Cooling buyers guide: active cooling
There are many ways to survive the summer heat, and even do it comfortably and sustainably. In our first ever cooling buyers guide in Renew 146, we look at cooling options for your home.
Keeping a home cool can be done through both passive and active methods. Passive methods include things like shading, which prevents heat entering the home, and ventilation, which removes it once it has entered the building envelope. Active methods mostly involve systems such as air conditioning, designed to remove heat once it has entered the house envelope or to cool the occupants even if much of the home remains hot.
In this extract from our cooling buyers guide, we look in detail at your options for active cooling. You can find the full buyers guide with information on passive cooling options such as shading and ventilation in Renew 146. The tables for this guide are available here.
Active cooling options
As passive cooling generally has no ongoing costs and is often cheaper to setup than an active system, you should always do as much passive cooling as possible to reduce the need for active cooling. But if you have done everything you can to keep your house as cool as possible, but the house still heats up, then you will need to look at some form of active cooling if the house is too hot to be liveable. Let’s look at the options available, in order of simplicity.
Moving air has a cooling effect for us humans, simply because we sweat. When moisture evaporates, it absorbs heat as it turns from liquid into a gas, as gases are naturally more ‘energetic’ than liquids. In the case of sweat, the heat mostly comes from the skin itself. Air movement increases the rate of evaporation of sweat, and this increases the rate that heat is removed from the skin, making you feel cooler and more comfortable. Indoors, we have to make our own breezes using fans, but don’t forget that fans cool only you, not the room, so only use them while the room is occupied, otherwise they are just creating heat, like any other appliance.
Portable fans range from the tiniest USB-powered fans that plug into a spare USB port on your computer and draw just a few watts, through to 230 V fans, which come in a considerable range of sizes, in three common types: box fans, oscillating desk fans and pedestal fans, with power ratings up to 40 watts or more. Smaller fans will use a lot less energy, with tiny personal fans using as little as a few watts.
Desk fans range in size from little 12 cm units designed to take minimal space on a desk and just cool one person, through to much larger units with blade diameters up to 40 cm or so, which can provide quite a breeze and help cool anyone nearby. Desk fans usually have the option to oscillate, so they produce a varying airflow and can cover a wide angle rather than just blowing air in one direction. Box fans are similarly sized, but they usually have a fixed motor and fan blades, with a moving grille in front of the fan directing the airflow in a variable manner. Pedestal fans are basically larger versions of oscillating desk fans, with their own in-built height-adjustable stand so they can be placed on the floor without taking up furniture space.
Not all fans have visible blades: some use internal ducted fan systems to provide a stream of air without visible movement. Dyson fans are the most well known. These types are generally more expensive for what you get and, because of their higher motor/fan speeds, can be noisy, although Dyson at least have re-engineered their latest models to reduce noise.
Regardless of the fan chosen, try to see one running in-store before buying. Some fans with folded metal blades might look cool, but they can make a lot more noise than units with more aerodynamically designed blades—an important consideration if you are trying to watch TV with the fan in close proximity. For a fan in a living area, it’s good to keep the decibel rating under 35 dBA. However, noise is all relative to the situation the fan is used in and, in bedrooms, some people like the white noise given out by a fan.
Also, don’t buy a fan based on price: a cheap fan might be a bargain initially, but if it breaks after a few years of use, it isn’t a bargain at all. Remember though that higher prices don’t always mean good quality either, so, like any other appliance, check out reviews before handing over your cash.
While portable fans are great for renters or where you need to minimise purchase cost, they do take up floor or desk space, can be noisy and often don’t last that long. The best way to move the air in a room in a quiet, efficient manner is to install ceiling fans.
A suitably sized ceiling fan can turn a hot, stuffy room into something much more liveable, even though it is only a perceived change (remember, fans don’t cool a room, they only cool you).
Ceiling fans come in a wide range of sizes, styles, colours and blade designs, and two different motor types—AC and brushless DC. In general, brushless DC fans (often just called DC fans) are the more efficient of the two and will usually have a lower energy consumption for the same size and style of fan compared to an AC unit. But energy consumption varies quite a lot and better quality AC units will have more efficient motors and will be closer in consumption to a DC fan. Both motor types can provide varying speeds, but DC fans will often have a greater number of speeds; AC fans may have just three or so speeds.
