Written by James Teague
Is the largest source of greenhouse gas emissions from an average Australian home and the second largest segment of household energy use in Australia, after space heating and cooling. It accounts for about 21% of the energy and generates about 23% of the greenhouse gas emissions (DCCEE 2010). In Australia, about 48% of the energy used for water heating comes from natural gas, 45% from electricity, 3% from liquefied petroleum gas (LPG) and 4% from solar (DCCEE 2012). Electric water heaters in particular contribute to these emissions: only half of Australian homes use electric water heaters, but they contribute 80% of hot water greenhouse emissions. Reducing your hot water use and using renewable energy sources to heat water are great ways to reduce your environmental impact. By installing the most appropriate and efficient water heater for your household size, water use patterns and climate you can save money and reduce greenhouse gas emissions without compromising your lifestyle. An efficient hot water service (HWS) can also add value to your home and help meet state, territory or local government regulations. 21% of energy used in the home heats water. Energy use in the Australian residential sector 1986–2020. Data are projected energy use for 2012.
Household energy use %
- Heating and cooling 40
- Water heating 21
- Appliances and equipment including refrigeration and
- cooking 33
- Lighting 6
More than half of hot water use is in the bathroom, a third in the laundry and the remainder in the kitchen. One of the best ways to reduce energy bills is to reduce hot water use by installing water efficient showerheads and taps — and save on energy and water. Behaviour change also saves energy and water: take shorter showers, use cold water for clothes washing, use water-efficient appliances, rinse dishes in cold water, and use mixer taps in the cold water position when hot water is not required (see Reducing water demand).
Water heaters Both of the two basic types of water heater
— storage systems and continuous flow (or instantaneous) systems — can use a variety of energy sources to heat water including solar, gas (LPG and natural gas) and electricity.
Storage water heaters Water is heated and stored in an insulated tank for use when it is required. These systems can operate on mains pressure or from a gravity feed (constant pressure) tank.
Mains pressure— Hot water is delivered at a similar pressure and flow rate to cold water so more than one outlet can usually be turned on without greatly affecting pressure. The storage tank is usually located at ground level inside or outside the house. Mains pressure systems have been the most popular systems in recent decades but heat losses from storage tanks and their associated fittings and pipes can be substantial. Large electric storage tanks and their fittings can waste up to 1,000kWh each year; a typical 5 star gas storage HWS wastes 3,500MJ.
This is equivalent to the energy required to heat 50–60L of hot water each day. Constant pressure or gravity feed
— Hot water is delivered at lower than mains pressure from a tank located in the roof of the house. Pressure depends on the height difference between the tank and the point of use. Gravity feed systems are most common for older properties and properties not connected to mains water.
In a gravity feed system, the storage tank is installed in the roof cavity, still above the collectors. Active (or pumped) systems In active systems (also known as pump or split systems), solar panels are installed on the roof and the storage tank is located on the ground or another convenient location that does not have to be above the solar collectors. Water (or another fluid) is pumped through the solar collectors using a small electric pump. Because active systems do not require a roof-mounted tank they have less visual impact, particularly when the solar collectors are mounted flush with the roof. However, active systems are usually more expensive to purchase and require more maintenance than passive systems. Active systems generally use more energy than passive systems because extra energy is required for pumping. There are also additional heat losses in the pipes between the tank and solar collectors. However, if renewable energy is used to power the pump and a high level of insulation is used for the pipes and tank, active systems can reduce greenhouse gas emissions as much as a passive system (see Renewable energy).
For either type of system, storage tanks may be made of copper, glass (enamel) lined steel or stainless steel. Copper and glass- lined tanks typically have a sacrificial anode to reduce tank corrosion, which needs to be replaced every few years. Warranties offered for tanks typically range from five to ten years.
Continuous flow water heaters Continuous flow or instantaneous systems heat only the water required and do not use a storage tank, so do not suffer the heat/energy losses of storage systems. They can operate on natural gas, LPG or electricity. Gas models are available with either electronic ignition or a pilot flame. They can be mounted externally or internally if suitable ventilation is available. Because continuous flow systems heat the water as it is used, they cannot run out of hot water. Continuous flow water heaters can be fitted with sophisticated temperature controls, including controls that allow the user to set the desired water temperature at the point of use (e.g. in the shower).
Water is not overheated and hot water does not need to be diluted with cold water to achieve a suitable temperature, thus saving energy and reducing the risk of burns or scalding. Types of hot water service The main types of water heaters on the Australian market are: heat pump gas (natural or LPG)
— storage or instantaneous solar
— electric or gas (natural or LPG) boosted electric
— storage or instantaneous.
