Cooling Your House
Use passive design principles to increase comfort and reduce the need for cooling. Insulate your home and shade windows from summer sun. Never use mechanical cooling as a substitute for good design (see Passive cooling). However, many existing homes do not include good design features, and climate change is bringing higher night-time temperatures, so cooling equipment may be needed at times.
The better insulated, shaded and draught-proofed a house is, the smaller (and cheaper to buy and run) the cooling unit needed, and the less often it needs to run.
Mechanical cooling devices
Consider these questions when choosing cooling systems:
- Does the air require cooling or will creating a cooling breeze be enough?
- How big an area needs to be cooled? A single living area is often sufficient to survive a few days of a summer heat wave in many climates.
- How often and for how long is cooling needed?
- Is space cooling or a whole house ducted system required? Whole house systems are more expensive to buy and generally more costly to run.
There are many variables to consider. Seek expert advice before proceeding with the design or purchase of a mechanical cooling system.
The three major methods of mechanical cooling are fans, evaporative coolers and air conditioners.
Fans should be the first choice for mechanical cooling.
With good design and insulation, fans can often supply adequate cooling for acclimatised residents in all Australian climates. They circulate air but do not reduce temperature or humidity. Typically, the air flow created by a fan provides a similar improvement to comfort as reducing the temperature by around 3°C.
Fans can be portable (table or floor) or fixed (ceiling or wall)
Fans are the cheapest cooling option to run and have the lowest greenhouse impact; air conditioners are expensive to run and produce more greenhouse gas.
Make fans your first choice for mechanical cooling — they are the cheapest to run and have the lowest greenhouse impact.
Portable table and floor fans or fixed ceiling and wall models are available.
Fans have a wide variation in efficiency: a Choice test showed ceiling fans used from 54W to over 100W. Check power consumption before buying and make significant savings in long periods of operation.
Combine fans with an air cooling system for comfort at higher thermostat settings from the extra air movement. The reduced air conditioner running costs more than offset the fan energy use.
Your second choice for mechanical cooling should be evaporative coolers, except in humid regions.
Evaporative coolers work best in climates with low humidity as the air has greater potential to absorb water vapour. They are significantly less effective in climates with high humidity.
They cool the air to just above the ‘wet bulb’ temperature. Check to see if the wet bulb temperature is at a comfortable level for you in summer by searching on the Bureau of Meteorology website (www.bom.gov.au) for wet bulb temperatures in your local area.
Evaporative coolers work best in climates with low humidity
Some doors and windows must be open for evaporative cooling to allow hot air to escape from the house. Smaller and older units do not use a thermostat, just a fan speed control. Newer, whole-house systems can be fitted with electronic thermostats and timers.
Operating costs can be low as only the fan and a small water pump use energy. However, many units have inefficient fans and fan motors that consume more energy than necessary. Some modern evaporative coolers use far less energy than older models: check with manufacturers.
Purchase costs are moderate.
Evaporative coolers use evaporation of water as the cooling mechanism. Check with your council to see if there are any restrictions on using water for evaporative cooling.
Take care when using portable units not to place them next to open windows and doors that can let in a lot of heat on a windy day.
Portable units have to be topped up with water at a rate of about 4L/hr. For central systems, water use can be 25L or more per hour on hot, dry days and may have implications in water restricted situations. Make sure the bleed-off rate isn’t excessive — ask the installer to set it to the recommended minimum.
Systems can also be mounted in windows and doors.
Evaporative coolers can increase heating bills and allow a house to heat up faster when not operating, because large volumes of air can be sucked out of the house through the evaporative unit. Many modern units have automatic seals when not in use. Otherwise, close off ducts and cover the roof unit in winter to reduce heat losses.
An indirect benefit of an evaporative cooler is that it tends to pressurise the house, keeping out bugs and dust.
Air conditioners/refrigerated coolers
If thermal comfort cannot be achieved with passive design, fans or evaporative cooling, consider air conditioning.
Air conditioning can give a higher degree of comfort in any climate. However, it consumes more energy and creates more greenhouse gases than fans and efficient evaporative cooling systems unless the building and air conditioner are very energy efficient.
