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It's 40 degrees in the shade and your house feels like a furnace, even with the fans on. In a perfect world, we'd all live in well-designed homes with excellent passive cooling features, but that's not the reality for most.
On those extreme summer scorchers, some form of mechanical cooling is a tempting option. Increasing numbers of Australians certainly think so, and the ownership of air-conditioners in Australian households has more than doubled in the last 10 years, to about 65 per cent. The problem is, air-conditioners are energy guzzlers and the increased use, especially during heatwaves, is placing a burden on energy supplies.
"For most of us, heating and cooling accounts for 38 per cent of our energy use," says Alexandra Graham, GreenHome NSW coordinator for the Australian Conservation Foundation.
On peak summer days, air-conditioners make up almost half of household electricity use in Australia.
"It's leading to 'load shedding' in areas like Victoria, where they often have to shut down electricity supply in certain areas so the system doesn't collapse," says sustainability expert Stuart McQuire, of Green Makeover, a Melbourne-based eco-consultancy.
So if you want the chilled air, make sure you buy and use the greener options.
COOL CHOICES . . .
Refrigerative air conditioning
The most common type of air-conditioner sold in Australia is a refrigerative system. It works just like a kitchen fridge, sucking warm air through an indoor unit, blowing it outside, and returning chilled air back into the house. It comes in portable, fixed or ducted form, though the biggest sellers are fixed split-wall models with one or more indoor units connected to an outdoor condenser. Reverse-cycle options can also be used as a heater in winter.
Refrigerative air-con is suitable for any climate, but is especially efficient in steamy, humid areas as it dehumidifies the air. However, it rates the worst in terms of energy use, running costs and greenhouse gas emissions. A large wall air-conditioner, for example, uses an average 1,100 kWh a year and costs about $100 a year to run, compared to 20 kWh and $2 a year for a fan, according to the Canberra-based Home Energy Advice Team (HEAT).
Look out for products with 'inverters', which vary their output to adapt to ambient conditions. They use up to 30 per cent less energy than standard models.
A more eco-friendly option is a heat pump or geothermal cooling system. It works on the same principle as refrigerative cooling, only the warm air is being transferred via pipes and a condenser buried in the ground. They can be pricey to install, depending on the depth of excavation, but they do offer big energy savings and running costs 20 to 50 per cent lower than traditional air-conditioners, according to Geo Climate Systems, a Melbourne-based heat pump installation specialist company.
Cheaper and considerably more efficient than refrigerative air-conditioners, evaporative coolers draw hot air through a wet filter and blow it back through the house. They come in portable, fixed or ducted options and use about 75 per cent less energy than a conventional air-conditioner. On average, they cost about $20 a year to run and use 220 kWh of electricity. They don't work well in humid areas, though, so they're not suitable for northern, tropical areas or places prone to high-humidity summers. They can use a lot of water - up to 25 litres an hour for central systems - but you can offset that by connecting it to your rainwater tank if you have one.