The filter question comes up every spring, usually in the same form: the system smells or the airflow is poor, someone has read that the filters need doing, and then nobody can find them. It is a fair confusion, because ducted heating and cooling is laid out almost the opposite way to how people picture it.
Quick answer: In most Melbourne homes there is only one filter in a ducted system, and it sits behind the large return air grille — not behind the outlets in each room. Some installations put it at the unit itself instead, in the roof space or under the floor. The return grille filter is a homeowner job; anything at the unit is not.
The short version is that most Melbourne homes have exactly one filter for the whole ducted system, and it is nowhere near the vents.
Why the filter question is confusing in the first place
A house with ducted heating might have eight or ten outlets. It is reasonable to assume that air is being filtered at each one, in the way a car has a filter for the cabin air that comes out of the dashboard vents.
Ducted systems do not work like that. Air is drawn out of the house at one point, passed through the heat exchanger or coil, and pushed back out to every room. Filtering only needs to happen once, on the way in, before the air reaches the equipment. So the filter sits at the intake — and the intake is the one grille in the house that pulls instead of pushes.
The return air grille, and how to be sure you have found it
The return is the big one. It is usually a wide grille in a hallway wall, a living area wall, or a ceiling, and it is noticeably larger than any of the outlets because all the air for the whole house has to get back through it.
The tissue test settles it in seconds. With the system running, hold a sheet of tissue paper flat against the grille. At an outlet it blows away. At the return it stays stuck to the grille. Once you have found the one that holds the tissue, you have found the filter — on most systems it is directly behind the grille, held by clips or a hinged frame you can open by hand.
Two things are worth knowing before you open it. Turn the system off first, so you are not pulling loose dust straight into the equipment. And look at how the old filter sits before you take it out, because most have an airflow arrow that has to point the same way it did.
Where to look, system by system
Not every Melbourne home has the same setup, and a few have more than one of these running at once. It is worth knowing which of them you actually have before going looking.
The pattern that runs through all of it: anything you can reach from inside the house standing on the floor is generally yours to maintain, and anything that requires getting into the roof space, under the floor or inside a cabinet is not.
What a filter does not protect
A clean filter is worth having, but it is easy to over-credit it. It protects the equipment immediately behind it — the heat exchanger, the coil, the blower — and it catches a share of the dust drawn in from the house. That is the job.
What it does not do is keep the ductwork clean. The ducts are downstream of the filter, they were installed in a roof space or subfloor that was not clean to begin with, and they accumulate over a working life measured in decades. Anything that gets into the system past the filter is not filtered at all, and in older installations there is often more of that than people expect: joints that have loosened, a return that draws partly from the roof space rather than only from the room, or a flexible duct that has come away at a collar.
That is the distinction between filter maintenance and ducted heating cleaning. One is a monthly-ish housekeeping task. The other deals with what is already in the system.
When a clean filter still means poor airflow
If you have just cleaned or replaced the filter and the airflow is still disappointing, the useful thing is that you have eliminated the cheapest cause. What tends to be left:
- Something on the return side. A couch pushed against the return grille, or a return that was undersized for the system when it went in, starves the whole house regardless of how clean the filter is.
- A duct problem in the roof space. Flexible ducting gets crushed by stored items, pulled off at a collar, or left kinked around an obstruction. One bad run affects the rooms it feeds and steals pressure from the rest.
- Build-up on the coil or blower wheel. Both sit behind the filter, but a filter that has been missing or bypassed at any point in the system’s life lets material through, and it settles where the air changes speed.
- Dampers still set for winter. Zoned systems and some manual damper setups deliberately favour certain rooms. A setting that made sense in July is worth revisiting now.
What is yours and what needs a technician
Draw the line at access rather than difficulty. Washing or replacing the filter at a return grille, taking the front cover off a split system indoor head and rinsing its mesh filters, clearing debris out of floor outlets, and making sure nothing is parked in front of a return — all of that is homeowner territory, and doing it before the cooling season starts is the right time.
Anything that involves opening the unit, getting into the roof space, working on evaporative pads, touching a coil or blower wheel, or opening the ductwork is not, and not because of difficulty alone. It is live equipment in an awkward place, and the components most worth cleaning are the ones easiest to damage. If the filter is clean and the system still is not right, that is the point to stop and have the rest of it looked at.
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Frequently asked questions
How do I tell the return air grille from an ordinary outlet?
By size and by direction. The return is noticeably larger than the outlets and there is usually only one of it, often in a hallway or the main living area. The simplest test is to hold a tissue against it while the system is running. An outlet blows the tissue away, the return holds it flat against the grille. If a grille sucks rather than blows, that is where your filter is.
Is there a filter behind each floor or ceiling vent?
No. The outlets are delivery points only, and there is nothing behind them to filter. What collects in a floor outlet is almost always material that fell in from the room, such as dust, hair and crumbs, rather than anything that arrived through the duct. Cleaning them out is worth doing, but it is housekeeping rather than filtration.
My filter is clean and the airflow is still weak. What else could it be?
A filter is the first restriction to rule out, not the only one. Weak airflow with a clean filter points further into the system: a crushed or disconnected flexible duct in the roof space, a build-up on the indoor coil or blower wheel, a return grille that is too small or partly blocked by furniture, or dampers left in a winter setting. Each of those needs the system looked at rather than the grille.
Does having a filter mean the ductwork stays clean?
Not on its own. A filter protects the equipment behind it and catches a share of what is drawn in, but the ductwork downstream still accumulates over years, and anything that enters the system past the filter is not filtered at all, whether it came through a gap at a joint, a poorly sealed return, or a duct run left open in the roof space. That is why homes with a well-maintained filter can still have ducts worth cleaning.
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