For efficient air conditioning, the house or room should be sealed and highly insulated with bulk and reflective insulation. Windows must also be shaded from the summer sun. (see Shading; Insulation)
Purchase costs vary depending on the size and type of air conditioner, and efficiency varies widely between units and models.
Choose the most efficient model of the correct size for air conditioning.
Systems using inverter technology and advanced design can show energy savings of up to 40% over standard units. The Energy Rating website (www.energyrating.gov.au) lists the products regulated by energy labelling programs and Minimum Energy Performance Standards.
Always choose the most efficient model for your application.
Air conditioners are available as portable, wall, window, split and ducted systems. Fixed systems need to be installed by a licensed refrigeration mechanic/electrician.
Ensure your air conditioner is correctly sized by having an expert calculate the cooling load before purchasing. Use the Australian Institute of Refrigeration, Air Conditioning and Heating’s online calculator for cooling requirements based on specific room characteristics.
Portable air conditioner units
Portable split units consist of separate indoor and outdoor components connected by a flexible hose passed through a partially open window or door. They plug into a standard power outlet and are generally not as efficient as other types of air conditioners but suitable for small rooms up to about 20m2. Always check the Energy Rating Label.
Portable single duct units consist of a single indoor unit which can be on rollers and a duct to exhaust the condenser air outside, usually through a window. These types of portable air conditioners are less effective than portable split units and currently are not required to carry an Energy Rating Label.
Photo: Paul Ryan
Portable air conditioners are not as efficient as modern fixed systems.
Portable single duct units draw the air from the room and exhaust the hot air out the window. The outlet often doesn’t seal tightly and heat can enter the room. The units draw air from other rooms in the house as they vent air, typically heating them up, and they do not work well in large rooms. Take care to empty the water that condenses or they do not work effectively.
Through wall/window units
Through wall/window units are placed in an existing external window or a hole made in an external wall. Smaller units can use a standard power outlet but larger ones may need a dedicated electrical circuit installed.
They are suitable for single rooms up to about 50m2 but are generally less efficient than fixed split systems.
A split system unit can be away from the outdoor compressor.
Fixed split systems
Fixed split systems, especially those using inverter technology, are generally the most efficient domestic air conditioners. The indoor wall or floor mounted unit can be up to 15m from the outdoor compressor.
Multi-split systems have more than one indoor unit running off the outdoor compressor.
Ducted air conditioning units cool large areas or an entire house.
Ducts must be well insulated, to at least R1.5, and joints sealed to prevent condensation and leakage. The roof should have reflective foil insulation installed and be vented to dispel hot air.
Zone systems to cool only occupied areas and allow different conditioning in living and sleeping areas.
Alternative heat exchangers
Reverse cycle air conditioners, in both cooling and heating modes, mostly use an air-to-air heat exchanger, like a refrigerator. They dissipate heat extracted from the room to the outside when cooling or from the outside air into the room when heating.
Ducted units can cool an entire house.
In colder climates, heating and cooling modes must be appropriately selected, as some units may ice up, reducing both efficiency and heating capacity in cold conditions.
Air-to-water or air-to-ground (also called geothermal) exchangers are far more efficient. Heat exchange pipes are run through a body of water or deep into the ground where the temperature is relatively stable all year round.
Geothermal systems are very efficient heat exchangers.
Geothermal systems are highly efficient, producing up to four units of heat output for each unit of electricity input, a performance comparable to the most efficient conventional air conditioners. However, their main benefit is that they can continue to operate efficiently in extremely hot or cold conditions, where peak capacity and efficiency of air-to-air units can be significantly affected. They can also be used to run hot water services.
Geothermal systems are highly efficient, even in extremely hot or cold conditions, and can run hot water services.
They are expensive to install and so are more suitable for housing in extremely hot or cold conditions and with large heating and/or cooling loads, e.g. multi-housing developments.
