March 2007
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A properly designed and installed solar water heating system can slash your power bill
A solar water heating system can provide between 50 and 75 percent of your yearly hot water needs. It can be effective anywhere in New Zealand, even in less sunny areas. Over time, it can pay for itself through lower power bills. By using solar water heating, you’ll also be helping to reduce New Zealand’s greenhouse gas emissions.
We explain what to look for, and how to use the system to get the best return in ‘free’ hot water.
Costs and benefits
Reduced energy bills
In summer, it may be possible to heat all the water you need with solar energy. In winter, or on cold cloudy days, solar water heating will meet part of your hot water needs – you’ll also need some supplementary heating from your booster system. The exact amount you save from solar water heating will depend on a wide range of factors including how much hot water you use, the solar water heating system you install, and the quality of the installation.
Installation costs
Because each installation is different, the costs of solar water heating can vary widely. At present, solar water heaters (including the cylinder) typically cost between $4000 and $8000 installed.
Environmental benefits
If you install solar water heating, you’ll be helping to reduce New Zealand’s dependence on non-renewable energy sources, and you’ll be helping to cut greenhouse gas emissions. Every residential solar water heating system installed is estimated to save, on average, about 1.4 tonnes of carbon dioxide emissions a year (assuming a mix of coal and gas fired electricity generation is avoided).
It’s been estimated that, for an average household, an effective system will:
- provide between 50 percent and 75 percent of annual hot water needs
- cut about 2200kWh from annual electricity use
- provide savings of $350–$450 a year (depending on the cost of your electricity or gas supply).
In general, it’s more expensive to install solar water heating in an existing house than in a new house. This is because, in an existing house:
- you may need to add pipes in parts of the house that are hard to access
- you may need to add structural framing in the roof so it can carry the weight of the solar water heating cylinder – if you choose to have the cylinder on your roof.
Hot water use
Solar water heating is generally more economically viable the more people in the household and the more hot water used. This means the more hot water you use, the better your savings will be.
House design
The design and location of your house are also important – they will determine how easy it is to have solar water heating installed, and how well the solar water heating system performs.
The solar panels need to face north or as close to due north as possible, so you’ll need a section of roof that:
- faces due north or close to due north
- has good year round exposure to the sun (ideally with no shading – and remember shading is likely to be greater in winter)
- is large enough to accommodate the collector panels (usually between 2m2 and 6m2)
- is preferably located near the hot water cylinder, or near where you use the hot water (e.g. bathroom or laundry).
How does it work?
A solar water heater works by absorbing energy from the sun in collector panels located on your roof. This energy is then transferred to water stored in a hot water cylinder.
At times when there isn’t enough solar energy to heat the water, ‘booster’ heating is used to keep the water in the cylinder at the right temperature. The booster heating can be provided by electricity, gas or a wetback. The main components of a solar water heating system include collector panels, the hot water cylinder, and equipment such as pumps and controllers.
Choosing a system
Solar water heating systems are sized according to demand for hot water. The more hot water you use, the larger the storage cylinder and collector panel area should be. The number of people living in your house is a good guide for how large the system should be. These numbers below are a rough guide only. The actual cylinder size and collector area you need will depend on your individual household situation, the system you choose, and the way it is configured.
| Number of people | Tank size (litres) | Collector panel area (m2) |
| 3-4 people | 200-270 | 2.5 to 5 |
| 5-6 people | 270-360 | 3.75 to 6 |
We recommend you discuss this with your supplier or installer to ensure you get a system that will meet your requirements.
‘Open loop’ and ‘closed loop’ systems
In some solar water heating systems, water is circulated through the collector panels and heated directly before being returned to the hot water cylinder. These are known as ‘open loop’ systems.
In other systems, a heat transfer fluid (usually a mixture of water and glycol) circulates through the collector panels. This fluid then passes through a heat exchanger in your hot water cylinder, heating up the water in your cylinder. There’s no mixing between the heat transfer fluid and the water stored in your cylinder. This type of system is known as a ‘closed loop’ system.
Discuss with your supplier or installer which option is more suited to your requirements.
Collector panels
There are two main types of collector panels for solar water heating systems – flat plate panels, and evacuated tube panels:
- A flat plate panel looks similar to a skylight. It absorbs sunlight and transfers the heat into the water or fluid flowing through the collector panel. The common size for a house is one to three panels, between 2.5m2 and 6m2 in total.
- An evacuated tube panel is made up of a series of glass tubes (between 1.5m and 2m long) sloping lengthwise up and down the roof.
In New Zealand’s temperate climate, both types of panel are equally efficient.
Pump or no pump?
The water or fluid can be circulated around the solar water heating system using a pump, or it can be circulated naturally using a thermosiphon system.
Systems that use natural circulation by thermosiphon are often called ‘passive’ systems. In this type of system, the hot water cylinder has to be located above the collector panels, usually on the roof . Cold water or fluid moves down from the cylinder into the collector panels; then, once it is heated by the sun, it rises back up into the cylinder.
Natural circulation by thermosiphon has the advantage that it does not require a pump and therefore is not dependent on electricity, whereas a pump system is.
