Key takeaways
- A balanced MVHR system supplies and exhausts equal air volumes, so it needs a pressure-balanced duct design and sealed penetrations rather than the single-fan approach used for exhaust-only ventilation.
- Heat recovery cores transfer heat between exhaust and supply air streams without mixing them, so the unit must be installed where condensate can drain and the core can be accessed for cleaning.
- Duct runs should be as short and straight as practical, insulated on the supply side where required, and sized to the fan curve rather than chosen by eye from the unit spigot size.
- Commissioning must include measured airflow at each supply and exhaust terminal, adjustment of balancing dampers, and a record of the settings so the system can be rebalanced after later building works.
- The system only performs as designed if the building envelope is airtight and the ductwork is sealed; leaks in either undermine the heat recovery benefit and can draw unconditioned air through the building fabric.
Mechanical ventilation with heat recovery is a balanced system: it supplies and exhausts air at the same rate, passing the two streams through a heat exchange core so that most of the warmth in the outgoing air is transferred to the incoming air without mixing them. On an airtight home, that balance is what stops the building from being depressurised by exhaust-only fans and pulling untreated air through gaps in the envelope.
The installation is not complicated, but it is unforgiving. A unit that is undersized, ducted with sharp bends, or commissioned by ear will not deliver the ventilation rate the design assumed. The homeowner notices stuffiness or noise; the builder notices a callback. Getting the mechanical side right from first fix is cheaper than fixing it after the ceiling is closed.
This article covers the installation and commissioning of balanced MVHR systems on NSW residential projects. It assumes you are working to a design that already specifies airflow rates and terminal locations, and that you need to execute it on site without compromising the building envelope.
What makes a system balanced, and why it matters on an airtight build
A balanced system has a supply fan and an exhaust fan inside the same unit, moving equal volumes. That equality is the point. In a leaky house, an exhaust fan can pull replacement air through wall vents and window frames without much consequence. In a house built to modern airtightness expectations, the same fan creates a pressure difference that draws air through the building fabric, bypassing filters and carrying dust and insulation fibres into the living space.
Balanced ventilation avoids that pressure difference. It also allows heat recovery: the exhaust stream warms the supply stream through a core, so the incoming air is closer to room temperature than the outdoor air. The core does not mix the streams, which is why it can recover heat without transferring odours or moisture from the wet areas to the bedrooms.
For the installer, the practical consequence is that the ductwork has to be designed as two separate but equal networks. You cannot treat the supply side as an afterthought and hope the exhaust side balances it out. The system is only balanced when the measured airflow at every terminal matches the design, and that is a commissioning task, not a guess.
Where the unit goes, and what it needs around it
The unit itself is usually the least difficult part of the installation, provided it is located where it can be serviced. In a roof space, that means a stable platform, enough clearance to slide the core out, and a safe walkway for future maintenance. In a cupboard or plant room, it means acoustic separation from bedrooms and living areas, because the fans run continuously on low speed.
Condensate is the detail that catches people out. In cool weather the exhaust stream can condense inside the unit, and the drain has to carry that water away with a continuous fall and a trap that will not be sucked dry by the fan pressure. If the drain is run uphill or terminates in a location that can freeze, water will pool in the unit and eventually damage the core or spill into the ceiling cavity.
Access matters for the same reason. Filters need cleaning at intervals the manufacturer specifies, and the core needs periodic inspection. If the unit is boxed in behind a fixed ceiling or a tiled wall, the homeowner will not maintain it, and the system will gradually lose performance. Plan the access panel at the same time you plan the unit position.
Commissioning sequence for a balanced MVHR system
- 1Measure at terminalsUse a calibrated flow hood or balometer at every supply and exhaust terminal to record actual airflow.
- 2Adjust balancing dampersTrim the dampers on each branch until supply and exhaust volumes match the design figures within the tolerance the designer set.
- 3Check total system pressureConfirm the unit is operating on a sensible part of its fan curve and that filters are clean before final adjustment.
- 4Record settingsPhotograph or write down the damper positions and controller configuration so the system can be returned to these settings later.
- 5Verify controls and boostTest the boost function, timer and any humidity or CO2 sensor to confirm they trigger the intended airflow change.
- 6Hand overWalk the homeowner through filter access, cleaning frequency and what the indicator lights mean.
Duct design and first fix: the decisions that are hard to reverse
Ductwork is where most of the performance is won or lost. The fan curve of the unit tells you how much air it can move against a given resistance, and every metre of duct, every bend and every transition adds resistance. A run that looks acceptable on a plan can be far too restrictive if it is installed with tight elbows and flexible duct that is sagging or kinked.
Keep runs as short and straight as the structure allows. Use rigid duct where you can, particularly on the supply side, and support flexible duct so it stays fully extended rather than compressed. Compression reduces the effective diameter and increases resistance, which shows up as low airflow at the terminal furthest from the unit.
