HVAC Odours That Keep Coming Back: When the Filter Is Part of the Problem

If an HVAC odour keeps returning after cleaning vents, changing cleaners or checking for obvious spills, the filter is worth a close look. A clogged, saturated or poorly matched filter can make odours linger by restricting air movement or by failing to capture the particles and contaminants that carry smell through the system.

For homeowners, office managers and commercial buyers, the key is to treat the filter as part of the whole air-handling system, not an isolated item. When it is doing the wrong job, the system can struggle to move air properly and may keep recirculating unwanted odour sources instead of removing them.

Why a filter can make odours seem worse

A filter does not create odours, but it can influence how they travel through a building. If the filter is heavily loaded with dust and debris, air has to work harder to pass through it. That can reduce airflow across the evaporator coil, through return grilles and into occupied areas, which may make stale or musty odours more noticeable.

A saturated filter can also trap moisture, dust and organic material. In some systems, that build-up can become a place where odour is held and then released back into the airstream. The result is often a smell that seems to come and go, especially when the system starts up after being idle.

Another issue is bypass. If a filter is the wrong size, installed poorly or not seated correctly, air can move around the filter instead of through it. That means particles linked to odour are not being captured as intended, and the system may keep circulating the same smell source.

Signs the filter may be involved

• •The odour is strongest when the system first turns on
• •Airflow feels weaker than usual at supply registers or diffusers
• •The filter looks dark, dusty or damp
• •There is dust build-up around the filter frame or housing
• •Odours return soon after cleaning the room or duct area
• •The system has been running with a filter that looks intact but does not seem to fit tightly

Clogged filters and restricted airflow

When a filter becomes loaded, the pressure drop across it rises. In practical terms, that means the fan has to work harder to pull air through the system. If the unit is already operating near its limits, reduced airflow can affect how well the system mixes, filters and moves air.

That matters for odour control because air movement is part of how contaminants are diluted and carried to the filter. If airflow is restricted, odours can collect in rooms, corridors, meeting spaces or plant rooms and then reappear later when the system cycles again.

For commercial buyers and building managers, this is one reason to check pressure drop where gauges are fitted and to monitor system condition instead of waiting for obvious complaints. Inspection frequency depends on the system, site conditions and dust load. Follow the filter and equipment manufacturer instructions, and for building ventilation matters confirm site requirements with the mechanical services engineer or building owner. If the system is part of a larger ventilation design, filter servicing or commercial filter servicing may also be worth arranging.

When the filter type is the real mismatch

Not every filter is suited to every odour problem. A coarse filter may capture larger debris well enough, but still allow finer dust and aerosol particles to pass through. In workplaces with cooking odours, cleaning chemicals, printing emissions or heavy foot traffic, the filter may be mechanically sound yet still be the wrong grade for the load.

The same applies when the system needs a better balance between air movement and filtration. A very fine filter can improve particle capture, but if it is not suitable for the fan, housing or duct design, it can raise resistance too much and reduce airflow. That can leave odours more noticeable even though the filter itself looks like a better product on paper.

In Australian buildings, you may hear older references to G3/G4 or F5-F9 classes. These are EN 779 classes and are still widely used in Australia, even though ISO 16890 has superseded EN 779. MERV, EN 779 and ISO 16890 do not convert directly, so it is better to choose the filter based on the actual system requirements rather than trying to match numbers across standards.

Moisture, mould and filters that have already loaded up

Odour complaints often start with moisture. If a filter is damp from condensation, leaks, a wet return path or poor storage before installation, it can hold smell and dust at the same time. In some systems, that can make a stale or musty odour return long after the original moisture source was addressed.

This is especially important in bathrooms, kitchens, laundries, basements, plant rooms and any area with variable humidity. A loaded filter in a damp environment can become a smell reservoir, particularly when the system switches off and the air sits stagnant.

If you suspect mould or persistent moisture in the system, the filter may not be the only issue. The underlying source should be checked, and any wet components or ducting should be assessed by the appropriate trades or mechanical services personnel. The filter should then be replaced with a suitable product that matches the system and environment.

Choosing the right filter for odour-prone spaces

The best filter choice depends on the application, the air handler and the type of contaminant load. A small office with regular housekeeping needs is different from a commercial kitchen, retail tenancy, warehouse office or healthcare-adjacent space. In some settings, the filter needs to focus on dust capture. In others, a higher performing filter arrangement may be needed to better manage fine particles that carry odours.

When comparing options, look at:

• •Filter dimensions and frame type
• •Required airflow and resistance in the system
• •The contaminant type, including dust, lint, smoke residue or general airborne particles
• •Whether the housing seals properly and prevents bypass
• •Whether the filter is designed for the environment, including moisture or heavier dust loading

Some filters are designed to be washable - check the product instructions. If you are not sure whether a washable option is suitable, the safest approach is to confirm compatibility with the equipment and maintenance plan before fitting it.

If your existing filter size is unusual or you have a non-standard frame, how to measure an air filter can help before you order. For unique housings or specialised applications, custom air filters may be the better option.

Practical checks before you assume the odour is from ducts

Before opening up ductwork or replacing other components, start with the basics:

• •Check whether the filter is visibly dirty, damp or damaged
• •Confirm the filter is installed in the correct direction, if airflow arrows are marked
• •Make sure the filter sits snugly in the frame with no gaps around the edges
• •Inspect the surrounding housing for dust build-up, moisture or corrosion
• •Review whether the filter type still matches how the space is being used

If the odour only appears when the system runs, the filter, fan and coil should be considered together. A filter issue can mask a deeper problem, but it can also be the simplest reason the smell keeps returning.

For some sites, AS 1668.2:2024 is part of the ventilation conversation. That said, site requirements should always be confirmed with the mechanical services engineer or building owner, because the right approach depends on the building and its use.

Getting the filter right the first time

If odours keep coming back, it is often worth replacing the guesswork with a proper filter match. Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, established 2009, and we can help supply or make the right filter for your system.

Whether you need a standard replacement or a custom solution, start with our products or industries we supply, then request a quote. You can also contact us on 07 3800 3448 to discuss the filter issue and the best next step.