What you should focus on when choosing a fan is the total energy consumption specifications and the effectiveness of air movement for that energy use (airflow specs will be in the fans’ brochures or datasheets). Dividing the airflow rating by the wattage for your chosen fan will give you a number you can use to compare to other fans. For example, a fan that flows 12,000 m3 per hour and uses 60 watts (W) gives you 200 m3/hr/W. Another fan might flow 8000 m3/hr and draw 24 watts, so flows 333 m3/hr/W—clearly more efficient. Total airflow (unrelated to the energy used) is also important and a typical fan should have a minimum airflow on high of at least 7000 m3/hour. Most are better than this, so most will do the job properly. Generally, for a given blade design and fan speed (in revolutions per minute, or RPM), the larger the diameter of fan, the higher the airflow.
Smaller caged fans (like a wall- or ceiling-mounted desk fan style unit) will have much lower flow rates, but they are designed to be more directional, so this is less critical. Several units are often clustered together in a single fitting to increase airflow.
A high efficiency, common ’4 foot’ (1.2 m) ceiling fan can use as little as 12 watts on high, whereas lower efficiency units can use as much as 100 watts. While 100 watts isn’t a great deal of power, if running for 24 hours a day it equates to 2.4 kWh per fan per day. Plus, ultimately all of that energy ends up in the room as heat.
Selecting a fan size depends on the size of the smallest dimension of the room, the ceiling height and the desired airflow (larger fans generally move more air). A fan’s diameter should generally be around a quarter to a third of the smallest room dimension. For example, a room which is four metres in its smallest dimension would need a fan at least one metre diameter and preferably larger—a standard 1.2 m diameter would be suitable. You can go bigger and just run the fan at a lower speed, but anything too large may be too imposing in the room.
A fan needs to be mounted such that the blades are at least 2.15 metres above the floor for safety, so rooms with low ceilings will need a low-profile ceiling fan (these have a central hub which mounts directly to the ceiling, with no downrod), or, if the ceiling is too low to allow for safe installation of a ceiling fan, a wall-mounted oscillating fan. Or, if you have a desire for something more unusual, consider punkah blade fans, which use one or more blades hinged from the wall or ceiling moved backwards and forwards by an electrical actuator. These eliminate the issue of rapidly spinning blades, while still producing a cooling air movement. They can be expensive and hard to source though, but specialty fan and lighting stores may be able to supply them.
Ceiling fans come with a wide range of features, including a large range of blade materials (including wood, metal and plastic), blade styles (modern, traditional, Tahitian (palm leaf) and even retractable), the number of blades (which can range from one to a dozen or so) and blade and hub colours (including designer colours, brass, gold, silver, black and white). There are also a range of technical features available, including reversibility, remote control and smart home compatibility.
For most people, the style of fan/blade will be chosen to match the room, but there are some other factors to consider. Metal blades can be a hazard in rooms with low ceilings, especially if kids play in the room, so wood, plastic or moulded blades (which tend to be not as sharp as pressed or folded metal blades) should be used. Fans with moulded blades are usually the most aerodynamically efficient.
Fans with fewer blades generally run faster than those with greater numbers of blades, so consider this if you want a slow-moving fan. Check datasheets for fan speed and airflow ratings to confirm.
Most ceiling fans are reversible which will push accumulated warm air down from the ceiling in winter. It is likely that any fan you buy will have this option, but it pays to double-check.
You also need to decide if the fan will have a light, as ceiling fans often replace light fittings in the centre of the room. Some fans still require the use of a bulb, while others have purpose-made LED panels built in.
Remote control fans, while adding yet another remote to the home, can eliminate the need for added wiring if replacing a light with a fan/light combo. The remote allows separate fan and light switching to be done by the remote, eliminating the need for separate circuits from wall switches.
