Solar water heating options
What should you consider when choosing the most effective solar water heating system for your household.
If you're thinking of installing a solar water heating system, you will need to consider how big the system needs to be and make decisions about the location of panels and cylinder, and the type of system and cylinder you want to use.
You will need professional advice about these things when you're shopping around for quotes, but the more informed you are the greater your chances of getting a system that works best for your household.
How big should the system be?
The size of your solar water heating system will depend on your demand for hot water. An average 3-4 person household will use about 200-270 litres of water a day, so your cylinder will need to be at least that size and your solar panels will need to cover an area between 2.5m2 and 5m2. An average 5-6 person household will need a 270-360 litre cylinder and solar panel area of 3.75-6m2.
These numbers 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. Discuss this with your supplier or installer to ensure you get a system that will meet your requirements.
It is not a good idea to skimp. Choose a system that will meet your home’s needs in the foreseeable future. If your home has four bedrooms, it’s a good idea to have a system big enough for 4-5 people even if only two people live there now.
Booster systems
You will need booster heating to keep the water hot when the sun isn't shining. The most common types of booster are electric, gas and wetback. It is also possible to use a diesel booster system.
With gas, the booster can either be in a hot water cylinder or in a separate instant hot water unit downstream from the cylinder.
Controls
A controller manages the use of booster gas or electric heating, and controls the pump in a pump system. It has a significant effect on the overall performance of your solar water heating system, so it’s important that it 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.
The controller often comes as part of standard installation.
Note: to keep dangerous legionella bacteria in check, water must reach 60°C for at least one hour each day.
Solar 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 2.5 square metres and 6 square metres 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 climate, both types of panel are equally efficient.
Siting the panels
Solar panels should be placed where they will get maximum exposure to the sun. This usually means they'll be on a roof. They should be sited so they aren't shaded by hills, trees or buildings.
Panels should ideally face north, although directions between north-east and north-west are usually satisfactory.
Panels should be tilted towards the sun. The optimum angle is equivalent to your latitude: in Auckland, for example, panels should be about 37% from horizontal and in Dunedin about 46%. If you tilt the panels up more than these figures, you will gain more heat during winter and less during summer. Tilting the panels down will give more summer heating and less in winter. If your roof doesn't slope correctly, your installer can supply a frame to give the right tilt. The frame will have to be well secured - you don't want it to take off in a strong wind.
The exact orientation and tilt are not critical. If you can't get the orientation quite right, you can use more panels.
Your roof and framing will have to be strong enough to support the weight of the cylinder if it is on the roof. Some strengthening may be needed.
'Open loop' and 'closed loop'
In some systems, water is heated as it flows through pipes in the solar panels. These are known as 'open loop' systems.
With other systems, a fluid (usually a mixture of water and glycol) passes through the solar panels and absorbs the heat, which is then transferred to the water through a heat exchanger within the cylinder. These are known as 'closed loop' systems.
Discuss with your installer or supplier which option is best for you.
- Open loop system with pump circulation. Graphic: EECA
- Closed loop system with pump circulation. Graphic: EECA
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 which are 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.
- Systems that use pumps to circulate the water or fluid are often called 'active' systems.
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.
Hot water cylinders
- Standard hot water cylinder Graphic: EECA
- Solar hot water cylinder. Graphic: EECA
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. Performance can be increased with the use of a controller for the supplementary heating.
If you are planning to get a system that includes a cylinder, check the quotes carefully - a cheap price may mean the cylinder and other components aren't included.
Use the right cylinder for the system
Check with your solar supplier that the cylinder you are already using is fit for purpose if you are adding solar panels. If you are installing a new system, check the expected lifespan of the cylinder first.
Location of the cylinder
For passive systems, the cylinder has to be located just above the collector panels. In this case, the roof will have to be strong enough to support the weight of a full cylinder. Some strengthening may be needed.
For pumped systems, the cylinder may be in the roof space or in the hot water cupboard.
Other factors
Insulation
To get the best performance from your system, all components will have to be insulated including the pipes. This is particularly important where there is a long distance between the cylinder and the hot water taps. It is critical in cold climates.
High temperatures
All pipes and cylinders will need to be able to withstand temperatures above 100 °C.
More information
From Smarter Homes
- Installing and using solar water heating
- Is solar water heating for me?
- Solar water heating case studies
- Water heating
- Heated swimming pools
From consumer.org.nz
Note: you may need to be a subscriber to access some of this information.
From other sites
For more information, the best place to start is the Energy Efficiency and Conservation Authority’s Solarsmarter solar water heating website. You can also find out more from the websites of the New Zealand Solar Industries Association. The Energywise website has information about water heating options.