Insulate the supply ducts where they pass through unconditioned spaces, and seal every joint. A leak on the supply side dumps conditioned air into the roof space; a leak on the exhaust side draws roof space air into the system. Both undermine the heat recovery and can introduce dust or insulation into the airstream. Where ducts penetrate the building envelope or the ceiling line, the vapour barrier and airtightness layer must be maintained and taped, not just pushed aside.
Terminal placement is part of the design, not a site decision. Supply terminals in habitable rooms and exhaust terminals in wet areas and kitchens is the usual arrangement, with separation between supply and exhaust terminals so the system does not short-circuit. If the design calls for a specific separation, install it; if you are unsure, check the design documentation before you cut the hole.
Condensate, frost and the climate you are building in
Heat recovery cores can frost in cold conditions if the exhaust air is warm and moist and the incoming air is very cold. Some units have a pre-heater or a frost protection strategy that temporarily reduces the supply fan speed. If the design includes one, it needs a power supply and a control connection, and it needs to be commissioned so the homeowner knows what it does.
Condensate management is not just about the drain fall. The trap has to be deep enough to resist the fan pressure, and the discharge point has to be appropriate. Running condensate into a roof space or onto a pathway is not acceptable; it needs to go to a suitable drainage point, and in some cases that means a condensate pump. If a pump is used, it needs a power supply and an alarm or overflow path that will not cause water damage if it fails.
In warmer parts of NSW, frost is less of a concern, but the core still produces condensate when the exhaust air is humid. The same rules apply: continuous fall, correct trap, accessible for cleaning. A drain that clogs with dust and biofilm will eventually overflow, and the first sign is often a stain on the ceiling below.
Pre-start checks before commissioning an MVHR system
- Confirm the unit is mounted level and the condensate drain has a continuous fall to a suitable discharge point.
- Check that all duct joints, penetrations and access panels are sealed and that the vapour barrier is continuous around them.
- Verify that supply and exhaust terminals are not short-circuiting, with at least the separation the designer specified.
- Inspect filters for shipping debris and confirm they are seated correctly in their frames.
- Confirm the electrical supply and controls are terminated and the unit can be isolated for service.
- Check that the heat exchange core is fitted in the correct orientation and the bypass damper, if fitted, moves freely.
- Ensure the roof space or plant area has safe access and enough clearance for future core removal.
Controls, boost modes and the handover that prevents callbacks
A balanced system runs continuously at a low rate to provide background ventilation, and boosts to a higher rate when needed. The boost can be triggered by a switch, a timer, a humidity sensor or a CO2 sensor, depending on the design. Whatever the trigger, it has to be wired and tested, because a boost mode that does not work is a system that will not clear moisture from a bathroom or a kitchen.
The controller also needs to be understandable. If the homeowner cannot work out how to change the speed or what the filter indicator means, they will either leave it alone and let the filters clog, or switch the unit off entirely. Both outcomes defeat the purpose of the installation. The handover should cover filter cleaning, what the controls do, and why the unit should not be switched off in summer. On a balanced system, running the unit in summer still provides filtration and ventilation, and many units have a summer bypass that brings in cooler night air without heat recovery.
Documentation matters here too. Record the commissioned airflow figures, the damper positions and the controller settings. If the homeowner later finishes the ceiling, adds a room or changes the layout, the system will need rebalancing, and having the original figures saves time and argument. For builders working towards their continuing professional development requirements, the NSW builder CPD guide explains how this kind of practical knowledge fits into the annual cycle.
Commissioning: what to measure and what to record
Commissioning is the step that separates a system that works from one that merely runs. You need a calibrated flow hood or balometer to measure airflow at each terminal, and you need the design figures to compare against. Start with filters clean, then measure every supply and exhaust terminal. Adjust the balancing dampers on each branch until the measured airflow matches the design within the tolerance the designer specified.
Balancing is iterative. Closing one damper to reduce flow at a terminal increases pressure in the rest of the system and changes the flow elsewhere, so you may need to go around the system more than once. Once the terminals are balanced, check the total system pressure and confirm the unit is operating on a sensible part of its fan curve. If the unit is running at maximum pressure to achieve the design flow, the ductwork is too restrictive and the system will be noisy and inefficient.
Record everything: the measured airflow at each terminal, the final damper positions, the controller configuration, the boost settings and the date. Photograph the damper positions if that is easier. This record is what allows the system to be rebalanced after future works, and it is also your evidence that the installation was commissioned properly if a question arises later.
Common defects and what they cost to fix
The most common defect is ductwork that is too restrictive, usually because of excessive flexible duct, tight bends or crushed sections. The symptom is low airflow at the furthest terminals, and the fix may involve rerouting ducts, which is expensive once ceilings are lined. The second most common is condensate drain problems: no fall, no trap or a trap that is too shallow, leading to water in the unit or the ceiling.