Another thing to remember when sizing and locating a fan is to allow for any existing light fittings. You don’t want to fit a fan too close to a downlight as it will cause flickering in the room as the blades block out the light from the light fitting as they rotate. If you can’t avoid covering a downlight to some degree, then you might be better off using a different fan type, such as wall-mounted units or ceiling-mounted wall fan style units like the Atlas Acqua.
Blade material choice may be affected by environmental factors. For example, you don’t want textured wooden blades on a fan fitted too close to the kitchen, as moisture from cooking may affect the wood and any build-up on the blades will be difficult to clean. Also, in tropical areas with high humidity you should be looking at fans rated for tropical conditions.
Noise can also be a factor for ceiling fans. Generally speaking, timber and plastic bladed fans are the quietest and metal-bladed fans the noisiest, but a metal blade that is well designed can still be quiet. Ideally, a fan will have a noise level rating on high of 30 dBA or lower. Many suppliers don’t supply noise level data for their fans; the simplest solution is to go and listen to one running in the shop—fan showrooms can be invaluable for comparing noise levels and airflow rates.
Another issue in some areas that still use mains electricity switching tones to control water heaters is that of motor buzz—particularly annoying as these tones are sent at night and early morning to control hot water system switching. Some AC fans are susceptible to this, whereas DC fans generally aren’t as the tones are filtered out by their internal circuitry.
Other aspects to consider when looking for fans include:
- fans generally work best in lower humidity environments, as sweat can evaporate faster, providing a better cooling effect
- avoid plastic-bodied fans if possible, particularly desk and pedestal fans. They can fatigue over time, especially if the plastic is thin at a critical point, such as the motor mounting point, and replacement parts are rarely available
- fans are the lowest energy use option, but also the least effective option on extreme heat days.
Reverse-cycle air conditioning
Reverse-cycle air conditioning is the most common cooling type in Australia. It works by extracting heat from inside the home and dumping it outside, much like a fridge does. In winter, it can be used for heating simply by operating in the opposite direction, collecting heat from outside, concentrating it and distributing it inside the room. Because they source or sink their heat into the outside air, they are known as air-sourced heat pumps.
A high efficiency reverse-cycle air conditioner is the most efficient form of active heating and can be the most effective cooling in many situations, particularly in high humidity conditions. Their combination of high efficiency, zero water consumption, minimal maintenance and the ability to double as high efficiency heating in winter makes them very cost-effective (as they eliminate the need for separate heating and cooling systems), especially when combined with a solar array, which can cover part or even all of the running costs at least in summer. The water that condenses from the indoor unit in summer can be used to water a small garden too! As winter heating, they produce lower emissions than gas heating (see our report Are We Cooking the Planet with Gas?; note the report is to be updated in early 2019).
While a wholesale shift to reverse-cycle air conditioning can place high peak loads on the grid at certain times of the day (such as the evening, when people come home from work and solar generation has tapered off), there are ways to mitigate even this problem. These include steadily improving air conditioner efficiency, improved housing stock efficiency reducing the need for heating and cooling, and smarter grid systems, including increasing storage capacity, especially behind the meter (in homes).
Because reverse-cycle air conditioners simply move heat from inside to outside the home, they can have efficiencies greater than 500%, or an Energy Efficiency Rating (EER) of 5. What this means is that for every unit of electricity in kWh used, they move several times that much heat energy from inside to outside (or vice versa in winter). For example, a unit with an EER of 5 means that it will move 5 kWh of heat energy for every kWh of electrical energy used.
EER is measured at specific conditions as laid out in AS/NZS3823, and those conditions are mild compared to extreme summer days, so the actual EER will sometimes be lower than stated. For example, EER is measured with an indoor temperature of 27 °C dry bulb (19 °C wet bulb) and an outdoor temperature of 35 °C (24 °C wet bulb).
Because air conditioner efficiency depends on the differential between indoor and outdoor temperatures, it’s not impossible for an air conditioner with a rated EER of 5 to drop to as low as 3 on a hot summer’s day (and the same in winter) or be higher than rated in milder weather—it works both ways.
