Aircon CFM: what the airflow figure on a spec sheet assumes
A spec sheet prints two numbers for the same machine, and most buyers read only one. The air volume figure decides how much of the cooling reaches the room, and it was measured under conditions no flat reproduces.
By Team Snowflake | Updated 9 Aug 2026
What the figure counts, and in what units
CFM counts a volume of air per minute. The letters stand for cubic feet per minute, and the figure says how much air the indoor fan moves past the coil. How cold that air is, and how far it reaches, are other lines on the same page.
Singapore sheets often print the same quantity in metric. Litres per second and cubic metres per hour are the two common forms, and cubic metres per minute turns up on data written for Japanese and Korean ranges. One cubic foot per minute works out at roughly 1.7 cubic metres per hour, or a little under half a litre per second.
Some catalogues print both units on the same row. The Daikin FTKM-PVMK catalogue gives air flow at high fan for its 18,000 Btu/h wall unit as 342 litres per second, then repeats it as 723 CFM on the line below. One fan, two ways of writing what it moves.
There is a reference density buried in the volume as well. ISO 5151 defines standard air as dry air at 20 degrees Celsius and 101,325 kilopascals, with a mass density of 1.204 kilograms per cubic metre. The standard then asks for every air quantity in a rating test to be expressed in cubic metres per second of that air. So a published volume is a statement about mass, dressed up as something a person can picture.
| How the sheet writes it | What one unit of it means | Roughly, in CFM |
|---|---|---|
| How the sheet writes itCFM, cubic feet per minute | What one unit of it meansOne cubic foot of air each minute | Roughly, in CFM1 |
| How the sheet writes itL/s, litres per second | What one unit of it meansOne litre of air each second | Roughly, in CFMAbout 2.1 |
| How the sheet writes itm3/h, cubic metres per hour | What one unit of it meansOne cubic metre of air each hour | Roughly, in CFMAbout 0.6 |
| How the sheet writes itm3/min, cubic metres per minute | What one unit of it meansOne cubic metre of air each minute | Roughly, in CFMAbout 35 |
The same air, normalised a different way
One other figure describes moving air and answers a different question. Air changes per hour asks how often a room's whole volume gets replaced, and it counts outdoor air arriving. CFM asks only how much air a machine pushes across its own coil.
A split unit scores well on the first and contributes nothing to the second. It recirculates the same room air, so the volume can be large while the exchange stays at zero. Anyone reading a spec sheet for ventilation is reading the wrong line.
Capacity and air volume are separate specifications
Capacity and air volume answer different questions, and the sheet sets them side by side without saying so. Capacity states the rate of heat the machine can take out. Air volume states how much air is available to carry that heat away from the coil and into the room.
The two do not scale together, and one maker's own data shows it plainly. The Mitsubishi Electric service manual for the MSZ-GE range publishes the same cooling air volume for three separate machines. The 06, 09 and 12 bodies each list 321 CFM in cooling at the high setting, while their rated cooling capacities are 6,000, 9,000 and 12,000 Btu/h.
One fan, three capacities. Work the arithmetic on those three sheets and the air supplied per ton of cooling falls from about 640 down to about 320. Nothing about that is a fault. The indoor body is shared across the range, and what changes between the models sits in the coil and the outdoor unit.
Two quotes matched on kilowatts can therefore be unmatched on air. The machine with less air will draw a room down more slowly and will feel weaker at the same setting, and that difference never appears in the capacity line people compare. Ask for the air volume alongside the capacity, and ask which setting it belongs to.
| Model, one series | Rated cooling capacity | Cooling air volume, high setting |
|---|---|---|
| MSZ-GE06NA | 6,000 Btu/h | 321 CFM |
| MSZ-GE09NA | 9,000 Btu/h | 321 CFM |
| MSZ-GE12NA | 12,000 Btu/h | 321 CFM |
The air-per-ton figure that will not verify
A single number circulates through trade writing as though it were settled law: every ton of cooling wants about 400 cubic feet per minute of air. Chasing it back to a design source turns up nothing. It does have a home in a rating standard, and the home is not the one the phrase suggests.
ANSI/AHRI Standard 210/240-2026 uses 400 standard cubic feet per minute per ton of cooling as a sorting boundary. A low-static blower coil system is one whose indoor units stay inside a stated pressure band when operated at a full-load airflow "not exceeding 400 scfm per specified ton of cooling". Mid-static systems carry the identical clause. The figure files equipment into test categories. It says nothing about what a room needs.
That same document rules out any universal ratio. Its small-duct, high-velocity definition specifies a full-load airflow of "at least 220 scfm per specified ton of cooling", roughly half as much air for the same cooling. Both categories sit inside one standard, describing equipment that is entirely legitimate.
So treat 400 as a category marker and leave it there. A sheet whose ratio sits well away from 400 carries no verdict on the machine. A quotation that cites the rule at you has borrowed a standard's authority for a claim the standard never made.
Air volume, coil temperature and delivered cooling move together
Three quantities are locked to each other, and no two of them fix the third on their own. Air volume, the temperature drop across the coil, and the cooling that actually lands in the room all move as a set. Push one down and something else has to shift.
The rating standards work in exactly that direction. ISO 5151 derives indoor-side cooling capacity from the indoor air volume flow rate together with the specific enthalpy of the air entering and the air leaving. Volume, multiplied by the change in the air's heat content. Those two terms are the entire calculation.