Heat pumps Air-sourced heat pumps are an efficient type of water heater that extracts heat from the environment (air, water or ground) to heat water. Electricity is not used directly to heat water (unless the heat pump is fitted with an electric boost element); it runs the compressor and thus its use is much less than for traditional resistive electric systems and of similar efficiency to an electrically boosted solar system. The pumps operate like a refrigerator but in reverse. Ambient air is used to heat a refrigerant, which converts to a gas. The gas is then compressed, expelling heat, which is transferred to the water. The refrigerant is expanded back to a liquid and the cycle repeats.
Solar hot water systems Solar hot water systems
Use roof mounted solar collectors to absorb energy from the sun to heat water which flows to a storage tank.
There are various system options available, allowing choice of: boosting options gas (natural or LPG) electric collector types flat plate panels evacuated tube collectors system configuration thermosiphon integrated systems split systems split systems. Solar HWSs are storage systems and, depending on your climate, can provide up to 90% of your hot water for free, and without greenhouse gas emissions using the sun’s energy (DCCEE 2010).
They cost more to buy and install than conventional HWSs but can save energy and reduce bills. Rebates and incentives are offered around Australia to reduce the up-front cost of solar units (see www.yourenergysavings.gov.au/rebates). The time required to break even (the payback period) depends on the climate, type of system installed, hot water use and energy tariff applied.
Solar water heating have additional benefits: they last longer than conventional water heaters and add to the value of your home. Seek expert advice to help you choose the most cost effective solar water heater for your needs. Consider the energy source for boosting (gas or electricity), energy efficiency, energy tariffs, ease of installation and product cost. The two usual types of solar collector are flat plate units and evacuated tubes. Flat plate units are most common and have been well proven in Australia for over 50 years. Evacuated tubes, which work more efficiently in cold climates, are more common in Europe and China. Flat plate collectors have been well proven over 50 years of use in Australia.
Roof mounted flat plate solar collectors and hot water tank.
To provide hot water on cloudy days or when demand exceeds supply, most solar water heaters come with a gas or electric booster. A gas booster usually produces fewer greenhouse gas emissions unless renewable electricity is used or the booster is a heat pump. Automatic booster systems located inside the storage tank can be inefficient — cutting in and pre-empting the sun. Override switches and timers can correct this problem if well managed.
A popular approach is to use an inline gas booster that works like a continuous flow water heater but the system must be specially designed to work as a booster for solar hot water (see Continuous flow water heaters above). The solar collector is generally located on the roof of your home, facing north. Make sure it is not overshadowed for long periods.
The storage tank can be located on or inside the roof or at ground level. Installation of a solar HWS is often more complicated than for a traditional HWS, and may incur time delays. In urgent situations, it is possible to install the solar tank and/or booster unit quickly, which can deliver reasonable hot water supply from just the booster.
The solar panels can then be added a few days later. Some suppliers also offer to install a temporary HWS, which is removed when the solar HWS is installed. How do they work? Most solar HWSs use solar collectors or panels to absorb energy from the sun. Water is heated by the sun as it passes through the collectors. It then flows into an insulated storage tank for later use.
In passive systems, water flows due to a thermosiphon effect between the collectors and the tank. As the water heats in the collectors, it becomes less dense and rises to the tank above the collectors, and cold water replaces it. In active systems, water is pumped between the collectors and the tank. The storage tank is usually fitted with an electric, gas or solid fuel booster that heats the water when sunlight is insufficient.
Some solar water heaters also have frost protection to prevent damage in frost prone areas. Solar HWSs are required to comply with Section 8 of AS/NZS 3500.4:2003, Plumbing and drainage — heated water services. For further information see the Building Code of Australia (BCA) Volume 2, Part 3.12.5. Solar collectors Solar collectors trap and use heat from the sun to raise the temperature of water.
The two main types are flat plate and evacuated tube collectors. Flat plate solar collectors — are the most common type and comprise: an airtight box with a transparent cover a dark coloured, metallic absorbing plate containing water pipes insulation to reduce heat loss from the back and sides of the absorber plate. One slight disadvantage of flat plate collectors is that they only operate at maximum efficiency when the sun’s rays strike perpendicular to the flat plate. They also suffer some heat loss in cold weather.
Evacuated tube solar
Collectors are more efficient than flat plate systems. Evacuated tube solar collectors consist of a series of transparent outer glass tubes that allow light rays to pass through with minimal reflection.
Each tube contains an inner water pipe coated with a layer that absorbs the sun’s rays. Water runs through the pipe and is thus heated. A vacuum (hence ‘evacuated’) between the outer tube and the water pipe acts as insulation, reducing heat loss. Evacuated tube systems are more efficient than flat plate systems, particularly in the cooler months and on cloudy days.
