Vacuum cleaners: airflow, sealed suction and the leaks between them

This is less a set of instructions about how vacuum cleaners are specified than an argument, and it is worth saying so at the start.

The two measurements and what each answers

Sealed suction, often quoted as water lift, measures the pressure difference a motor can produce with the inlet blocked. Airflow measures the volume of air moved per unit time with the inlet open, which is what carries dust along a hose.

A machine needs both, because suction lifts debris from a carpet pile and airflow transports it to the bin without dropping it. Motor input wattage measures neither, and it was the headline figure for years precisely because it is trivially easy to raise. Regulations in several regions now require standardised performance figures instead, which is why older and newer specifications look different.

Where the performance is lost

Every bend, seal and filter between the floor head and the motor costs pressure, and a poorly sealed machine loses more than a weak motor does. Bagless cyclonic separation is a pressure loss by design, traded for the convenience of not buying bags.

On the bench, filters clog progressively, so a machine that performs well when new can perform badly after a month without any fault developing. Hose diameter and length matter substantially, which is why a canister machine and an upright with identical motors behave differently. The most informative thing about a machine is often how easy it is to keep the air path clear, since that is what preserves performance.

Filtration and the trade it forces

Fine filtration captures small particles that would otherwise pass through and be blown back into the room, which matters for allergies. Finer media resist airflow more, so high filtration and high airflow pull in opposite directions and must be balanced by a larger motor or a larger filter area. A sealed system matters more than filter grade, because air that leaks around a filter has not been filtered at all.

Filter cost and replacement interval are part of the purchase, and washable filters need genuine drying time between uses. Claims about particle capture percentages refer to defined particle sizes under defined test conditions, and the size stated is the important part.

The floor head does the mechanical work

On carpet, most dirt is held in the pile by friction, and a rotating brush agitates it loose so that airflow can remove it. On hard floors a brush can scatter debris, which is why heads either lift the brush, change speed or switch to a soft roller.

After the warranty ends, head sealing against the floor determines how much of the available suction reaches the dirt rather than leaking around the edges. Hair wrapping around the brush bar is the most common maintenance problem, and designs differ enormously in how easily it is removed.

Because the head is where the work happens, a good head on a modest motor generally beats the reverse.

Cordless changes the constraints

A cordless machine trades sustained power for convenience, and its suction usually falls as the pack depletes unless the electronics regulate it. Runtime figures are typically quoted on the lowest power setting with a non-powered head, which is not how most cleaning is done.

Judged against the category, bin size is far smaller, so emptying frequency rises and the dust release mechanism becomes a daily interaction worth examining. The battery is a consumable with a finite life, so whether packs are replaceable and still sold in five years is a central question. Weight distribution decides whether a stick machine is usable overhead, and the specification sheet rarely captures how it balances.

What to establish before buying

Find standardised performance figures rather than motor wattage, and check whether they were measured on carpet, hard floor or both. Establish the ongoing cost and availability of bags, filters and brush bars, since these decide the running cost more than energy does.

In the small print, check how the machine is serviced: whether the head opens without tools, whether the hose can be cleared, and whether parts are listed for sale. Match the machine to the floors you actually have, because carpet and hard floor reward almost opposite designs. This site does not test cleaning performance, so standardised comparative tests from consumer organisations are the evidence to read alongside this.

The takeaway

Judge a vacuum by how little it loses between the floor head and the motor, not by the number on the motor.

Buy for the failure you can live with, not the feature you will use twice.