Setting up
The airflow is set for the house, and the controller set to slow or stop the fan when the roof space is colder than the rooms.
What you can do, and what to leave
You can
- Watch the windows on cold mornings for a couple of weeks before and after, so you can tell whether it's helping.
- Keep using the bathroom and kitchen extractor fans: the system doesn't replace them.
- Use the controller as the manual describes, and don't cover the ceiling outlets.
Leave to a licensed installer
- Fitting or moving the fan unit, ducts or filters in the roof space.
- Wiring, controller sensors and the Certificate of Compliance.
- Changing the air source or adding an outside intake.
What we'd do
What we'd ask for
We'd settle the air source before anything else. If it's roof space only, we'd expect a controller that stops the fan when the roof air is colder than the house, and a straight answer about fresh air. For an older house with timber joinery and wet windows every morning, positive pressure is the type Consumer NZ points to. For a newer, tighter home, we'd get a balanced heat recovery quote as well and compare the two.