This efficiency comes from the vacuum insulation, which minimises heat loss, and the curved surface of the tubes that allows the sun’s rays to strike perpendicular to the water pipes for a greater part of the day. Evacuated tube systems weigh much less than flat plate systems but cost significantly more. Individual tubes can be replaced in the event of damage, making long term maintenance potentially less costly.
In warmer climates, such as Darwin, the additional cost of evacuated tubes is usually not warranted over flat plate solar collectors. Properly maintained solar thermal collectors should outlast the life of the storage tank. When the tank needs replacing, the existing collectors can be connected to the new tank.
Frost protection Frost protection for solar collectors is essential in frost prone areas. During a frost, water can freeze in the solar collector and damage it unless preventative measures are taken. Common types of frost protection include: knock valves (mechanical drain down valves), which can be problematic as they often jam open and drain the tank, or fail to operate, causing severe damage electric heating elements, which are vulnerable in the event of power failure closed circuit systems, which separate the heating fluid from the water and are usually the best option as water does not flow through the solar collectors and therefore cannot freeze there.
Open circuit versus closed circuit In an open circuit system, water flows directly through the solar collectors, into the storage tank and then through pipes into your home. In a closed circuit system, a fluid other than water flows through the collectors, picks up heat from the sun and transfers this heat to water in the storage tank through a heat exchanger.
Closed circuit systems are most commonly used for frost protection. A fluid with a lower freezing point than water is used to prevent ice forming in the solar collectors and damaging them as it expands. Choose the fluid carefully as some become ‘gluggy’ and reduce efficiency. Most fluids need to be checked or replaced every five years. Some closed circuit systems pump hot water through the collectors when temperatures approach freezing.
Avoid systems with this feature if frosts are likely because this action lowers efficiency significantly in cold weather. Passive versus active systems Passive (or thermosiphon) systems In passive (or thermosiphon) systems the tank is placed above the solar collectors so that cold water sinks into the collectors, where it is warmed by the sun, and rises into the tank. A continuous flow of water through the collectors is created without the need for pumps.
Passive solar systems create a continuous flow of water through the collectors. Passive systems come in two types: close coupled and gravity feed. In a close coupled system the horizontal storage tank is mounted directly above the collector on the roof and supplies heated water at mains pressure. The roof must be strong enough to hold the weight of the tank of water.
A flat plate solar, close coupled system supplies heated water at mains pressure. This arrangement is the most cost effective to install but efficiency is reduced in cool and cold climates by heat loss from the tank.
Additional insulation of tanks is desirable in these climates. Alternatively, tanks can be detached and moved inside the roof space, although this increases the cost. Passive solar systems have a continuous flow of water through the collectors without the need for pumps. In a gravity feed system, the storage tank is installed in the roof cavity.
These systems are cheapest to purchase but household plumbing must be suitable for gravity feeding, including larger diameter pipes between the water heater and taps. A common alternative is to use a closed circuit gravity feed system to heat mains pressure water using a heat exchanger.
In a gravity feed system, the storage tank is installed in the roof cavity, still above the collectors. Active (or pumped) systems In active systems (also known as pump or split systems), solar panels are installed on the roof and the storage tank is located on the ground or another convenient location that does not have to be above the solar collectors.
Water (or another fluid) is pumped through the solar collectors using a small electric pump. Because active systems do not require a roof-mounted tank they have less visual impact, particularly when the solar collectors are mounted flush with the roof. However, active systems are usually more expensive to purchase and require more maintenance than passive systems.
Active systems generally use more energy than passive systems because extra energy is required for pumping. There are also additional heat losses in the pipes between the tank and solar collectors. However, if renewable energy is used to power the pump and a high level of insulation is used for the pipes and tank, active systems can reduce greenhouse gas emissions as much as a passive system (see Renewable energy).
An active system can place the tank conveniently at ground level. Careful checking of an active system is required to ensure that it is working as designed.
If the pump or sensors fail, it may not be obvious as the booster (gas or electric) continues to heat the water. Turn off the boost in summer to check if the pump is still working as designed. If the water temperature is cold, call the service agent to check the system. Higher energy bills may also indicate a faulty system. Select a product with a warning light that shows the pump is working, if possible.
Active systems are often used for solar conversions, when solar collectors are added to an existing HWS, or when the roof can’t support the weight of a passive system. Storage tanks Tanks are manufactured from stainless steel, copper or mild steel coated with vitreous enamel. Copper tanks are suitable only for low-pressure systems. The other tanks are suitable for mains pressure.
