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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.
A ducted cooling system is a whole-house option, like central heating. It cools the home via ducts or vents in every room, connected by pipes to a main condenser. Ducted systems can use refrigerative, evaporative or geothermal cooling, and have high installation costs. Running costs vary according to the type of cooling system used and the best systems are zoned, so you can control which rooms or parts of the house you cool.
Before you hit the showrooms, do your homework to make sure you buy the most energy-smart model available.
Most fixed refrigerative air-conditioners have a star-rated energy label, with more efficient products getting more stars. The potential energy savings are enormous. For example, a 6-star rated cooler will use 28 per cent less energy than a 2-star equivalent, according to HEAT. See the air-con section at www.energyrating.gov.au for a full range of products and ratings.
It's also worth getting expert advice on the size of unit needed to cool your space.
"As a guide, allow 125 watts or 0.125 kW per square metre of floor space for living areas and 80 watts per square metre for bedrooms," says McQuire.
How you install and use your air-conditioner also has a big impact on its efficiency. With refrigerative systems, avoid installing the outside unit on a hotter north or west-facing wall where they'll have to work harder. You also need to ensure the room is completely sealed.
"Minimise heat loss by placing draught-stoppers on exhaust fans around door and window frames," says the ACF's Graham.
In comparison, evaporative coolers require good ventilation, so check there is a good cross-breeze through open windows and doors.
Cut down on heat getting in through large areas of glass such as the walls of bi-fold doors. Draw curtains or blinds when using an air-conditioner to stop up to 16 per cent of heat transfer through the glass.
Avoid the temptation to set your air-conditioner's thermostat to super-chilled. Setting the temperature between 25 and 27˚C in summer should be just right.
"Every degree below that will make 10 to 15 per cent difference in running costs," says Green Makeover's McQuire.
Last of all, make sure you clean your air-conditioner's filters regularly to reduce dust circulation and maximise airflow.
Refrigerative air-con uses 1,100 kWh of energy per year, and will cost $100 per year
Heat pumps 385 kWh of energy per year, and will cost $35 per year.
Evaporative air-con 220 kWh of energy per year, and will cost $20 per year.
Ceiling fan uses 20 kWh of energy per year, and will cost $2 per year.