Five Air Purifier Sizing Mistakes

The five mistakes#

1. 1Buying on the coverage claim. "Covers 1,500 sq ft" is not a standardised figure, no single test or set of assumptions sits behind it. CADR is the standardised number, and it is the one EPA tells you to size with. Coverage claims vary between brands for the same delivery; CADR does not.
2. 2Rounding down between published sizes. The published table comes in 100 square foot steps, and a 350 square foot room does not appear in it. Take the larger row, 260 cfm, the 400 square foot minimum, not 195, the 300 square foot figure. CADR is a minimum, so rounding down leaves the room permanently under-served.
3. 3Ignoring ceiling height. Every published figure assumes an eight-foot ceiling. A 200 square foot room with a ten-foot ceiling holds 2,000 cubic feet, not 1,600, a quarter more air than the row was written for. EPA's advice for higher ceilings is a higher CADR.
4. 4Trusting MERV as a room-size rating. MERV rates how well a furnace or HVAC filter captures particles. It says nothing about how large a room a portable unit can serve, and EPA publishes no MERV-by-room-area table. A filter rated MERV 13 and above must demonstrate at least 50% removal of the smallest particles tested, that is a capture figure, not a size.
5. 5Expecting CADR to cover odours. CADR is defined for particles: smoke, dust and pollen. There is no published CADR for gases or odours, so there is no minimum to size against. A purifier sold on odour removal is not offering a figure you can check against your room.

Why the mistakes cluster#

The five mistakes share an engine: they all start from the box rather than the room. Coverage claims, MERV numbers and odour promises are all printed on the box. The published minimum is matched to the room, which is the thing you actually own.

The habits reinforce each other, too. Buy on a coverage claim and you will round down, because coverage claims encourage treating sizes as suggestions. Size on CADR and the round-up rule takes care of itself.

The habit that replaces all five#

1. 1Measure the room's floor area.
2. 2Find the published minimum, round up between rows, and remember the table stops at 600 square feet.
3. 3Check the tobacco-smoke CADR, which represents the smallest particles.
4. 4If the ceiling is well above eight feet, lean towards the next row up.
5. 5Place the unit with clearance, near the source, and close the door.

If you have already bought small#

A small unit in a large room is not useless; it delivers less clean air than the room deserves. Run it continuously at its highest speed, place it near where you sit or sleep, and close the doors so it serves one room rather than a leaky flat.

Then treat the arithmetic as a shopping instruction. The gap between the CADR you have and the published minimum for the room is the specification of the second unit, or the replacement, worth buying. compare HEPA air purifiers on Amazon with the published row in hand rather than a coverage claim.