Evaporative cooling

Evaporative coolers draw hot air through a series of wet filter pads that are supplied with water from a tank at the base of the unit. Ducted and wall mounted systems draw in hot air from outside, while portable units draw in the room air. Water from the filter pads evaporates, drawing heat out of the air and humidifying it. The cooled, moist air is then blown into the room being cooled (wall mounted and portable units) or throughout the house (ducted systems).

Evaporative coolers work partly by cooling the air inside the house and partly by creating a cooling breeze. To work effectively, they need favourable weather conditions, and some windows and internal doors need to be left open so the cooled moist air can be exhausted from the house.

Ducted and wall mounted evaporative coolers draw in outside air, providing a continuous supply of fresh cool air when in use. However, it’s important to note that they should not be used when it is smoky outside as they will draw smoke into the house and lead to poor indoor air quality.

Ducted evaporative systems also require additional maintenance to weatherise them for winter to avoid hot air rising through the ceiling vents and ducts and escaping through the cooling unit on the roof, as this will increase winter heating costs.

Types of evaporative coolers

Ducted evaporative coolers have the main cooling unit located on the roof and use ductwork to carry cooled air throughout the home to ceiling outlets located in different rooms. They are available in both standard and inverter models. Inverter models have a variable speed (or inverter-driven) motor to control the fan speed, resulting in lower running costs than the standard systems, especially when operated at lower fan speeds.

Window/wall mounted, and portable evaporative coolers are available to cool small rooms or to provide spot cooling. These systems are cheaper to buy but provide limited cooling potential and can be quite noisy when running. Portable units really only provide spot cooling for people located in front of the units and the water tank must be filled manually.

Ideal conditions

The efficiency and effectiveness of evaporative cooling systems can vary greatly depending on outdoor weather conditions. Their cooling effect relies on evaporating moisture from the wet filter pads, and this means that they will not work as effectively in humid conditions. They work best in hot dry conditions. Under good conditions, they cool the outside air by a certain amount – typically around 10^oC – but, unlike refrigerative air conditioners, they do not cool the room air to a specific thermostat setting. So, if it’s 38^oC outside, they might only cool the inside air to 28^oC.

If the outside air is humid, the cooling effect of the unit is limited because the water will not evaporate as easily from the pads, and a higher fan speed may be required to feel comfortable. As a guide, Melbourne's average humidity on a summer afternoon is between 40 and 50%, while in Mildura it's between 20 and 30%. Both areas can use evaporative coolers, although they are more effective in Mildura's drier climate.

Running costs

Evaporative coolers use both electricity and water. Actual running costs will depend on the cooling capacity of the unit and the fan speed that you choose, the type of water management system used, and the outside weather conditioners – water use will by higher on the hotter, drier days. Evaporative coolers are not subject to any energy or water efficiency labelling requirements making it difficult to compare the performance of different models.

Running costs over the summer can be reduced by operating the evaporative cooler only when required, using the lowest fan speed that allows you to feel comfortable, and operating only the fan when it’s humid outside but not very hot.

Regular maintenance, according to the manufacturer’s guidelines, will also help to keep running costs down. In particular, the filter pads may need to be cleaned at the start of each summer, to ensure effective operation and reduce the workload on the fan.

Water consumption

Evaporative coolers can use a significant amount of water over summer, especially ducted systems in hot dry climates. How much water is used depends on the size of the unit, the temperature and humidity, and the fan speed that the unit is set to operate on.

In addition to the water used for cooling the outside air, ducted evaporative coolers must regularly refresh their water supply, otherwise it can become too salty, and a range of water management systems are used by different brands/models. This could include a regular ‘bleed’ of the water in the tank and replacement with fresh water, a complete dump and replacement of the water in the tank after a certain number of operating cycles, or replacement of the water on an ‘as needs’ basis under the control of a water quality management system. If you are purchasing a ducted evaporative cooler, ask about the water management system used and what the typical hourly or daily water consumption is likely to be in your area. You can use this to estimate the cost of the water used.

Sizing

Sizing guidelines for evaporative coolers are based on their rated airflow rate – expressed in cubic metres per hour - and the total volume of the house to be cooled. A typical rule of thumb used is to base their size on 30 air changes per hour for the area of the house being cooled. It is important to seek an accurate sizing quote from a local retailer or installer who will also consider your local climate.

Choose energy-efficient cooling

When selecting the right evaporative cooling system for your home, it is important to compare different models as efficiency can vary greatly. Evaporative systems aren’t subject to energy or water efficiency labelling requirements so ask the retailer or installer about the key features of the models they are selling:

1. Is it a standard or inverter system – inverter systems have lower power consumption on the lower fan speeds and use less electricity over a summer than standard models
2. What is the power consumption of the evaporative cooler at full, half and the lowest fan speeds. Evaporative coolers will spend a lot of their time operating at the lower end of the fan speed range, so it’s important to have a low power consumption here
3. What type of water management system is used and what is the expected daily water consumption of the unit in your location.