Reverse-cycle systems come in several types, including split systems (the most common), multi-head splits, ducted, portable and box coolers.
Split systems, which have one indoor unit (the air handling unit) per outdoor unit (the compressor unit), allow you to cool one room or area for a fairly low installation cost. You can start with a single split system and add more to other rooms as budget permits.
Multi-head splits allow the fitting of two or more indoor units per outdoor unit, so they can be more cost- and space-efficient, although usually with a somewhat lower EER than a single split of similar size. For maximum efficiency, separate single splits are the way to go, but they will require an electrical circuit for each outdoor unit, which may add to installation costs.
Ducted systems use a much larger compressor unit and a single or dual indoor unit which is usually fitted in the ceiling, or sometimes under the floor or outside. Cool air is distributed from the indoor unit via insulated ducts to each room. Such a system can simplify electrical installation as it only needs a single electrical connection. However, this is offset by the requirement for duct installation, which can often be difficult depending on the floor or roof cavity space available, and lower overall system efficiency compared to separate split system units.
In days gone by, most reverse-cycle air conditioners were of the window/wall mounted box type, which were a simple metal box which contained the compressor unit at the back and the indoor unit part at the front. It was fitted to a window or a hole in the wall, with a metal support bracket outdoors. These units still exist and can be useful for renters (fitted temporarily into a suitable window) or for those on a budget, but they do have lower EERs than split systems, generally speaking. Because the compressor is acoustically coupled to inside the home by the metal case, they are also noisier. As they are simple boxes with no pipework to be done, they can be installed by the homeowner if a suitable power point is located near the installation point.
Portable reverse-cycle air conditioners are another option for renters or for quick DIY installations. They are a self-contained unit, much like a box air conditioner, just more vertical (taller than wide), usually on castors for mobility. To expel heat outdoors, they use a large diameter (usually around 150 mm) flexible duct and an adjustable window adaptor that lets you seal the outdoor end into most windows with minimal air leakage. There will usually also be a water drain hose to remove condensed water to outdoors.
Because they exhaust hot air out through the duct, they also cause some warm air to be drawn into the home to replace it. This is one of the reasons they have lower EERs, usually between 2 and 3 for typical operating conditions. Some units have dual ducts, one an intake, the other the exhaust, so they effectively bring outside air in to extract the heat, then dump it outside again. This should make them more efficient than single duct units, but EERs for dual duct units are only slightly better than for single duct units.
Portable units can also be a lot noisier than split systems, as they have their compressor inside the room being cooled. Their main advantage, aside from portability, is that they are cheap. Portable reverse-cycle units can often be found at around $500 and sometimes even cheaper.
One huge advantage reverse-cycle air conditioners have over evaporative air conditioners is that they can be used to control humidity. In particular, especially in the tropics, they will reduce the humidity to much more comfortable levels. Some units have active humidity control when cooling (which can add considerably to the cost), while others dehumidify as a byproduct of their normal operation. If you find that air conditioning tends to dry out your skin excessively in summer then you might want to consider active humidity control.
Which indoor unit type?
Split and multi-split indoor units come in a wide range of types. The most common is the wall-mounted unit, usually mounted close to the ceiling, but there are also floor-mounted units (mounted on the wall, but at floor level), ducted (like a mini whole-house ducted system, but usually ducted to one or two rooms), cassette (mounted in the ceiling, so they just look like a ceiling vent), and concealed units, which can be mounted in the wall or ceiling, with cool air exiting via a relatively narrow vent.
Wall-mounted units are the most common, but floor-mounted units may be preferred if your main use is for heating rather than cooling. Wall-mounted units often come with airflow patterns to ensure better air mixing. In general, wall-mounted systems tend to be the most efficient (have the highest EERs) and in most cases they are likely to be the best option. Floor-mounted units also take up some floorspace and furniture can reduce their effectiveness to disperse air evenly.
Choosing an air conditioner of the right size requires several considerations. The amount of cooling required varies greatly depending on who you ask, but figures seem to range from 60 to 160 watts of cooling capacity per square metre of room area. If you are also planning to use the unit for heating, consider your heating requirements when sizing as well.
