Aircon CFM: What the Airflow Figure on a Spec Sheet Assumes

CFM counts a volume of air per minute: cubic feet per minute, the 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 common forms, with cubic metres per minute on Japanese and Korean data. One CFM is roughly 1.7 cubic metres per hour.

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 below. One fan, two ways of writing what it moves.

A reference density is 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. Every air quantity in a rating test is then expressed in cubic metres per second of that air. A published volume is a statement about mass, dressed up as something a person can picture.

Capacity and air volume answer different questions, and the sheet sets them side by side without saying so. Capacity is the heat the machine can take out; air volume is how much air carries that heat 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 and the air supplied per ton of cooling falls from about 640 to 320. That is not a fault: the indoor body is shared, and what changes sits in the coil and the outdoor unit.

Two quotes matched on kilowatts can be unmatched on air. The machine with less air draws a room down more slowly and feels weaker at the same setting, and that difference never appears in the capacity line people compare. Ask for the air volume with the capacity, and which setting it belongs to.

Three quantities are locked to each other, and no two fix the third on their own: air volume, the temperature drop across the coil, and the cooling that lands in the room. Push one down and something else has to shift.

The rating standards work in that direction. ISO 5151 derives indoor-side cooling capacity from the air volume flow rate and the specific enthalpy of the air entering and leaving: volume multiplied by the change in heat content, and those two terms are the calculation.

Which is why a machine can lose air and still blow cold. Less air over the same coil spends longer in contact, so each cubic foot comes off colder. The reading at the grille improves while the room receives less, and a hand 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 outlet air still feels properly cold, describes a shortage of volume, not of cooling.

Conditions sit behind every published volume, written down but never on the same line. ISO 5151 requires a cooling capacity test to run "with 0 Pa static pressure maintained at the air discharge of the equipment". Nothing sits in front of the outlet: 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: super high, high, medium, low and quiet. Its cooling series for the 06 to 12 bodies reads 399, 321, 237, 170 and 145 CFM, so the quietest setting moves a little over a third of the top.

One more qualifier sits in brackets. 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 here, 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 less. 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. The published number marks the top of a range, and the route settles where a system lands inside it.

The figure is easy to obtain before a purchase and awkward afterwards. Every maker publishes it somewhere, in a catalogue or service manual, and the AHRI standard requires the cooling full airflow to be specified before any test begins. Ask for it in writing with the quotation, and the setting it belongs to.

Measuring it on site is a different proposition. ANSI/ASHRAE Standard 41.2 covers air velocity and airflow measurement, and rating laboratories use nozzle apparatus; none of that fits in front of a wall unit in a bedroom. On a ducted system the measurement belongs in the handover paperwork; on a wall unit, the published figure and the setting are what anyone has.

Whatever loads up afterwards has no baseline to be judged against. The published volume assumes a clean filter, was taken once in a laboratory, and nobody restates it. A system quietly shedding air over two years sheds it against a number never checked in the room. That lifts filter servicing out of housekeeping: cleaning the filter is the one routine act that gives volume back.

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 attention the capacity line already gets.