Solar air cooling
Solar air cooling systems use a fan or ventilator to extract hot air out of the roof space for tile/metal roofs or the gap between the sarking and metal roof sheets. They work by extracting hot air from the roof space and replacing it with ambient air, to minimise heat transfer to the ceiling space below. The effectiveness of removing hot air from the roof space is very sensitive to roof colour and presence of reflective foil under the roofing. A dark (or unpainted steel) roof absorbs an enormous amount of heat. The temperature in the roof cavity is significantly reduced only by a very large air flow, well beyond the capacity of most fans and vents. A light roof or reflective foil (see Insulation) under the roofing dramatically reduces heat gain, so ventilation systems are more likely to make a noticeable difference.
Systems using a solar panel as the only source of electricity have no running costs but work only in sunny conditions. Some systems combine daytime and night-time cooling. The advantage of a system using grid electricity to power the fan is that night-time cooling can flush heat out of the building overnight.
These types of systems are many and varied. Some also draw cool/warm air into the building when required.
Cooling systems compared
The cooling comparison table assumes well designed and efficiently operated systems and gives general running costs and greenhouse gas emissions. Obtain expert advice before making decisions on the type best for you.
|Cooling systems comparison|
|System type||Running cost||Greenhouse gas emissions*|
|* Using GreenPower can reduce emissions from all systems but increases running costs.|
|Solar air cooler (using electricity to power fan)||Low||Low|
Cooling system operating tips
Shade outdoor components of air conditioners from direct sun — but don’t limit air flow around them.
Some units are noisy in operation. Split systems (where the compressor is outside) are quieter inside but consider your neighbours when selecting and locating external components.
Reverse cycle models can also be used for heating and can provide low cost, low emission heating (see ‘Heating’ above). Units that use electric heating elements cost more to run and produce more greenhouse gases.
For ducted systems, install a zoning system so only rooms requiring air conditioning are cooled. Ensure that ducts are well insulated and consider installing reflective foil or painting the roof a light colour and ventilating it to reduce the roof space temperature.
Purchase a system that has controls such as a timer to schedule activation and shut-off.
Set the thermostat as high as possible on your cooling system.
Never set the thermostat at a temperature below what you require — that does not make the unit cool faster.
Always aim to set the thermostat as high as possible.
Avoid leaving air conditioning running when no-one is home. It is cheaper to cool the house down when you arrive home, or to set a timer so that the house begins cooling shortly before people return home.
Practical tips for heating and cooling
Do not leave heating and cooling appliances on overnight or when you are out, although slow combustion stoves can be left on in very cold weather. If you must have the house comfortable when you arrive home, install a timer and turn your system on about 15 minutes before your return.
Locate thermostats in the most used rooms and away from sources of heat and cold.
Each degree of extra heating in winter or cooling in summer increases energy consumption by about 5–10%. Set the thermostat to 18–20°C in winter and 25–27°C in summer.
Dress appropriately for the weather. Putting on a jumper is better than turning the heater up.
Maintain your heater. Keep reflectors shiny and free of dust. Clean air filters regularly.
Service all heaters and coolers according to the manufacturer’s instructions. Pay special attention to air filters.
Close windows and doors in areas where a heater or air conditioner is on unless ventilation is required for unflued gas appliances.
Close drapes or blinds, especially in the evening when you are heating.
|References and additional reading|
|Contact your state, territory or local government for further information on energy efficiency: www.gov.au|
|Australian Institute of Refrigeration, Air Conditioning and Heating. Fairair online calculators for air conditioners. http://www.fairair.com.au/Calculator.Size.aspx|
|Choice. 2012. Heating options for your home, buying guide. www.choice.com.au|
|Department of the Environment, Water, Heritage and the Arts (DEWHA). 2008. Energy use in the Australian residential sector 1986–2020. Canberra. [additional reading now found on industry.gov.au]|
|Energy Rating. www.energyrating.gov.au|
|Your Energy Savings. Rebates and assistance. www.yourenergysavings.gov.au|
|Queensland Department of Local Government and Planning. 2011. Air conditioner guide: a guide to assist with the design, installation and maintenance of air conditioners. [additional reading now found on www.brisbane.qld.gov.au]|
Principal authors: Geoff Milne, Chris Reardon
Contributing authors: Paul Ryan, Murray Pavia