Sizing a system for a room

Why the size matters

Bigger is not the safe choice. Energy Rating’s buying tips note that bigger appliances generally use more energy, so there is no point paying for capacity you won’t use. energy.gov.au’s guide for northern Australia puts it more sharply: an incorrectly sized unit will be very inefficient, and it pays to get professional help.

Five things to settle before the kilowatts

1. Which space, how often, for how long. Your Home suggests thinking about the area you want cooled and how long you need it. Its example: where summer nights cool sharply, you may only need to cool a small living area through the hot day, then open bedrooms to cooling breezes at night. For an energy-efficient home, it says, the best option is cooling only the rooms where it is needed, because a central system in a home with unused rooms can waste energy cooling empty spaces.
2. A room that can hold the cool. energy.gov.au suggests installing in an easily sealable, insulated room, so you feel the effect sooner, avoid air leaks and keep energy costs down. Your Home says “for efficient air-conditioning, the house or room should be well sealed and highly insulated”.
3. The sun on the glass. Your Home says unprotected glass is often the greatest source of heat entering a home, and that shading the glass is the best way to cut unwanted heat gain. Effective shading can block up to 90% of the heat of direct sun. East and west windows are harder, because the low morning and afternoon sun is more difficult to shade. The outdoor unit should be shaded from direct sun too.
4. The insulation above and around it. Your Home calls insulation critical to passive cooling. It estimates that insulating the roof and ceiling can cut heating and cooling costs by 45 percent or more, and insulating walls by around 15 percent. It also warns that insulation without shade can trap built-up heat inside, an ‘oven’ effect. Before adding insulation, energy.gov.au says to have a licensed electrician check that your wiring is safe to cover.
5. Your climate. energy.gov.au’s northern Australia guide suggests cooling-only units for tropical climates, and reverse-cycle units for sub-tropical and hot arid climates where both heating and cooling are needed. Your Home adds that improving how well a home holds its temperature may mean you need less heating or cooling than you think. Its passive cooling page notes that buildings in every Australian climate need some cooling at some time of year, and that it is especially useful in hot climates, humid or dry.

The room’s own facts

When an installer works out the size, the Queensland electricity network Energex says they will take into account how big the area is, whether its walls and roof are insulated, and how much sun it gets. Its checklist also names the direction the room’s windows face. These are worth writing down before anyone visits:

• the room’s floor size, and whether it opens onto other rooms;
• which way its windows face, and what shades them;
• whether the ceiling and walls are insulated;
• where the outdoor unit could sit sheltered from the afternoon sun, which energy.gov.au suggests discussing with a licensed technician.

Getting to the number

For an idea of the size you may need, energy.gov.au points to the FairAir website of the Australian Institute of Refrigeration Air Conditioning and Heating. Your Home suggests the institute’s online calculator, or quotes from air-conditioning installers, to make sure the system is correctly sized for your space.

The number you end up with is a capacity in kilowatts, and it is the figure at the top of the energy rating label. Energy Rating says it matters both for making sure the unit can cool the space and for comparing like systems. Reading the energy rating label shows where to find it, and split, ducted or evaporative covers which kind of system to size.