There are so many variables that it can be impossible to know for sure just how much air conditioner capacity is required. Variables include the house design, orientation and materials used, the performance of insulation, the colour of the roof and walls (darker colours get hotter), the colour and type of materials surrounding the house (for example, light-coloured paving may reflect heat into the home through the windows, bypassing the shade from the eaves, while dark paving may re-radiate heat it absorbs into the house as well as heating the outdoor air near the windows), the level of surrounding vegetation and shading, the size and location of windows, and the type of glass and frames used in them, the level of thermal mass in the home, the degree of thermal coupling to the soil under the home, accessibility to cooling breezes in the evening, and a host of other parameters that will vary from home to home. For an approximate estimation of air conditioner sizing and energy use, see the calculators at www.fairair.com.au, but note that these calculators do not take into account all of the variables mentioned above—they just provide a guiding estimate.
After living in the home for a while, you can get an idea of the amount of cooling required by monitoring the home’s thermal performance. For example, if the home barely gets above 30 °C, even after a string of hot days, then it is performing reasonably well and a system near the lower end of the range would likely be fine; a 4 x 5 metre room (20 m2) might be fine with 2 kW of cooling capacity. But for a house that heats up considerably, you could need considerably more—though first you could work on cutting demand by improving building thermal performance.
Sizing larger systems, such as ducted systems, will generally follow the same rules, just for the entire home. If your home is, say, 200 m2, then you will probably need a ducted system in the 15 kW range or larger. Although the actual cost will depend on how much of the house you cool—careful use of zoning can reduce costs considerably—a system this size, with an EER of 4, is going to draw close to 4 kW from the grid, which can rapidly add up on your power bill. This is where the previously discussed passive cooling measures really come into play—if you can make the home perform better thermally, then you should do those changes before selecting an air conditioner.
Some experts suggest the standard sizing approach used by suppliers is based on lower/higher temperatures than needed, and so will tend towards (possibly excessive) oversizing. Such sizing estimates may also not apply to well-sealed, shaded and well-insulated homes. As already mentioned, FairAir’s calculator takes some but not all of the possible variables into account.
A larger unit will cost more to buy and likely use more energy and cost more to run, so consider that in your decision. However, a unit will be less effective if it is undersized, being unable to remove heat fast enough. On extreme heat days, it may not be able to bring the room temperature down to a comfortable level, at least not in a realistic timeframe. A larger unit may also run its compressor at a lower rate, reducing wear and tear and potentially increasing lifespan. Indeed, not constantly operating near its design limit means it may operate more efficiently than its EER rating, which is measured at full load.
Some experts prefer undersizing. Going smaller will be more likely to prompt you to improve the efficiency of your home to make the system work more effectively. Smaller units generally have higher efficiency and some have a boost function you can use to give short-term increased cooling or heating. To compensate for its lower effectiveness, you could also sit closer to it to cool down (or warm up in winter), but not everyone will want to do that. Running a ceiling fan at the same time will also help, making it feel like the temperature is up to 3 °C lower than it actually is.
To keep running costs down you want to maximise air conditioner efficiency. One of the primary ways to do this is to locate the outdoor unit in as cool a place as possible. This means a southern wall, or a well-ventilated, shaded area or protected by plants. An outdoor unit in full sun is going to struggle to expel heat and will run far less efficiently than one that is protected from direct sunlight. An outdoor unit in full sun may be running at 20 °C more than one in full shade. Efficiency can drop dramatically if the outdoor unit is too hot, to the point where the system may not be able to cool the room effectively, and it will certainly use more electricity and work a great deal harder to do so. One paper by the Australian Institute of Refrigeration, Air conditioning and Heating (AIRAH) showed that a white roof dramatically reduced inlet air temperature for a roof-mounted air conditioner and improved efficiency!
The opposite is true in winter, so if you are also going to use your air conditioner for heating, then the best location is one that is shaded in summer but receives sun in winter. This is generally going to be on a north wall under an eave or elsewhere with a removable shade (which can be an artificial shade device or simply a deciduous plant).
