Choosing the right size air conditioner for your home


In today’s climate, it’s common for Australian households to install some form of air conditioning.

The most practical time to install a quality air conditioning system is while you are building a new home, or completing a renovation – this will likely allow the easiest access for your builder or installer to access the roof and wall cavities to best position your cooling unit.

With hundreds of brands, systems, sizes and units available, how do you know which product is right for your home?

Here are a few guidelines to follow when deciding what size air conditioner you need for your home:

  • To calculate how big your home is in square metres, you can check your house plans or use some basic maths to measure and multiply out the dimensions of each room you want air conditioned, and then add them up to figure out the total house size.
  • For reverse cycle air conditioners, the general equation to work out what size air conditioner your home requires is between 125 watts and 145 watts per square metre. Very energy efficient homes may require as little as 80Wm2. Commercial office or retail space may increase up to 180 watts per square metre.
  • For evaporative air conditioners (also known as evaporative coolers), the size of air conditioner you need is determined by the volume of area (cubic metres) to be cooled, as the evaporative cooler displaces air in the home out through open windows and doors.
  • If you are looking at a ducted reverse cycle system it is a good idea to get the system zoned so you don’t air condition your whole home at one time. This will save on running costs and means you don’t have to buy such a large air conditioner.
  • A good way to split the zones is to have the bedrooms on one ‘zone’ and living areas in another. If you have a two storey home, splitting zones across levels is another practical way to spread the load on your system.

A good air conditioning dealer will measure your home and design the zones to suit your home and your lifestyle, and give you advice on the most efficient way to structure any ‘zones’. Generally air conditioning is sized to air condition all your daytime living areas (family room, lounge and theatre) at the same time.

House Size Guides

As a general guide, the following size air conditioning units are most suitable to these house types:

1 or 2 bedroom unit
In most situations a 2.5kW split system will be big enough for the master bedroom and a 5kW split system for the living area. If you chose a larger unit, some of the cool air would “spill over” into other areas.

3 bedroom home
If there are only one or two people living in the house, you could opt for two split systems – one in the master bedroom and the other in the main living area.

For a family of three or more though, it is generally more economical, and better looking for the overall house decor, to upgrade to a ducted system if you want three or more rooms air conditioned.

Ducted evaporative air conditioning is a good choice for people on a budget as it so much cheaper to buy and to run than ducted reverse cycle air conditioning. The downside with an evaporative cooler is that it provides cooling only, not heating, and it does not work so well on humid and extremely hot days ( hotter than 36 degrees celsius). A 7kW evaporative cooler with up to 7 outlets (outlets are the vents the air comes out of) would normally be sufficient.

Ducted reverse cycle systems for a three bedroom house would normally be 10kW or 12.5kW with two zones – a day zone (living areas) and a night zone (bedrooms).

4 bedroom home
Today the average new home has four bedrooms and two living areas. Air conditioning this size of house requires a 7 or 8 outlet evaporative cooler capable of cooling 160sqm of living space with 2.4m high ceilings, OR a 14kW ducted reverse cycle system with 4 zones and 7 or 8 outlets.

Houses with more than 4 bedrooms

Many air conditioning manufacturers do not produce reverse cycle air conditioning units larger than 14kW.

For big family homes 16kW, 18kW and 20kW ducted reverse cycle air conditioners are available in leading brands like Daikin. Units 18kW and above usually require the home to have three phase power capacity. Most air conditioning controllers can operate up to 8 zones but specialist controllers like the iZone allow 12 zones.

Evaporative coolers generally treat up to 220sqm. Coolbreeze manufactures a model capable of treating 255sqm (with standard 2.2m high ceilings).

Adding ducted and split system air conditioning to an existing home

In single storey homes it is easy to add ducted air conditioning after the home is built.
With existing double storey homes it is often not possible to provide ducted air conditioning to the ground floor. This is because the ducted air conditioner is placed in the roof and ducting needs to get from the roof to each room.

In this instance the only air conditioning option for the ground floor is split systems. Multi-split systems can be installed where several split systems are run by one air conditioner compressor. This looks tidier, but will end up costing more than ducted reverse cycle air conditioning as you are paying for multiple units plus their installation. Advantage include being able to set different temperatures for individual rooms though and running each unit separately, which would be more energy efficient than a zoned system.

Roof space

If your home has a flat roof there is often not sufficient roof space to install a ducted air conditioner and the ducting. In this case split systems or multi-split systems are the most practical solutions.


About Author

Julie Curnow is the co-owner of Air and Water Residential (shortened to Air & Water), a leading supplier and installer of home air conditioning and heating products in Perth, Western Australia. Air & Water pay particular attention to designing the most energy efficient system for their customers, using the best technology and products on the market. They stock the most up to date cooling systems from quality brands including ducted, split system, and evaporative air conditioning, as well as gas and wood fired and ducted heating solutions. Julie’s years of experience in the heating and cooling industry have given her a wealth of knowledge on the best design solutions to create comfortable living environments under the varying Australian climate. She shares her knowledge of effective and efficient heating and cooling solutions on the Air & Water blog, and is pleased to be writing for My Renovation Magazine.

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