Which is why a machine can lose air and still blow cold. Less air over the same coil spends longer in contact with it, so each cubic foot comes off colder than before. The reading at the grille improves while the room receives less. A hand held at the vent reports the one term of three that moved the wrong way.
The room's own behaviour is the honest reading. A space that used to settle and now creeps down slowly, while the air at the outlet still feels properly cold, is describing a shortage of volume and not a shortage of cooling.
What a cold vent does not prove
A technician reporting cold air at the outlet has measured one term out of three. That is a real measurement and it clears several faults off the list. What it cannot settle is whether the room receives anything close to the published figure.
The protective question is short. Cold compared with what, and at which fan setting? A temperature drop only carries meaning next to the volume it was taken at, because a starved coil produces a deep drop just as readily as a healthy one does.
Every published figure comes with conditions attached
Conditions sit behind every published volume, written down but never on the same line. ISO 5151 requires a cooling capacity test to be run "with 0 Pa static pressure maintained at the air discharge of the equipment". Nothing sits in front of the outlet during that test. No grille, no duct, no half-closed vane.
A fan setting is attached to the number as well. The Mitsubishi Electric MSZ-GE specification names five of them: super high, high, medium, low and quiet. Its cooling series for the 06 to 12 bodies reads 399, 321, 237, 170 and 145 CFM. The quietest setting moves a little over a third of what the top one does.
One more qualifier sits in brackets beside it. The same row prints a wet-coil series against the dry one, at 364, 286, 201, 134 and 109 CFM. Wet is the ordinary state of a coil in Singapore, because a machine pulling humidity out of the air runs wet nearly all the time. At the quiet setting, the wet figure lands about a quarter below the dry.
Some catalogues publish considerably less than that. The Daikin FTKM-PVMK sheet gives air flow at high fan only, while giving sound pressure at high, medium, low and quiet. Four settings documented for noise, one for air. Someone comparing units on quiet running has numbers for how they sound and none for what they deliver.
On a ducted body the figure moves again once resistance stands in front of it. Duct, bends, grilles and the filter each take a share of the fan's external static pressure allowance, and the volume reaching the rooms drops as they do. So the published number marks the top of a range. The route settled long ago where inside it a system would land.
| What moves the delivered volume | Which way it goes | Whether the sheet shows it |
|---|---|---|
| What moves the delivered volumeThe fan setting in use | Which way it goesDown at every step below the top | Whether the sheet shows itUsually, as a series of figures |
| What moves the delivered volumeA coil running wet, as it does here | Which way it goesDown against the dry figure | Whether the sheet shows itSometimes, bracketed on the same row |
| What moves the delivered volumeAnything standing in front of the outlet | Which way it goesDown, since the test faced nothing | Whether the sheet shows itNo, the test condition sits elsewhere |
| What moves the delivered volumeA filter holding what it has caught | Which way it goesDown, and further as weeks pass | Whether the sheet shows itNo, the figure assumes a clean one |
| What moves the delivered volumeDuct and grilles on a ducted body | Which way it goesDown as the route claims more allowance | Whether the sheet shows itOnly as a pressure rating, not a volume |
The setting the remote picks for you
Auto fan mode selects from that series without announcing which entry it took. A unit left on auto, or on a quiet or sleep programme, spends much of its running time at a setting whose published volume is a fraction of the top one. The machine is doing as told. The room is getting the small number.
Ruling this out costs nothing, and it gets skipped more often than anything else here. Before a fan motor falls under suspicion, or a larger machine gets quoted, establish which setting the unit has been living on. Compare at the top setting instead. A service report saying airflow was checked, without naming the setting it was checked at, has reported an opinion.
What to ask for, and what an answer should contain
The figure is easy to obtain before a purchase and awkward to obtain afterwards. Every maker publishes it somewhere, in a catalogue or a service manual, and the AHRI rating standard requires the maker to have specified the cooling full airflow before any test begins. Ask for it in writing alongside the quotation, with the setting it belongs to.
Measuring it on site is a different proposition. ANSI/ASHRAE Standard 41.2 covers methods for air velocity and airflow measurement, and rating laboratories use nozzle apparatus to do the job. None of that fits in front of a wall unit in a bedroom. On a ducted system the measurement is practical and belongs in the handover paperwork; on a wall unit, the published figure and the setting are what anyone has to work with.
Whatever loads up afterwards has no baseline to be judged against. The published volume assumes a clean filter, it was taken once in a laboratory, and nobody restates it later. A system quietly shedding air over two years is shedding it against a number that was never checked in the room. That is what lifts filter servicing out of housekeeping and into performance. Cleaning the filter is the one routine act that gives volume back, which is why the interval earns more attention than it usually gets.
Compare machines at matching settings or leave the comparison alone. Top setting against top setting, wet figure against wet figure wherever both are printed, and the volume line read with the same attention the capacity line already gets.
Three questions worth asking before anything is ordered
Three questions cover most of it, and a supplier who has read the data answers them without looking anything up. What air volume does this model publish, and at which fan setting? Is there a wet-coil figure printed as well as a dry one? Which setting does the unit fall to when it is left on auto?
The middle question is worth the most. A wet-coil volume at a named setting is the closest thing on any sheet to what a Singapore room will genuinely be handed. It strips out the two assumptions the laboratory figure makes without ever saying so.
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