Air conditioner efficiency is described on each system’s energy rating label. At present, labels give a star rating, separate rated energy input and output figures for heating and cooling, and an average energy usage figure for a typical installation, but the energy used will vary depending on many factors, including the cooling load (how much heat needs to be removed), the position of the outdoor unit, the climate the system is used in, operating hours per day, the outdoor air temperature and humidity. The energy efficiency label really is just a good way to compare the relative efficiency of different units at full load, so that you can select the most efficient unit that meets your other criteria. To calculate the EER of a particular unit, just divide the rated cooling capacity by the cooling energy input figure.
Because the installed location can have a significant impact on the energy efficiency and performance of air conditioners, the Energy Efficiency Advisory Team (EEAT, who set the ratings requirements for appliances) is examining a move to a zone-based energy efficiency labelling system for some products, including air conditioners. The proposed new ratings would provide star ratings and energy use figures for three zones—hot (e.g. Brisbane and Darwin), average (e.g. Adelaide, Sydney and Perth) and cold (Canberra, Hobart, Melbourne and New Zealand). See here for more on energy rating labelling.
While the indoor units of most split system air conditioners are very quiet (levels vary from 20 dBA through to 50 dBA, depending on fan speed, mode and make/model), the outdoor units can be considerably louder (from less than 40 dBA in quiet mode to over 70 dBA). While keeping refrigerant hose lengths short should be considered (a shorter hose reduces losses slightly), locating the outdoor unit where it won’t disturb you or the neighbours (which can lead to council complaints) needs careful attention.
When units are placed between two buildings, or between a building and a fence, noise can echo, making it sound louder than if the unit were in a more open area. Try to locate outdoor units such that they will work most effectively given the shading criteria stated previously, but also such that noise will not be an issue. This can be tricky, and you may need to include a noise barrier around the outdoor unit if the available location is restricted to a less than ideal position. It is important, however, to abide by the manufacturer’s clearance requirements around the unit, to ensure efficiency isn’t reduced by airflow being hampered. The same goes for box (in-window) units and even portable units.
The main maintenance requirement is cleaning of air filters, which can become clogged with dust, inhibiting efficiency.Cleaning schedules can be found in the air conditioner’s user manual, but for regular use, checking the indoor unit’s filters once a month is good practice. Some units are self-cleaning but if they have accessible filters they should still be checked. The efficiency of these systems depends on free airflow through the indoor and outdoor units, so any buildup of dust should be rectified.
Outdoor units are generally maintenance-free, but check them occasionally for any buildup of leaves, dust and critter debris such as spider webs, which can all reduce efficiency. This is especially true if the outdoor unit is surrounded by vegetation or you live in a dusty area. A soft brush is usually all that’s needed to clear any buildup. Don’t be tempted to hose away debris as most outdoor units are rated for rain, not horizontal high pressure jets of water. Some designs of outdoor unit can be hosed down—refer to the system user manual for the best way to clean your unit.
If you live near the sea, corrosion of the heat exchanger on outdoor units can reduce efficiency considerably, so this should be looked for at least annually.
If performance seems to be degraded and filters have been cleaned or replaced, then the system may have lost some of its refrigerant gas. In this case it will need checking and regassing, which can only be done by a qualified refrigeration mechanic or installer. See the end of this article for more on refrigerants.
Ducted systems may require the occasional duct cleaning, which has to be done professionally, but generally the main problem is damage to the ducts, which allows air leakage and will considerably reduce system efficacy. Duct damage can occur over time, as duct tape separates as it ages, or it can occur through rodent damage, especially for ducts under the floor. A visual inspection of ductwork should be done once a year, if possible. If you can’t access the underfloor space to inspect the ducts, it’s worth spending $20 on a cheap USB endoscope camera which you can feed into each duct in turn to check for damage. These are readily available from eBay and similar websites, with cable length up to five metres or more.
If you can view the ducts, then a thermal camera can let you see leaking ducts by showing cold or warm spots on the ductwork. You can hire these cameras, and they are also available as a plug-in adaptor for many smart phones (the FLIR One) and some smartphones even have them built in (the Cat S60).
