Evaporative Air Coolers

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Image of an evaporative air conditioner on the roof of a house

Evaporative coolers range from small mobile units which cool one room up to large units that can cool a whole house. They consist of a fan or blower that draws in outside air and passes it through a wet filter. As hot dry air moves through the filter, water evaporates, cooling and humidifying the air. The cool air is then blown through the room or house.

Evaporative systems cool best when the air is drawn slowly over the wet filter pad, which allows maximum evaporation to occur. Systems with larger total filter areas will provide better cooling because the air can flow more slowly over the filter pads.

Care should be taken when purchasing evaporative systems to ensure that the unit is adequately sized for your particular requirements. As a guide, the entire volume of the space(s) to be cooled should be flushed through every 2 minutes (or around 30 air changes per hour). Make sure you obtain an accurate sizing quote from your air-conditioning specialist before purchasing your system.

The performance of evaporative systems deteriorates as the humidity increases. Consequently, evaporative systems are not suitable for the humid northern, coastal regions of Western Australia. They are generally suitable for areas with dry summers, such as inland and southern regions (including Perth), although their effectiveness will be reduced during the occasional periods of humid weather that these areas can experience.

Like all electrical appliances there are ongoing running costs. Water is required for the cooling process and electricity is required to move the air and run the control system, but the amount of electricity required to run an evaporative air conditioner is much less than for equivalent sized refrigerative air conditioners.

Portable units
These units cool one room at a time and are usually on a stand with legs and wheels so you can move them around the home. They plug into a mains wall power socket and need to be filled with water. Although a few models have electric heating elements to provide winter warmth, most do not have this capability. The best location for portable units is near a slightly open window or door with an opening on the other side of the room.

Fixed or Fascia Units
These units are mounted through a wall or window and have permanent wiring, water connections and fresh air supply. They can cool larger areas than the portable units.

Ducted systems
Ducted systems are usually mounted on the roof. They typically use 10 to 30 litres of water per hour, depending on size, water hardness, air humidity and operation. After filtering, humidification and cooling, the air is ducted through the roof space to all rooms in the house. Most roof-mounted systems are quite large and some can be noisy outside so you should consider the visual and noise aspects when choosing placement.

Operating tips
When operating an evaporative cooler, always ensure that some doors or windows are slightly open so that the fresh, cool air produced by the air conditioner can easily flush out the hot air inside your home. Some ducted coolers have self-closing ceiling vents which allow the air to be exhausted into the roof space. These may provide security advantages over opening windows or doors, but may also increase heat loss in winter.

With all evaporative coolers always make sure that the water supply is clean and fresh. In areas where the water is hard (containing minerals), frequent filter changes may be necessary. In severe cases of water hardness corrosion of the unit can occur. This is due to water evaporation inside the unit, allowing dissolved salts or minerals to leave a residue on the filters. To overcome this problem, most systems use excess water to flush the filter pads. In most areas of the south west, including the Perth metropolitan region, water quality is perfectly acceptable and only minimal flushing is necessary.

Where portable units have not been used for a week or more the water reservoir should be drained so that bacteria and algae do not grow in the reservoir or filter pads, causing a health hazard. The reservoir should also be flushed out regularly to remove dirt and residues.

Ducted systems are permanently connected to power and water and require little maintenance apart from cleaning the air filters, which is usually done by a maintenance contractor. Some ducted systems have a fresh air mode where no evaporative cooling occurs. This mode can be used to flush cool outdoor air through a warm home (eg summer nights).

The vents of ducted systems should be closed off in winter. Some vents do not seal well and it may be worthwhile placing an extra cover over the vent. Most ducted systems should also be covered in winter which will assist in reducing heat loss.