Choosing MVHR for a Passivhaus in South Australia

MVHR sizing depends on the total internal volume of the home, the occupancy pattern, the moisture load in wet areas, and the target air quality standard. A generic room-by-room selection produces an oversized or undersized system, both of which perform poorly.

A Passivhaus-accredited designer calculates the total ventilation rate required (typically 30 litres per second per person, adjusted for occupancy patterns), matches this to a specific MVHR unit capacity, and specifies the duct layout to deliver the airflow to each room at design conditions.

For a typical Adelaide four-bedroom Passivhaus, this design work takes a Passivhaus designer around two to five days and costs in the range of a few thousand dollars. It is not a step to skip. Undersized MVHR ruins the internal comfort of the whole build; oversized MVHR consumes more energy than necessary and increases noise.

Three MVHR brands have strong presence in the Adelaide market. Each has different characteristics.

Zehnder is the premium European brand, imported through Australian distributors. High heat recovery efficiency (typically 85 to 90 percent), quiet operation, and excellent filter access. Higher initial cost but strong long-term reliability. Service network in Adelaide is limited; most warranty work is via Sydney or Melbourne distributors.

Fantech is Australian-manufactured, with a strong local support network including Adelaide-based technicians. Slightly lower heat recovery efficiency than Zehnder but competitive pricing and easier ongoing service. Well-suited to smaller Passivhaus projects on tighter budgets.

Stiebel Eltron is another European brand with growing Australian presence. Integrated with heat pump water heating on some models, which appeals to all-electric homes. Service network is expanding in Adelaide.

The right brand depends on total budget, priority on efficiency versus serviceability, and access to a specific installer who knows the brand well. Discuss options with your Passivhaus designer at concept stage.

MVHR duct runs must be short and generously sized. Long runs lose airflow to friction; undersized ducts create noise and reduce efficiency.

The typical layout places the MVHR unit centrally in the home (often in a plant room or the roof space) with short, direct duct runs to each room. Supply air goes to living rooms, bedrooms and studies. Extract air is drawn from wet areas (kitchen, bathrooms, laundry).

Duct diameter matters. For a residential Passivhaus, main trunk ducts are typically 150 to 200mm diameter, branch ducts 75 to 125mm. Undersized branches are the most common installer mistake and create audible noise at the diffuser.

Rigid ducting is preferred over flexible ducting for main runs. Rigid ducts have lower friction and lower turbulence. Flexible ducting is acceptable for short final connections to diffusers.

Duct routes must be shown on architectural drawings early, not left for the installer to work out. A duct route conflict with a structural beam or a ceiling downlight found during installation is expensive to resolve.

Three practical usability factors are often overlooked at selection stage but become critical during occupation.

First, filter access. MVHR filters need to be replaced every three to twelve months depending on the filter grade and the local air quality. The filter access point must be reachable without a ladder, and ideally without moving furniture. Filter replacements are inexpensive but must actually happen; hard-to-reach filters get skipped.

Second, condensate handling. MVHR units generate a small amount of condensate as warm exhaust air is cooled during heat exchange. This must drain safely to a plumbing outlet. In roof-space installations, the drain route needs to be planned at design stage.

Third, acoustics. MVHR fans are generally quiet but can transmit noise through ducts if the design is poor. Acoustic attenuators (silencers) on supply and extract trunks reduce noise transmission. Discuss acoustic performance with the designer if the home has any acoustically sensitive spaces (music room, home office).

MVHR is a long-life system (typical component life 15 to 25 years) but it requires periodic maintenance to perform at spec.

The primary maintenance items are: filter replacement (every three to twelve months), fan motor inspection (annual visual check), heat exchanger cleaning (every two to five years), and duct hygiene inspection (every five to ten years).

Include a maintenance manual in the practical completion handover pack. It should show the location of every access point, the filter part number, the recommended replacement schedule, and the contact details of the installer.

Consider a service contract with the installer or an accredited maintenance provider. Fees are modest, typically a few hundred dollars annually, and ensure the filter changes actually happen. In owner-occupied homes, forgotten filter changes are the most common performance degradation cause.

Finally, provide a spare filter set at handover. The installer should supply at least one full replacement set that can be fitted by the homeowner. This bridges the first six to twelve months and gives the homeowner confidence in the maintenance routine.