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Suction is airflow, and the filter decides how much of it you get
Two different measurements get called suction, they trade against each other, and the one that matters changes with what you are cleaning.
Both approaches to vacuum cleaner airflow work. What differs is what they cost you, and the cost is what this sets out.
The difference in one place
- Sealed suction and airflow are measured with opposite conditions.
- Cyclonic separation uses inertia rather than a barrier to remove dust.
- A partially clogged filter chokes the whole system before it looks dirty.
Two numbers that fight each other
Sealed suction is measured with the inlet completely blocked, so no air moves and the figure describes maximum pressure difference. Airflow is measured with the inlet wide open, so pressure difference is minimal and the figure describes volume moved.
A real cleaning task sits between the two, with some restriction and some flow, and both extremes are misleading on their own. High sealed suction with poor airflow lifts a stuck item and clears a room slowly, which frustrates people considerably. The useful behaviour is the combination, and neither figure alone predicts how a machine performs on an actual floor.
The motor makes pressure, the nozzle makes it useful
A fan creates a lower pressure inside the machine, and atmospheric pressure outside pushes air in through the nozzle. Nothing is pulled; the surrounding air is pushed, which is why performance is bounded by atmospheric pressure itself.
Mechanically, narrowing the nozzle raises the air speed at the opening, which is what actually lifts particles off a surface. That is why a crevice tool feels far stronger than a wide floor head fed by the identical motor. A wide head covers more area per pass and lifts less firmly, which is the trade made every time a tool is changed.
Cyclonic separation removes dust without a barrier
Air entering a cyclone is forced into a tight spiral, and the particles carried in it are flung outward by inertia. Heavier particles strike the wall and fall into the bin while the air continues turning and exits from the centre.
At the protocol level, the mechanism requires speed, so a cyclone that is too large or fed too slowly separates far less effectively. Fine dust follows the air rather than the wall, which is why cyclones are followed by filters rather than replacing them. Multiple small cyclones in parallel handle fine particles better, because a smaller radius means a stronger outward force.
Filters are where performance quietly disappears
Every filter restricts airflow, and that restriction grows as material accumulates in and on the medium. The visible surface can look almost clean while the depth of the medium is loaded, which is why appearance misleads.
In the datasheet, as restriction rises, the motor moves less air, and less air means less lifting force at the nozzle. Some machines respond by drawing more current and running hotter, which shortens motor life over time.
Washing a filter and reinstalling it before it is completely dry damages it and encourages growth inside the machine.
Agitation does what airflow cannot
Carpet fibres hold dirt mechanically, and airflow alone cannot dislodge material gripped between them. A rotating brush strikes the fibres and flicks debris upward into the airstream, which is a mechanical action rather than a suction one. This is why a machine with a driven brush outperforms a stronger machine without one on carpet.
On hard floors the brush can scatter debris ahead of the nozzle, which is why many heads allow it to be switched off. Hair wrapping around the brush stops it turning, and a stalled brush removes the entire advantage without any obvious symptom.
Figures here are typical rather than guaranteed — check the spec sheet for your part.
Why it slows down long before the bin is full
In a bagged machine the bag is the filter, and its pores clog with fine dust well before the volume is used. In a bagless machine the pre-filter loads in the same way, with the same effect on airflow.
In practice, blockages in the hose are the other common cause, and they reduce flow while sealed suction stays unchanged. Testing by holding a hand over the nozzle only measures sealed suction, so it detects a leak and misses a restriction. Listening for a rising motor pitch is a better indicator, since a restricted machine unloads its fan and spins faster.
Side by side
| Consideration | What it means in practice |
|---|---|
| Two numbers that fight each other | Sealed suction and airflow are measured with opposite conditions. |
| The motor makes pressure, the nozzle makes it useful | Cyclonic separation uses inertia rather than a barrier to remove dust. |
| Cyclonic separation removes dust without a barrier | A partially clogged filter chokes the whole system before it looks dirty. |
The takeaway
The nozzle sets the speed, the filter sets the volume, and the brush does what neither can.
Once you know what it is trading away, the design stops looking arbitrary.
Questions readers ask
Does more power mean better cleaning?
Input power describes electricity consumed, not air moved. Efficiency of the fan, the sealing and the filter path matter far more.
Why does mine smell?
Fine organic dust trapped in a damp filter is the usual cause. Drying filters fully and emptying the bin promptly prevents most of it.
