Post-Storm Odours in Offices: When the Problem Is in the Air Filter Path

Storms can leave an office with a stubborn damp, musty or even sewage-like odour long after the rain has stopped. When that smell seems to move through the building rather than stay in one room, the issue may be in the air filter path, not just in the occupied space.

For office managers, the quickest way to narrow it down is to follow the air: where it is drawn back, where it passes through the system, and where moisture or debris may have been left behind after the storm.

Why storm odours can linger in an office

Heavy rain, wind-driven debris and local flooding can change how a building smells in ways that are hard to trace by scent alone. Damp materials may dry slowly inside wall cavities, ceiling spaces, roof plant areas or return air paths, while the HVAC system keeps moving air through those spaces.

Common sources include:

• •Wet insulation, ceiling dust or debris trapped near return grilles
• •Moisture in ductwork, filter frames or access doors
• •Organic matter pulled into the system during windy conditions
• •Backflow or contamination from nearby drains, pits or plant rooms
• •Musty contamination in the return air plenum or mixing box

If the smell intensifies when the system starts up, that is a clue that the odour is being drawn through the air handling path rather than coming only from a local room source.

Start with the return air path

Return air is often the best place to begin because it brings air from the office back to the system. After a storm, that path can pull in moisture, dust and debris from areas that are normally out of sight.

Look for these signs around return grilles and nearby ceiling spaces:

• •Discolouration on grilles, frames or nearby ceiling tiles
• •Damp patches, staining or visible mould growth on surrounding materials
• •Dust that appears caked, darkened or damp rather than dry and loose
• •Odours stronger close to one return point than another
• •Evidence of water ingress from the roof, façade or service penetrations

If one return grille smells worse than others, isolate that area and inspect the ceiling void, grille frame and adjacent duct connection. Sometimes the odour is being pulled from a local pocket of moisture rather than from the whole system.

Check the filter section, not just the visible filter face

Office managers often look at the filter media and stop there, but the filter section itself can hold the problem. After a storm, moisture can collect in the housing, around the frame, on access doors or in the gasketed edges, then create a persistent musty smell even after the filter media has been replaced.

What to inspect in the filter section:

• •Wet or stained insulation in the housing or access panels
• •Gaps where unfiltered air could bypass the filter bank
• •Rust, corrosion or debris in the frame tracks
• •Condensation or standing water near the unit
• •Dust loading that is uneven across the filter face

If the filter has absorbed moisture, it may need to be removed and inspected carefully. Some filters are designed to be washable - check the product instructions before cleaning or reusing anything. If the filter media is damaged, collapsed or contaminated, replacement is usually the safer path.

You can also compare the filter size and fit against the equipment requirements. If you need to confirm dimensions, our guide on how to measure an air filter can help.

How odours move through the system

An odour rarely stays where the problem started. In an office HVAC system, it can travel through several paths:

• •Through return air ducts from one zone to another
• •Across a leaking filter seal into the supply side
• •From a damp plenum into the fan section
• •Through a mixing box where outside air and return air combine
• •Around access doors or poorly seated panels

That is why a smell may be strongest in one part of the office but actually originate upstream. A musty odour near supply grilles, for example, may point back to contamination in the fan section or filter housing rather than the grille itself.

If the smell resembles sewer gas, do not assume it is only dust or mould. Check whether nearby plumbing, floor wastes, roof drains or plant room drainage may be feeding the issue into the return air path.

A practical tracing method for office managers

You do not need to dismantle the whole system to get useful clues. A systematic inspection can help you decide whether the issue is local, ducted or centred on the filter section.

Walk the building in sequence

Start in the area where the odour is first noticed, then move toward the nearest return grille, ceiling void or plant access point. Note whether the smell becomes stronger or weaker as you move.

Compare different zones

If one office bay smells damp while another does not, that can help distinguish between a local wet material and a system-wide issue. Check whether the return air path serves both areas.

Inspect after the system has been running

Some odours are faint when the system is off and become stronger once air starts moving. That can indicate contamination in the return path, filter section or ductwork.

Check for moisture first

Look for the source of dampness before focusing only on the smell. Moisture is often what allows an odour to persist, especially in ceiling spaces, around insulation and inside filter housings.

Review maintenance records

If there has been a recent storm, water leak, roof repair or filter change, that timeline can help identify where the problem began.

When to call in the mechanical services engineer

If the odour persists, keeps returning after cleaning, or appears to travel through multiple parts of the building, it is worth involving the mechanical services engineer or building owner. Site requirements should be confirmed with them, especially where the system must meet AS 1668.2:2024 or other building-specific ventilation arrangements.

You should also seek expert input when you find:

• •Repeated moisture in the filter section
• •Visible mould growth in the return path or housing
• •Damaged seals or bypass around the filters
• •Strong sewage-like smells near drainage or plant rooms
• •A filter bank that does not sit correctly in the frame

If the system uses higher-efficiency filters, the exact filter type and installation details matter. Ratings and classes do not convert directly between schemes, so match the product to the system specification rather than relying on rough equivalents.

Prevent the smell from coming back after cleanup

Once the source is found and cleaned, the follow-up matters just as much as the first inspection. If the filter path still has moisture, debris or leakage, the odour can return the next time the system runs hard after a storm.

A sensible follow-up plan includes:

• •Drying out any damp insulation or accessible surfaces
• •Replacing damaged or contaminated filters
• •Checking seals, frames and access doors for bypass
• •Clearing return grilles and accessible duct entries
• •Monitoring pressure drop where gauges are fitted

Inspection frequency depends on the system, site conditions and dust load, so follow the filter and equipment manufacturer instructions rather than relying on a fixed calendar. In storm-prone buildings, more frequent checks may be sensible after heavy weather or leaks.

Need the right filter or a custom replacement?

If you are tracing a post-storm odour and suspect the filter section, Total Air Filtration can help supply or make the right filters for your system. We are a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, established 2009.

Call 07 3800 3448 or request a quote. You can also explore our products or custom air filters if your office system needs a specific size or fit.