What Size Air Conditioner Do I Need

Bigger is not safer

The instinct when choosing a unit is to round up. Take the larger one and it will cope on the worst February afternoon. In Cairns that instinct is wrong, and it is the most common reason a room ends up cool and still uncomfortable.

For most of the year the useful work an air conditioner does here is not lowering the temperature, it is removing moisture. That work happens slowly. Air has to pass over a cold evaporator coil long enough for water to condense on the fins and run away down the drain. A unit that is too big for the room reaches the set temperature in a few minutes, shuts down, and restarts a short time later. Each of those brief runs barely wets the coil. The thermostat reads 24 degrees and the room still feels like a wet towel.

A correctly sized unit runs longer at lower output, which is exactly what an inverter compressor is built to do, and it is dehumidifying the whole time it runs. Comfort in the tropics is a humidity number as much as a temperature one.

What actually decides the capacity

Floor area is where sizing starts and it is nowhere near the end of it.

Ceiling height matters because you are conditioning a volume, not a floor. A living room with raked or three metre ceilings holds far more air than the same footprint under a flat 2.4 metre ceiling.

Glass matters more than almost anything else. A window is a hole in the insulation, and west facing glass in Cairns takes direct sun for hours at the hottest part of the day. Two identical rooms, one facing east under a deep eave and one facing west with nothing shading it, are not the same job.

Then there is the roof above the room, whether the ceiling is insulated, how many walls are external, how many people are usually in there, and whether the room can be closed off at all. An open plan area running into a hallway and a stairwell is not a room. It is a zone, and it has to be sized as one.

No single row of that decides anything. They compound, which is why two rooms of identical floor area can land two capacity steps apart.

Why the watts per square metre rule breaks here

Every calculator online multiplies floor area by a figure and hands you a number. As a rough sanity check that is fine. As a specification it fails twice here.

The first is that it ignores glass and orientation, which is where most of the variation actually sits.

The second is less well known. Nominal cooling capacity is measured at a standard test condition, and the indoor air in that test is drier than a Cairns afternoon in the wet. In real conditions here a larger share of the machine’s output goes into condensing water rather than dropping air temperature, so the sensible cooling you get is less than the plate suggests. The rating flatters a unit in a dry climate. It does not flatter it here. That is an argument for sizing carefully, not for going up a model.

Getting it right before you buy

Sizing is a decision made at quote stage and it cannot be corrected afterwards by turning the temperature down. What we need is simple, and you can usually give it over the phone:

1. Room dimensions, length and width, in metres
2. Ceiling height, and whether it is flat or raked
3. Which way the main windows face
4. Roughly how much glass, and whether anything shades it in the afternoon
5. Whether the room closes off with a door or runs into the rest of the house
6. How many people are normally in there

That is enough to specify a capacity properly. If you are unsure on any of it, say so rather than guessing, because a guess on orientation moves the answer more than a guess on floor area does.

From there we specify a capacity that will hold the room and pull the humidity down with it. See what is involved in split system installation in Cairns, or call 0489 266 523 and describe the rooms.