Vitreous enamel tanks are fitted with a ‘sacrificial anode’ that needs to be replaced every few years to protect against corrosion (more frequently where water quality is poor). Other tanks do not require this protection unless noted by the manufacturer. Outdoor storage tanks can suffer frost damage and significant heat losses in cool climates. In such climates they should be located indoors whenever possible, as part of a drying cupboard.
Booster systems Solar water heaters can be gas, electric or solid fuel boosted. Electric boosters use an electric element inside the storage tank to heat water. Gas boosters use a natural gas burner to heat water either in the storage tank or more commonly as a separate unit downstream from the storage tank. Inline gas boosters are becoming more common: when designed properly they provide
hot water at the desired temperature, while maximising the solar contribution. However, the inline booster must be designed to work with a solar water heater for boosting when the inlet water temperature is not sufficient. Check to make sure it is (see Continuous flow water heaters above).
Solar collectors and tanks can fit into building design. Solid fuel boosters heat water through a heat exchanger, commonly known as a ‘wetback’ system. Gas and solid fuel boosted systems produce fewer greenhouse gas emissions. Boosters can be manually operated or automatically controlled by a thermostat that cuts in when tank temperatures fall below desired levels.
If boosters are not appropriately designed and operated they can defeat the purpose of having a solar water heater by reducing the solar contribution. For example, thermostat controlled boosters inside the tank on off-peak electricity often cut in at night, which means that when the sun rises, there is little useful heating to be done. If residents mostly shower at night, or the storage tank has high heat losses, the problem is worse. Timers can also be used to manage boosters and ensure maximum solar contribution.
Talk to your supplier about the correct operation of timers. Positioning your solar water heater For optimum performance throughout Australia, a solar HWS should face solar north. Orientation can deviate up to 45° from north without significant loss of efficiency. Frames can help orient a solar HWS if necessary. Use a compass to check orientation or check the map in a street directory — true north is at the top of the map (see Orientation). For maximum efficiency, ensure that the solar collectors are not shaded by trees or nearby buildings, particularly in winter when the sun is low in the sky. Optimise your solar collector’s performance by facing it due north, keeping it in full sunlight and tilting it to the recommended angle for your latitude. For best performance, install solar collectors at an angle to the horizontal to maximise the annual amount of sunlight falling on the panels.
The recommended angle to the horizontal for installing solar collectors is the same as the angle of latitude at that location. In Australia, the angle varies from 17.5° in Darwin to 53° in Hobart. In some cases, it may be desirable to increase the angle somewhat to improve winter performance and reduce overheating in summer. In practice, many solar water heaters are installed at the roof pitch angle as it is cheaper and usually more aesthetically pleasing to install solar collectors flush with the roof, rather than use supports such as tilt frames to achieve a greater angle.
Roof pitch angles in Australia are commonly between 20° and 30°, so this often reduces performance in winter and increases the risk of summer overheating. In existing homes, the benefits usually outweigh the costs. In new homes, roof areas could be designed to accommodate a suitable solar collector angle. Other installation tips A complete thermosiphon system full of water can weigh several hundred kilograms. Most roofs can support a storage tank without reinforcement but check with your builder, designer or engineer before installation. Be sure to insulate all components, including pipes and valves, to get the best performance from your system.
This is particularly important for thermosiphon systems where there is a long distance between the tank and hot water taps. It is critical in cold climates. Make sure the booster control is in an accessible location and has an indicator light you can see from inside to remind you to turn it off when not required. Operating and maintaining your system Follow the manufacturer’s maintenance recommendations. Set the temperature of your booster thermostat to above 60°C. A lower setting may allow growth of harmful Legionella bacteria. Make sure you turn the booster off when going on holidays. When you return ensure that the water the water is heated to above 60oC for at least 35 minutes before use.
This will kill any bacteria that may have grown. It could take several hours for the water to heat above 60oC. In favourable climates during summer, water temperatures in a solar water heater can approach boiling point. Heat dissipation devices may be required to prevent water from boiling. It may also be necessary to fit a mixing valve to reduce water temperatures at the tap to safe levels during summer. When some solar systems overheat (usually when high temperatures, strong sun and low usage combine, such as when residents are away), they may dump significant amounts of hot water to protect themselves.
This can be a safety issue and can impact on water consumption. Some people with low hot water usage hear strange noises from pressure build-up in summer, due to overheating. Some shade a part of their solar collector in summer to reduce overheating problems. Carry out activities that need hot water early in the day so that the water left in the tank will be reheated by the sun, ready for use at night. Regularly clean solar panels to remove dust. Flush out solar collectors to remove sludge. Heat pump systems do not require flushing.