While there are cooling-only units available, most areas of Australia will benefit from the ability of a reverse-cycle unit to operate as a heater as well. In addition, currently the best cooling-only model available is a 5.5-star device, while reverse-cycle machines currently exist up to 7-stars, and cooling-only models are usually around the same price as reverse-cycle.
Ground-source heat pumps
These are a variation on reverse-cycle systems. Instead of using air as the source or sink of heat, ground-source heat pumps use the higher heat capacity of the ground. For the outdoor unit, they use a compressor coupled to a series of pipes embedded either vertically in bore holes or horizontally in trenches, or, sometimes, coils are placed in a nearby body of water. While they have the potential to be more efficient than air-sourced heat pumps, the small gains in reduced running costs are greatly outweighed by the added complexity of installation and the much greater cost of a ground-source heat pump.
Evaporative air conditioners
This type of air conditioning relies on the evaporation of water to cool the incoming air stream which is drawn in from outside and displaces hot air in the home through open windows or vents.
When water evaporates it absorbs heat, in this case from the air that helps evaporate it. It also humidifies the air, and in hot, dry climates, where evaporative cooling is most effective, it can decrease the effective air temperature by 10 °C or more. Because it relies on evaporation, this form of air conditioning does not work well, if at all, in the tropics, where air humidity levels are usually very high, thus limiting the ability of water to evaporate.
Evaporative air conditioning has one big drawback—water usage. Manufacturers are a bit cagey, often giving vague references to water usage rather than hard figures, but it’s not uncommon for a whole-house ducted system to use several hundred litres of water a day, with typical rates ranging from 41 litres per hour in Hobart to 67 litres per hour in Adelaide according to this 2009 study. Check with your supplier for their up-to-date figures. To reduce buildup of minerals and bacteria, systems generally either use a continuous bleed-off of water or dump the remaining water at the end of each day. While work has been done on reducing water use in these systems, they are only as effective as the amount of water they can evaporate. In areas of water scarcity, they are generally not suitable.
Where these units excel is energy use. Because they are only running a large fan and a small water pump, power consumption is generally in the 200 to 400 watt range—a great deal less (about a tenth in fact) than a whole-house reverse-cycle unit—although this can vary greatly depending on features and capacity, with some domestic units drawing up to 2 kW or more.
Modern units have much more efficient fans and motors than older ones, and it is now mandatory for them to have dampers that shut off airflow when they are not running—important to limit air leakage in winter.
Maintenance requirements for these units is greater than for reverse-cycle air conditioners. Evaporative units have several porous pads through which the water flows and air is drawn. Many are made from wood ‘wool’ or a paper-based material, and these degrade over time and have to be replaced—a job that requires a trip onto the roof. Some have plastic-based pads which last longer. For natural material pads, replacement may be required as often as each year, depending on the level of air conditioner use, the water quality and the amount of dust buildup. This can get expensive, given that a set of pads can set you back $250 or more, depending on brand and model.
Generally, servicing should be done by professional installers due to the risks involved. They may also need the occasional cleanout of the water reservoir, and regular disinfection (once or twice a year) is recommended. While many models are self cleaning, the effectiveness of this does vary and cleanliness of the system should be checked regularly—after all, the air from the system is flowing directly into your home.
To address this issue, some manufacturers make indirect systems that use the evaporative system to cool air that then absorbs heat from a separate air stream, which then flows into the home. This has the advantage of providing cool air without the humidity of a direct system, and indeed cooling efficiency of these systems can be greater than a direct system. This indirect system also allows the use of chlorination to control bacterial growth. The Seeley Climate Wizard is an example.
The water pump in evaporative systems is another point of failure, with some models seeming to be particularly unreliable. Check reviews of any proposed systems, especially reviews from owners who have had the systems for a while. A useful document on running and maintaining an evaporative cooling system can be found here.
