Most Melbourne homeowners do not have a maintenance schedule for their ducted system — they wait until something noticeable happens before acting. The problem is that by the time obvious symptoms appear, the system has typically been accumulating contamination for years. Understanding the early and definitive signs that cleaning is needed helps you act before the system is heavily soiled, before health effects become pronounced, and before a blocked system starts affecting performance.
This guide walks through every sign worth knowing — from the first subtle indicators to the clear signals that professional cleaning is overdue. It also covers how to do a basic self-inspection at home and when a camera inspection by a professional is warranted.
Visible Signs at Registers and Vents
The most accessible inspection points are the supply and return air registers throughout the home. What you can see here gives a reasonable indication of what is happening deeper in the system.
Dust discharge from supply vents
Hold a white tissue or sheet of paper near a supply register when the system first starts after a period of inactivity. If the paper shows a grey residue or fine dust within a minute of system startup, the duct system is actively discharging particles. This is the most direct confirmation that cleaning is needed. The discharge is heaviest when the system first starts because settled debris is re-entrained by the initial airflow surge.
Visible debris at register openings
Remove a supply register grille and look into the duct opening with a torch. You should see sheet metal or flexible duct material. If you see a grey-brown layer of compacted debris on the duct floor or walls within the first 30 cm, the system needs cleaning. Pet hair matting around the register frame or visible inside the opening is a particularly clear indicator in pet-owning households.
Ghost marking around ceiling vents
A grey or black halo on the ceiling surrounding a supply vent is called filtration soiling or ghost marking. Fine particles from the duct airstream deposit on the ceiling surface as air exits the register. This discolouration indicates a significant particulate load in the air being distributed by the system. It is also a useful indicator when comparing vents — a vent with pronounced ghost marking is likely receiving more contaminated airflow than others, which may indicate a particularly dirty section of ductwork upstream.
Visible mould or moisture at registers
Fuzzy growth or dark discolouration inside or around a supply register opening is a serious sign. Mould inside ductwork requires professional assessment and cleaning. Do not attempt to clean mould from duct openings yourself — disturbing mould inside a system without negative pressure extraction releases spores throughout the duct network. See our dedicated guide on mould in air ducts in Melbourne.
Smell Indicators
Odour from a ducted system is one of the most reliable early indicators that something in the ductwork needs attention:
Musty or stale smell from registers
A musty, earthy smell from supply registers — particularly noticeable when the system first starts in winter — typically indicates accumulated dust, biological contamination, or mould inside the ductwork. The smell may be subtle at first and intensify over weeks as the system distributes the contamination. See our guide on musty smells from air vents in Melbourne for detailed diagnosis.
Pet odour from vents
In homes with pets, a warm animal smell from supply registers is a specific indicator of dander and biological contamination accumulation. This is distinct from temporary odours caused by a pet being near a return air grille — the contamination smell persists regardless of where pets are located when the system runs.
Burning or dusty smell at startup
A brief dusty smell when the heating system first starts after a long dormant period is normal — dust on the heat exchanger burns off in the first few minutes. If this smell persists beyond the first 10 to 15 minutes of operation, or if it recurs regularly at startup, it indicates a significant dust accumulation in the system that warrants cleaning.
System Performance Indicators
Duct contamination affects system performance in measurable ways. These performance changes are often attributed to system age or fault, but frequently indicate cleaning is needed:
Reduced airflow from registers
Hold your hand near a supply register. Airflow should be clearly perceptible at 15 to 20 cm distance. Weak or barely detectable airflow from registers that previously performed well indicates duct restriction — either from debris buildup inside the duct run or a blocked filter. Check the filter first; if replacement does not resolve the issue, professional cleaning is the next step. See our guide on weak airflow from vents in Melbourne.
Uneven heating or cooling between rooms
If some rooms heat or cool noticeably faster or slower than others from the same system, and this was not always the case, debris restriction in specific duct branches may be the cause. Partial blockages divert proportionally more airflow to less restricted branches, creating imbalance. See our guide on uneven heating and cooling from ducts in Melbourne.
Rising energy bills without changed usage patterns
A ducted system working harder to push air through contaminated or partially restricted ductwork consumes more energy for the same conditioning output. If your gas or electricity bills have increased during the heating or cooling season without a change in usage patterns, duct restriction is one of the first things to investigate. See our guide on dirty ducts and rising energy bills in Melbourne.
When to Call a Professional
Some signs warrant immediate professional assessment rather than monitoring. Contact FreshDuct or a qualified technician promptly if you observe:
- Visible mould growth at or inside any register opening
- Signs of rodent or pest activity — droppings, nesting material, or sounds from the duct system
- A gas or chemical smell from supply registers (requires immediate attention — could indicate heat exchanger fault or gas leak)
- Water or moisture visible in or around supply registers (indicates condensation problem or system fault)
- System has never been professionally cleaned in more than 5 years of use
For a broader assessment of health and performance indicators from duct contamination, see our guide on air duct cleaning and indoor air quality in Melbourne.