A ventilation system that has been installed incorrectly does not announce itself immediately. It runs, it makes noise, and it moves some air. The problems show up over weeks and months: rooms that never fully clear, moisture that keeps returning, energy bills that do not make sense, and eventually mould, structural damage, or system failure that traces directly back to how the installation was done.
Most ventilation installation mistakes are not complex errors. They are straightforward oversights in planning, product selection, or installation practice that compound over time. Understanding what they are and why they happen is the most practical way to avoid them, whether you are managing a new installation or evaluating an existing one that is not performing.
Undersizing the Fan for the Space
The most common ventilation installation mistake is selecting a fan rated for a smaller space than the one it serves. Fan capacity is measured in litres per second (L/s) in Australia, and the required capacity is determined by the room's volume and the required air changes per hour for that room type.
Australian Standard AS 1668.2 sets the benchmark. Bathrooms require a minimum of 25 L/s for intermittent operation. Kitchens require significantly higher rates. When a fan is selected without calculating the room volume and required air change rate, it is almost always undersized.
Incorrect Duct Routing and Excessive Length
Duct routing decisions made during installation have a direct and permanent effect on system performance. Every metre of duct run, every bend, and every change in diameter adds resistance that reduces the airflow the fan can deliver.
The most damaging ducting errors involve excessive run length without upsizing the duct diameter, sharp 90-degree bends, flexible duct that has been compressed or kinked, and duct that sags, creating low points where condensation pools.
Terminating into the Roof Space
A duct that does not exit the building is not a ventilation duct. It is a moisture delivery system for the roof cavity. Terminating exhaust ducts into the roof space rather than through the roof or wall to the exterior is one of the most consequential ducting errors.
Humid exhaust air condenses on roof timbers, insulation, and sarking, causing timber rot, insulation degradation, mould growth, and structural damage to roof framing.
Ignoring Make-Up Air Requirements
Exhaust ventilation removes air from a space. For that air to be replaced, make-up air must be able to enter. In a tightly sealed room with no provision for make-up air, an exhaust fan creates negative pressure that limits how much air it can actually move.
This is a frequently overlooked aspect of ventilation installation. Installers focus on the fan and duct and treat the make-up air path as someone else's concern.
Poor Sealing at Duct Joints and Connections
Duct systems that leak at joints and connections lose a portion of the air they are meant to move before it reaches the terminal outlet. In exhaust systems, leaking joints discharge humid air into building structure rather than expelling it to the exterior.
Common errors include joints pushed together without proper sealing, duct tape rather than purpose-made sealing tape, and connections made without clamps or appropriate fasteners.
Incorrect Fan Placement
Where a fan is positioned within a space affects how effectively it removes air. Fans positioned directly above the primary moisture or odour source perform better than fans positioned across the room from it.
In rooms with complex layouts or multiple moisture sources, a single poorly positioned fan may leave dead zones where air does not exchange adequately.
Skipping Commissioning and Performance Verification
An installation that has not been tested is an installation with unknown performance. Commissioning means verifying that the installed system actually delivers the airflow it was designed to deliver, that ducts are sealed and correctly terminated, and that controls operate as intended.
Skipping this step means ventilation installation mistakes go unidentified until symptoms appear, when rectification is a separate job rather than a correction made before the wall was closed.
Installation Checklist
Before You Sign Off on Any Ventilation Installation
- ✓ Fan Sizing: Has the room volume been calculated and matched to the fan's rated capacity at operating static pressure?
- ✓ Duct Routing: Is the duct run as short as possible? Are bends gradual rather than sharp?
- ✓ External Termination: Does every duct actually exit the building through a weather-protected outlet?
- ✓ Make-Up Air: Is there a clear path for replacement air to enter when the fan runs?
- ✓ Joint Sealing: Are all duct joints and connections properly sealed with appropriate materials?
- ✓ Fan Placement: Is the fan positioned directly above the primary moisture or odour source?
- ✓ Commissioning: Has the system been tested to verify it delivers the required airflow?
Commissioning takes minutes on a simple installation. On a more complex system serving multiple rooms or a commercial space, it is a structured process that confirms the system performs as designed before the building is handed over or occupied.
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Frequently Asked Questions
The most common indicators are rooms that do not clear of moisture or odour despite the fan running, condensation that persists on surfaces well after the fan has been operating, and mould that returns in the same locations after treatment. A tissue test at the fan grille is a basic check: if the tissue is not held firmly by suction, the system is not performing adequately. The cause may be a sizing issue, a blocked or kinked duct, incorrect termination, or leaking connections.
A flexible duct carries more resistance per metre than rigid duct of the same diameter and performs poorly when kinked, compressed, or run in long unsupported sections. It is appropriate for short final connections and where rigid duct cannot practically be routed. For primary duct runs, particularly those of significant length, rigid duct provides substantially better airflow performance. An installation using flexible duct throughout a long run will deliver considerably less airflow than its fan rating suggests.
A tightly sealed room without adequate make-up air provision creates negative pressure that limits how much air the fan can extract. The fan works against the pressure differential it creates rather than freely moving air through the duct. Ensuring an undercut door, transfer grille, or other make-up air path is present allows the fan to operate at its rated capacity. This is one of the most overlooked aspects of ventilation installation and one of the most straightforward to address.
Duct connections in concealed locations are not practically accessible for routine inspection once the installation is complete, which is why sealing them correctly during installation is critical. For accessible duct sections in roof spaces or plant rooms, checking connections and termination points every few years as part of a general maintenance inspection is reasonable practice. A ventilation system that was performing well and has declined in performance without an obvious cause often has a duct connection that has worked loose over time.
Yes. The termination point affects both system performance and building protection. A termination that faces into prevailing wind creates back pressure against the fan. A termination that discharges under an eave or near a window risks re-entry of exhausted air. Terminations without backdraft dampers allow wind-driven air to enter the duct when the fan is off, which can introduce moisture and reduce the fan's effective operating capacity. The termination should be a weather-protected external outlet with a backdraft damper, positioned to discharge away from openings and prevailing wind where possible.
