How does balanced heat recovery work?
BRANZ describes heat recovery ventilation as a ducted system with two fans, one drawing air in from outside and one removing stale internal air, plus an air-to-air heat exchanger, generally in the roof space, that recovers heat from the air going out. Consumer NZ looks at it from the room's side: warm, damp air is extracted from living spaces, and its heat warms the dry air coming in.
Because the incoming air genuinely comes from outside, BRANZ says heat recovery systems meet the fresh outdoor air requirement of Building Code clause G4 Ventilation, and Consumer NZ says this type can fully meet Building Code requirements. That's the clearest difference from a roof-space positive pressure system.
Energy recovery ventilation (ERV) is a close cousin: BRANZ says it moves water vapour across as well as heat, which helps control humidity. Mitsubishi Electric NZ, for one, lists Lossnay heat recovery fresh air ventilation alongside its heat pumps.
How much heat does it really keep?
In a BRANZ test house trial, the heat exchanger core recovered around 73% of the heat in the outgoing air, in line with the typical 70% for cross-flow cores. BRANZ also warns that actual delivered efficiency can drop below 30% if heat and air losses in the ducting aren't properly considered.
That gap is the whole argument for a careful installer. Short, sealed, well-insulated ducts and evenly matched airflows get you close to the core's figure; long, leaky runs through a cold roof space drag it down. And check what you're actually buying: BRANZ's advice is to make sure a proposed heat recovery system really incorporates a heat recovery device.
Which homes suit it?
BRANZ says these systems make the most sense in a relatively airtight building, installed as part of a new build or a major renovation, and Consumer NZ agrees they work best in modern homes. In a draughty older house, a lot of air escapes through gaps without ever passing the heat exchanger.
BRANZ is clear that they aren't heating systems, though they do reduce heat losses from the building. Plan the heating alongside: a well-sized heat pump in the living area is a natural partner.
How a specialist designs and fits one
Survey
How airtight the house is, the roof space, and where an intake and exhaust can go outside.
Duct design
Supply ducts usually serve living areas and bedrooms, and extract ducts the kitchen, bathroom and laundry, with each run kept as short as practical.
Fitting
The unit and heat exchanger go in the roof space, with insulated ducts to a ceiling grille in each room.
Wiring
A registered electrical worker connects the unit and controls, and the fixed wiring comes with a CoC no later than 20 days after.
Balancing
Supply and extract airflows are measured and set to match, which is what makes the system balanced.
Renting it out?
For a rental whose building consent was first received on or after 1 November 2019, Tenancy Services accepts continuous mechanical ventilation in place of fans, but only where it continuously pulls air from the kitchen and bathroom to the outdoors. An older rental still needs extractor fans that vent outside. Checking what a particular system does is the landlord's job.
Day to day, and for the technician
You can
- Keep the outside intake and exhaust grilles clear of plants, working from the ground.
- Notice changes in noise, airflow or condensation, and mention them at the next service.
- Answer the controller's filter reminders by booking a service, not by climbing into the roof.
Leave to a licensed installer
- Designing, fitting and sealing the ducts.
- Cleaning the heat exchanger core and replacing roof space filters.
- Wiring and the Certificate of Compliance.
- Measuring and balancing the airflows.
What we'd do
Our view
If you're building, or gutting a house and making it tighter, we'd plan balanced heat recovery in from the start, with the duct runs drawn on the plans. In an older, draughty house it's harder to justify, so we'd look at draught-stopping and positive pressure first. Either way, we'd ask the specialist how they'll keep duct losses down, because that's where the recovered heat goes missing.