Air leaks in the ductwork or at penetrations are another frequent issue. They are hard to see but easy to measure: a pressure test or a duct leakage test will show them. Fixing them after the fact means opening up ceilings or roof spaces. Unbalanced systems are also common, often because commissioning was skipped or done by feel rather than measurement.
The cost of these defects is not just the repair. It is the callback, the disruption to the homeowner, and the risk of moisture damage or poor indoor air quality that could have been avoided. On a project where the ventilation system is part of the compliance story, a defect can also raise questions about whether the building meets its design intent. The ducted air conditioning design and installation article covers similar duct design principles, and the same discipline applies here.
Worked example: a builder in western Sydney
A builder in western Sydney is completing a new home designed with a balanced heat recovery ventilation system. The design calls for supply terminals in the bedrooms and living areas, exhaust terminals in the bathrooms, kitchen and laundry, and a unit in the roof space. The builder has installed the unit on a platform with a condensate drain to a nearby downpipe, and the ductwork is a mix of rigid and flexible duct.
At commissioning, the flow hood readings show that the exhaust terminals are close to design, but the supply terminals in the two far bedrooms are well below the required airflow. The builder checks the ductwork and finds that the flexible duct to those rooms has been compressed where it passes through a tight roof space, and there are two tight elbows close together. The dampers on the other branches are wide open, so the system is trying to push air through a restriction.
The fix is to replace the compressed section with rigid duct, reduce the number of elbows, and then rebalance the system. Because the ceiling is not yet lined, the cost is labour and materials rather than demolition. The builder records the final airflow figures, photographs the damper positions and hands over the system with a demonstration of the boost controls. The lesson is that the commissioning step caught a defect that would have been much harder to fix later, and the measured airflow is what proved the system was working.
Where this fits in your CPD and your next project
Ventilation is one of those areas where the regulations, standards and product technology keep moving. Keeping up with it is part of maintaining your registration, and the NSW builder CPD guide sets out how the annual requirements work. If you want to go deeper into the mechanical services side, the PointCPD course catalogue includes courses on building services and sustainability.
On your next project, treat the ventilation system as a coordinated package: the unit location, duct routes, condensate, controls and commissioning all need to be resolved before the ceilings go up. The split system air conditioner installation article covers some overlapping refrigerant and electrical considerations, and the heat pump dryer and laundry ventilation article looks at moisture management in a specific room. Together they form a picture of how air moves through a modern home, and why the mechanical systems that manage it deserve the same attention as the structure.
Commissioning is not optional. The most expensive mistake is skipping measured airflow and balancing, because a system that runs but does not deliver the design flow will create comfort and moisture problems that only surface after the owners move in.
Getting the details right from the start
Balanced mechanical ventilation with heat recovery is a straightforward technology, but it relies on discipline in the installation. Keep ducts short and sealed, manage condensate properly, commission by measurement and hand over with clear instructions. If you do those things, the system will do what it was designed to do, and the homeowner will never have to think about it. If you do not, you will be back to fix it, and the cost of that visit is always higher than the cost of doing it right the first time.
Questions builders ask
What is the difference between a balanced MVHR system and a normal exhaust fan?
An exhaust fan removes air from one room and relies on replacement air leaking in through the building fabric or vents. A balanced MVHR system supplies and exhausts air at the same rate through a heat exchange core, so the building is not depressurised and heat is recovered from the outgoing air.
Where should the MVHR unit be installed?
It should be installed where it can be serviced, with enough clearance to remove the core and access the filters. Roof spaces, cupboards and plant rooms are common, but acoustic separation from bedrooms and a safe walkway for maintenance are essential.
Does an MVHR system need a condensate drain?
Yes, in most installations. The exhaust air can condense inside the unit, so a drain with a continuous fall and a trap deep enough to resist fan pressure is needed. The drain must discharge to a suitable point, not into the roof space or onto a path.
How do I commission a balanced ventilation system?
Use a calibrated flow hood to measure airflow at every supply and exhaust terminal, then adjust the balancing dampers until the measured flow matches the design. Record the final damper positions and controller settings so the system can be rebalanced after future works.
Can I install flexible duct on an MVHR system?
Flexible duct can be used, but it must be fully extended and supported so it does not sag or kink. Compressed or sharply bent flexible duct adds resistance and reduces airflow, so rigid duct is preferred for long runs and tight spaces.
What maintenance does an MVHR system need?
Filters need cleaning or replacement at the intervals the manufacturer specifies, and the heat exchange core should be inspected periodically. The condensate drain and trap should be checked for blockages, and the terminals should be kept clear of dust and obstructions.
Sources
- Working in heat · Safe Work Australia
- Heat pump water heaters · energy.gov.au
Published 6 August 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.
