Two Small Air Purifiers or One Large One?

The minimums are per room#

The published table matches a CADR minimum to a room's floor area. Not a flat, not a floor, not a hallway, a room. A large unit standing in the hall serves the hallway, whose minimum is small, and then loses delivery to every doorway it is asked to reach beyond.

That is why "one big unit for the whole flat" is usually the wrong shape of answer. The minimums were never written per home.

Where two units win#

Two closed rooms at night, two bedrooms, or a bedroom and a nursery, is the clean case. Each room gets its own unit sized to its own row, each door stays shut, and each sleeper gets air cleaned to the published minimum. One large unit in the corridor serves neither room.

Two smaller units also shorten the journey particles must make to reach a filter. In a bedroom, a unit near the bed catches what the bed region produces before it spreads. compare small-room air purifiers on Amazon, that is the most common honest configuration.

Where one unit wins#

One large open space, an open-plan living area, a studio flat, is the other clean case. One unit sized to the space's row with the round-up rule, one set of filters to maintain, one machine to place and clean around.

There is a third case that surprises people: a single large room can also justify two units, not for coverage but for quiet. Two units whose combined CADR meets the room's minimum can each run at a lower speed than one unit doing all the work, and fan speed is what you hear. For a sleeping space, that trade is often worth the floor area.

The decision rule#

1. 1Sketch the floor plan. Mark every room with a door that closes at night or during use.
2. 2Each closed room gets its own unit, sized to that room's published row, round up between rows.
3. 3Shared open space gets one unit sized to the whole open area's row, not to the largest room inside it.
4. 4Two closed rooms never share one unit, however large the unit. The corridor is not a room.
5. 5If a single open room doubles as a sleeping space, consider two units meeting the minimum together, each running quieter as a result.

The honest costs either way#

Two units mean two filters to buy and two machines to maintain. One large unit costs more up front than a small one and cannot be in two rooms at once. Neither trade is hidden: the per-room minimums are published, and the filter counts are printed on the box.

On running cost we quote no figures. The published wattage of the speed you run is the only electricity number worth reading, and it sits in the manual rather than in an advert.