First, the names
Plenty of people say 'HRV' when they mean any home ventilation system. HRV and DVS are New Zealand businesses, and even Tenancy Services writes 'products like HRV and DVS systems' rather than treating either as a category.
So 'positive pressure vs HRV' is really two questions: which type of system suits your house, and whose product? On this site we name the type.
Positive pressure: air pushed in from above
BRANZ says these systems bring air inside to create a positive internal pressure, and the air then leaks out through gaps and open doors and windows. Consumer NZ says it's usually blown in from the roof space, or from outside on some types.
The sources pull in different directions on which houses suit it. BRANZ says the more airtight the home, the more effective the system, while Consumer NZ says it suits older houses with wooden joinery better than modern ones with sealed aluminium joinery. A ventilation specialist should look at your joinery and how draughty the house is before recommending one.
Winter is the catch. Consumer NZ warns that blowing cold roof space air in during winter means ramping up the heating, and BRANZ is plain that air from the roof space is not fresh outdoor air. It also cites EECA and University of Otago research finding that, most of the time, pumping roof space air into the house gave no heating or cooling benefit. In BRANZ's view, ducted warm air transfer systems that draw from the roof space don't meet Building Code clause G4, because they bring no outdoor air in.
Balanced heat recovery: fresh air in, warmth kept
Two fans do the work, one drawing air in from outside and one removing stale air, while a heat exchanger (generally in the roof space) passes warmth from the outgoing air to the incoming air. BRANZ says these systems meet the fresh outdoor air requirement in clause G4.
In a BRANZ test house the core recovered around 73% of the heat from outgoing air, but delivered efficiency can fall below 30% if duct losses aren't planned for. BRANZ says these systems are most cost-effective in a relatively airtight building, fitted during new construction or a major renovation.
They aren't heaters, BRANZ notes, though they do reduce heat loss. Check that anything sold as heat recovery actually contains a heat recovery device. Energy recovery ventilation (ERV) is a close relative that moves water vapour as well as heat.
The three types side by side
| Positive pressure | Balanced heat recovery | Heat transfer | |
|---|---|---|---|
| Where the air comes from | The roof space, or outside on some types | Outside, through one of two fans | A warmer room in the same house |
| How stale air leaves | Pushed out through gaps and openings | Extracted by the second fan | It doesn't: warm air is only moved around |
| Best suited to | Depends on the house, so get it assessed | Relatively airtight homes, new builds and big renovations | Homes where a log burner or pellet burner heats one room |
| Is it a heater? | No | No, but it cuts heat loss | No, it moves heat that's already there |
Heat transfer: moving warmth, not freshening air
A heat transfer system ducts warm air from a heated room to others. BRANZ says it isn't a heater, and it's most useful where a log burner or pellet burner heats one room and the air near that room's ceiling reaches around 30 degrees or more. Consumer NZ says it isn't ventilation either. See heat transfer systems.
If you rent the house out
The ventilation standard relies on windows that open, an extractor fan or rangehood venting outside in the kitchen, and an extractor fan venting outside in each bathroom. Tenancy Services says recirculating systems, including products like HRV and DVS systems, aren't suitable to meet it.
A continuous mechanical ventilation system that extracts air from the kitchen and bathroom to the outdoors can replace the fans, but only in a home first consented on or after 1 November 2019. Whether a system complies is measured against the standard, not promised by whoever sells it.
Ask before you sign for any ventilation system
- Which type is it: positive pressure, balanced heat recovery, or something else?
- Does the incoming air come from the roof space or from outside?
- If it's sold as heat recovery, where's the heat exchanger?
- Does it suit my joinery and how airtight the house is?
- Which filters does it use, and how often does it need servicing?
What you can do, and what's theirs
You can
- Run the rangehood and extractor fans while you cook and shower.
- Open windows when the weather allows.
- Clean or change filters as the manual says, where you can reach them without going into the roof space.
Leave to a licensed installer
- Anything in the roof space, including the fan unit and ducts.
- Wiring, controllers and extractor fan circuits.
- Servicing the heat exchanger and duct runs.
What we'd do
Our pick, and why
We'd deal with moisture at its source first, with extractor fans that vent outside in the kitchen and bathroom. Then, in a newer or reasonably airtight home, we'd look hardest at balanced heat recovery, because it brings in genuinely fresh outdoor air. In an older, draughtier house we'd ask a home ventilation specialist to look at the joinery and the roof space before choosing a type, and we'd be wary of anyone selling roof space air as a way to heat the house.