With a pump system, the hot water cylinder can be located at a level below the collector panels. This can be helpful if you’re installing a solar water heating system in an existing home and you want to use the existing cylinder, or if you would rather not see the cylinder on the roof. It is essential that the pump is used in conjunction with a controller to ensure the pump operates only when necessary.
Systems that use pumps to circulate the water or fluid are often called ‘active systems’
Hot water cylinders
The hot water cylinder can be part of the system on the roof, or it can be mounted separately – in another part of the house.
You can use a conventional hot water cylinder or a specialist solar water heating cylinder. The key difference is that specialist cylinders are larger and they are specially designed to maximise the use of solar energy. If you choose a specialist cylinder the system is likely to perform better. However, it is possible to achieve reasonable performance with a conventional cylinder, through effective system design.
If you choose to use a conventional cylinder, you’ll need to have a controller for the supplementary heating. Conventional hot water cylinders in houses are usually 180 litres or less. This storage capacity is generally too small for a solar water heater to achieve optimal performance for a household of three or more people.
If you are planning to get a system that includesa cylinder, check the quotes carefully –a cheap price may mean the cylinder and othercomponents aren’t included.
Controllers
A controller manages the use of supplementary (‘booster’) gas or electric heating, and controls the pump in a pump system. The heating controller has a significant effect on overall system performance, so it is important this is set up correctly.
There are two types of supplementary heating controllers – time trigger controllers and minimum temperature controllers:
- A time trigger controller can be used to keep the booster heating turned off during the day, to ensure you’re not paying for electricity or gas to heat your water when the sun could be doing the job for free. The timer can also be set to turn on the supplementary heating before periods of high water use. This can greatly increase the system’s efficiency.
- Minimum temperature controllers simply trigger the supplementary heating whenever the cylinder temperature drops below a minimum pre-defined temperature.
A controller will ensure that you’ll never be without hot water, regardless of the time of day. Make sure you talk to your installer or supplier about the type of controller you need.
It’s also worth reviewing your electricity tariff when you install a solar water heating system to ensure you are paying the best rate for supplementary heating.
The Building Code requires that the temperature of the water in your cylinder must reach 60 degrees at least once a day. Make sure your system is set up to meet that requirement.
Frost protection
In frost-prone areas, you’ll need to ensure that the system you buy has frost protection. Otherwise, the water or fluid could freeze in the collector panels. The water/glycol mixture used in most closed loop systems has anti-freeze properties.
In open loop pump systems, temperature sensors can turn the pump on to run water through the system before the collector panels freeze. However, this won’t work during a power outage.
Some open loop thermosiphon systems have frost valves, which let the water flow through the collector panels when the temperature is close to freezing.
Frost tubes could also be considered on some systems. These enable the water in the collector panels to freeze without causing damage. The devices absorb any pressure build-up created by the water expanding or freezing.
Talk to your installer or supplier about what method is appropriate for the system you choose.
Obtain quotes and information from several industry-accredited suppliers or installers to ensure you get the best system at a competitive price.
Choosing an installer
We recommend always using an industryaccredited supplier or installer. A database of accredited suppliers or installers can be found here. Ask what post-installation service or warranties they provide.
As part of their quote, installers should include an assessment of your house, roof orientation, and any structural support requirements for the cylinder.
If you choose to have a system with a cylinder on the roof, the installer will need to do an assessment of any structural
requirements to ensure the weight of the cylinder is supported. If you have a system with just collector panels on the roof, this is not likely to be a concern.
Ask them about their specific experience in installing systems in circumstances similar to yours, for example if you have a two-storey house, or if you live close to the sea or in a very exposed place.
Regardless of which type of system you choose, the installation will need to comply with the NZ Building Code and that means you will need a building consent from your local council.
Use and maintenance
Problems
As long as the solar water heating system has been sized and installed properly, it should perform well. If not, go back to your installer and ask for the system to be fixed. After the maker’s warranty has expired, you are still covered by the Consumer Guarantees Act for both manufacturing defects and lack of skill by the installer.
Maintenance requirements
A solar water heating system requires little maintenance, but, for maximum performance and energy savings, some maintenance is needed:
- Hot water cylinders should be maintained in a similar manner to normal electric hot water cylinders – hot and cold relief valves should be flushed every six months, and the anode in a glass-lined water container should be changed every five years (or more frequently in hard water areas).
- Collector panels that are out of sight should occasionally be visually inspected for leaks, which may occur in the collector case, glass or pipes. Condensation on the glass or wet insulation indicates the system may be leaking.
Collector panels should be washed if they get dirty. This is usually a problem only if the panels are protected from the rain or during long dry periods, near dusty roads or when pollen levels are high. - Check for shading at the same time – trees may grow unnoticed. Debris or bird droppings may gather on or around the collector panel, reducing solar absorption.
Although not common, broken glass or damaged glazing should be replaced immediately as water coming in will cause rapid deterioration of the absorber’s surface and insulation. - Frost protection methods that use frost plugs (which are increasingly rare) sometimes need resetting in freezing conditions. If the system uses glycol or a water-glycol mixture, this should be replaced about once every five years, according to the manufacturer’s instructions.
Manufacturers’ instructions should be followed for any maintenance issues or requirements over the life of the system. - If there is any discolouration or corrosion, discuss this with your supplier or installer.
Make sure you are given adequate operating instructions when your system is installed.
