Three duct types
Operators clean the lint filter and assume the duct is fine. But the filter catches only part of the lint, every bend and rise traps more, and restriction feeds on itself - so the run clogs far faster than a steady build-up would suggest.
The short answer
Most operators empty the lint filter and assume the ductwork behind it is looking after itself. It is not. The filter only ever catches part of the lint a load sheds; the rest passes into the duct and settles on every surface, and it settles fastest at bends, joints and vertical rises. Worse, the process accelerates - a partly restricted duct slows the airflow, and slower air drops more lint. So laundry ducts do not fill steadily; they compound, which is exactly why they clog faster than expected.
The filter myth
A lint filter is a coarse first defence, not a seal. Fine, airborne lint passes straight through it and travels into the exhaust duct, where it coats the walls, the joints and the fan. Because that lint is out of sight - behind the dryer, inside the wall, at the exterior termination - the operator sees a clean filter and assumes a clean system, while the duct quietly loads up. The visible part is reassuring and the hidden part is where the problem actually lives.
High-volume laundries make this worse simply through throughput. Back-to-back loads all day mean the lint load is relentless and cumulative, so even a small fraction slipping past the filter each cycle adds up quickly over a run that never really stops.
Geometry and the feedback loop
Duct shape decides where lint lands. Every ninety-degree bend, every joint and every vertical rise is a place the airflow slows and turbulence drops lint out - which is why guidance treats each bend as effectively shortening the usable duct length. Long runs and multiple turns, common where a dryer is tucked away and vented a distance to atmosphere, give lint many places to settle. Flexible foil or thin plastic ducting makes it worse still: it sags, kinks and traps lint far more readily than smooth rigid metal.
Then the feedback loop takes over. A partly clogged duct restricts airflow; slower air carries less and drops more lint; the restriction grows, slowing the air further. The build-up is not linear but accelerating, so a duct that seemed fine for months can tip into serious restriction quickly. Moisture compounds it - the warm, humid exhaust makes lint stick and compact rather than blow clear.
What it means for maintenance
The practical conclusion is that the filter is not the maintenance signal - the duct is. Emptying the filter every cycle is necessary but tells you nothing about the run behind it, and the visible symptoms of a clogged duct - loads taking longer to dry, a hot laundry room, weak airflow at the termination - only appear once restriction is already well advanced. Because the accumulation compounds, the safe approach is to inspect the whole run on an interval set by volume and clear it before restriction builds, rather than waiting for the drying times to climb. A duct cleaned to atmosphere - not just the filter emptied - resets the loop before it starts.
Questions
No. The filter catches only part of the lint; the rest passes into the duct and settles on the walls, joints and fan - out of sight behind the dryer and in the wall. A clean filter can hide a loading duct.
Airflow slows and turbulence drops lint out at every bend, joint and vertical rise, so each is a catch point. Guidance treats each bend as shortening the usable duct length, and long runs give lint many places to settle.
A partly clogged duct restricts airflow; slower air carries less and drops more lint; the restriction grows and slows the air further. Build-up is accelerating, not steady - so a duct can tip into serious restriction quickly.
Yes. Flexible foil or thin plastic ducting sags, kinks and traps lint far more readily than smooth rigid metal, so it clogs faster and is best replaced with rigid metal where possible.
Inspect the whole run on an interval set by volume and clear it to atmosphere before restriction builds, rather than waiting for drying times to climb - by then the clogging is already well advanced.
Phoenix Duct Clean · by the numbers
We clear the whole exhaust run - the parts the filter never sees - before restriction turns into downtime or a fire risk. Call to set an inspection interval that fits your volume.