“Why does a new AHU keep developing wet, dirty areas even though the filters and coils are being serviced?”
Microbial-control problems are often blamed on maintenance teams after handover, but many begin with design choices: poor drainage, wet filters, inaccessible coils, contaminated intake locations or no practical way to clean the equipment.
The practical answer
The Standard interfaces directly with AS 1668.2 for outdoor-air quantity and air-handling design. It adds the microbial-control perspective: intake location, filtration, humidification, evaporative equipment, condensate, drainage, duct cleanliness and access.
It also applies to cooling water systems and commissioning. New or altered systems should be prepared and commissioned so contamination risks are not built into service from day one.
The strongest design question is not just whether the plant works when new, but whether wet or contaminated components can be inspected, drained and cleaned throughout the building life.
Where this Standard fits
- New AHUs and air-handling systems
- Outdoor-air intake and exhaust location review
- Cooling coils, drain pans and condensate systems
- Humidifiers and evaporative air-cooling equipment
- Ductwork near moisture-producing equipment
- Cooling towers and cooling water systems
- A substitute for AS 1668.2 ventilation requirements
- An operation-only maintenance Standard
- A reason to treat every visible stain as proof of hazardous contamination
- Permission to install wet components without cleaning access
- A cooling-tower-only document
- A design that can be rescued later by more frequent cleaning
How should microbial-control risk be designed out?
Look for where contaminants or moisture can enter, accumulate, grow and become difficult to remove.
A good result needs the equipment definition, loads, design method, fabrication, inspection and operating limits to describe the same real system.
Key engineering concepts to understand
Moisture is a design variable
Wet filters, standing condensate and damp duct insulation are not just housekeeping problems; geometry and drainage determine whether moisture persists.
Access is part of hygiene design
Coils, trays, filters and wet components need safe access for inspection and cleaning throughout their service life.
Air intake location matters
Outdoor air can import contamination when intakes are poorly located relative to exhausts, cooling towers or other pollution sources.
Drainage needs to be observable
A drain that cannot be inspected or cleaned can fail silently while water accumulates inside plant.
Commissioning starts the lifecycle
Dirty construction debris, untreated water systems and incomplete cleaning can create problems before the building is occupied.
Design and maintenance standards are paired
AS/NZS 3666.1 establishes maintainable systems; AS/NZS 3666.2 defines the ongoing operation and maintenance pathway.
Detailed engineering case studies
The examples below show how the Standard changes a real engineering decision. They are not clause summaries or universal answers; each case starts with the equipment, task and evidence available.
Condensate remains in a drain pan after the unit stops
A new AHU develops odour complaints. Inspection shows water remaining in parts of the drain pan and evidence of wet debris downstream of the cooling coil.
AS/NZS 3666.1 brings the review back to drainage, grading, air pressure, access and cleanability rather than treating the problem as a filter replacement issue.
What made the job difficult
- The drain connection exists but the pan has local low points.
- The trap arrangement is difficult to inspect.
- High negative pressure may affect drainage performance.
- Access to the downstream side of the coil is poor.
How the engineering review should proceed
- Inspect the full condensate path from coil and tray to visible discharge.
- Check pan geometry, drainage, trap configuration and the effect of AHU pressure.
- Assess whether wet insulation or downstream components need cleaning or replacement.
- Modify the arrangement so future inspection and cleaning can be performed safely.
The repair should correct the drainage and access problem, not just clean the current contamination. The commissioning record should verify that condensate clears under actual operating conditions.
Outdoor-air intake located near an exhaust discharge and cooling tower
A building extension adds a new AHU intake on a roof where an existing exhaust outlet and cooling tower are nearby. The shortest duct route is attractive to the project team.
AS/NZS 3666.1 requires the microbial-control review to consider intake location, nearby contamination sources and wind effects. Duct convenience is not the only design criterion.
What made the job difficult
- Roof plant has been added in stages over many years.
- Prevailing winds and multiple roof levels complicate simple distance checks.
- The nearby exhaust may be objectionable under some operating conditions.
- The design team has not completed a site survey of adjacent openings.
How the engineering review should proceed
- Map all nearby intakes, exhausts, cooling towers, vents and natural openings.
- Review prevailing wind and building geometry rather than only plan distance.
- Coordinate the intake/discharge review with AS 1668.2 requirements.
- Relocate or redesign the intake if the contamination pathway cannot be controlled robustly.
The correct intake location may require a longer duct route or different roof arrangement. The design decision should prioritize air quality and lifecycle risk over the shortest installation path.
New cooling-tower system brought online without a robust commissioning record
A new cooling-water system is started quickly to meet a summer opening date. The owner receives limited records of flushing, cleaning, pretreatment and initial water-system setup.
AS/NZS 3666.1 treats commissioning and pretreatment as part of microbial-control design, especially for new cooling water systems and components.
What made the job difficult
- The construction program compressed the pre-start activities.
- Temporary water circulation occurred before final treatment arrangements were complete.
- The final operating manual does not clearly record the commissioning condition.
- Responsibility passes quickly from contractor to facility manager.
How the engineering review should proceed
- Reconstruct what flushing, cleaning and pretreatment were actually performed.
- Verify that the water treatment and control systems are operational and documented.
- Inspect the system condition before assuming the new installation starts from a clean baseline.
- Complete the operating and maintenance handover so AS/NZS 3666.2 activities have a reliable starting point.
The project should close the commissioning evidence gap before normal operation continues indefinitely. A new system is not automatically a clean or well-controlled system.
Common mistakes
Poor drainage and inaccessible components cannot be solved efficiently by more frequent cleaning.
Air intakes should be reviewed against nearby exhausts, cooling towers and pollution sources.
Wet filtration media can create a microbial-control problem rather than solving one.
Condensate systems need inspection and cleaning access so failures can be detected.
Construction debris and untreated systems can create a poor baseline at handover.
Plantroom layout should preserve realistic maintenance access around the selected components.
Information to gather before making the decision
Air-handling system layout
Outdoor-air intake and exhaust locations
Nearby cooling towers and pollution sources
Filter type and access arrangement
Cooling coil and drain-pan details
Trap and condensate discharge arrangement
Humidifier or evaporative equipment details
Duct access near wet components
Plantroom maintenance clearances
Cooling water system design
Commissioning and cleaning records
Operating and maintenance manual requirements
What should happen next?
Related AS Applied content
Source basis & limitations
- The page does not reproduce detailed construction requirements, water-treatment criteria or cooling-tower provisions.
- AS 1668.2 and AS/NZS 3666.2 are treated as explicit design and lifecycle interfaces.
Need the Standard applied to real equipment?
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