Some hydronic heating systems can also be used for cooling. Instead of running hot water through their radiators, they can run cooled water instead. This can provide several kilowatts of cooling capacity, depending on the system, and cooling can also be used with other hydronic radiator types. This type of system is known as radiant cooling, and is not as strange as it sounds. See en.wikipedia.org/wiki/Radiant_cooling for more information on the general concept.
Hydronic cooling can be part of a new heat pump hydronic system, and may even be able to be added to an existing system, but it will depend on the existing boiler and how the system has been set up as to whether it is cheaper to add a cooling system to an existing boiler (such as a gas boiler or a heating-only heat pump system), or upgrade the boiler to a reverse-cycle heat pump unit that can do both heating and cooling.
Slab cooling will suffer from the same thermal lag issues as slab heating—the high thermal mass means that changes in the cooling setting takes considerable time to reflect in the actual slab temperature. If for example, a cool change comes through, turning the slab cooling system off will still result in the slab cooling the home for some time, potentially cooling the home more than desired. A better solution may be to fit ceiling or wall-mounted radiators for the hydronic cooling system, or fan coil units, which have a heat exchange coil and fan in one unit. These all respond rapidly to changes in cooling requirements, but ceiling panels may not be visually desirable and are better suited to commercial installations. You may need to be careful about condensation of water on the panels if they are running at low temperatures; drip trays may be needed, especially in humid climates.
Ways to reduce cooling costs and impact on the grid
Active cooling systems invariably use electricity, and electricity can be expensive, especially if you are on a time-of-use tariff.
The first way to reduce running costs is to select the most efficient air conditioner. Check the Energy Rating website for the latest air conditioner models and don’t get too carried away with extra features that you don’t need.
Another way to reduce the running costs of active cooling systems is to use them less. This means being willing to have a warmer home in summer or making better use of passive cooling methods, as described earlier. This doesn’t mean that in extremes of temperature you should avoid using air conditioning to the point where the home is dangerously hot; just use common sense and don’t use it when you could simply open a window and let in a cool breeze instead.
Running costs can be offset by operating cooling systems when solar-generated electricity is at a maximum and, fortunately, maximum generation and maximum heat occur at the same time due to the same cause—the sun. However, if you are not at home during the day, you may not be making use of that solar generation.
To counter this, air conditioning can be set to run on a timer in the afternoon while there is still plenty of solar generation happening, pre-cooling the house for the evening and potentially reducing the need for cooling when you get home. Generally, this only works well for homes with good thermal performance that can retain the coolth into the evening—a house that rapidly heats up again will gain nothing from pre-cooling. Pre-cooling has another advantage for the electricity grid of reducing the evening load, placing less stress on the grid infrastructure. Almost all modern air conditioners come with at least a basic timer, and many have smart home compatibility so, combined with smart home devices and services, you may be able to control the level of cooling depending on conditions such as the amount of solar electricity being generated.
Cooling just one section of the home, rather than all of it, will also greatly reduce energy costs. Set up that cool refuge by cooling just the lounge or a bedroom and see how you go. You could save many hundreds to thousands in running costs each summer.
While it may not be obvious, appliances, including lighting, generate heat. Indeed, pretty much all the energy that goes into an appliance ultimately turns into heat that then warms your home. For example, if you have 10 x 50 watt downlights running, that’s 500 watts of heat being generated continuously. For an oven rated at 2400 W that runs at, say, a 1:3 duty cycle, that’s 800 watts while the oven is in use. All of this heat must be removed by the air conditioner, so, the less you use appliances on a hot day, the cooler the home will be and the less energy will be needed to cool it.
So turn off lighting and appliances if not being used, change out all incandescent and halogen lamps for LEDs and minimise cooking by preparing cold meals or, if cooking is required, consider a solar cooker or use the most efficient cooking appliance you can—microwaves and induction cooktops use less energy than other cooking methods. Make sure you use lids on pots: this can dramatically cut the amount of heat and water vapour produced when cooking. Or use a thermal cooker, where you heat the food initially on the stove then drop the pot into an insulated container, where it cooks with the retained heat.
The tables for this guide, listing suppliers for fans, reverse-cycle air conditioners, evaporative air conditioners and portable air conditioners, are available here.