What to know
What actually collects in a duct system
Most of it is ordinary and unglamorous. The bulk of household dust is skin cells, fabric fibres from carpet and upholstery, soil tracked in on shoes, and cooking residue. All of it circulates, and a share of it settles inside the ductwork rather than on your shelves.
On top of that sits whatever is particular to the house: pet hair and dander, insulation fragments where a run passes through an attic, drywall dust from any work that has been done, and in older systems, corrosion from the duct itself.
Why the return side fills first
Supply ducts push conditioned air out. Returns pull household air back, unfiltered, all the way to the filter at the air handler. Everything the house is carrying travels that route.
It is why the return grille is the first place to look when you want to know how a system is doing, and why homes with pets show it there long before anywhere else.
What makes one house fill faster than another
- Pets, particularly ones that shed, which is the single biggest differentiator between otherwise similar homes.
- Carpet against hard flooring, since carpet holds and releases fibres continuously.
- Remodelling, where fine drywall dust gets into everything and settles for years afterwards.
- A filter that is the wrong size for its slot, letting air bypass around the edges.
- Ducts running through an unconditioned attic or crawl space, where connections have more chance to draw in dust.
- How long the system runs, which in this climate means both a real winter and a real summer.
What accumulation does to the system
Material that settles narrows the effective size of a run, and a narrower run moves less air. That shows up as a room falling behind the rest of the house, or airflow that has quietly weakened over several seasons.
Where it matters most is the return side and the area right at the air handler, because everything the system moves passes through there.
What this does not mean
Cleaning a duct system will not make a house dust-free. Most of what settles on your furniture is being generated inside the house, and it will keep arriving whether the ductwork is spotless or not. Anybody promising otherwise is selling something.
What cleaning addresses is what has accumulated inside the system itself, which is a real and specific thing rather than a general improvement to the house.
So how often?
When accumulation has reached the point where you can see it, or where airflow has changed and the filter does not explain it. For many settled homes that is a long interval. For a home with several pets, or one that has just been through construction, it can be considerably shorter.
Rather than guess, have somebody look. Ours costs nothing, and being told the system is in reasonable shape is a perfectly good outcome.