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Solar water heating options

What to consider when choosing the most effective solar water heating system for your household?

Hot water from the sun

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. 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 and the size of the panels. The Solar Association recommends that the cylinder size is approximately 75 litres volume of hot water for every 1m² of collector area. Typically, you should have 1m2 of solar panel collectors for every person in the house.

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’s 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 four to five 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, or combinations of those. With gas, the booster can either be in a hot water cylinder or in a separate instant hot water unit downstream from the cylinder. Options for electric boosters include electric heating elements or heat pump heat exchange coils within the cylinder, or a separate electric instant hot water unit downstream from the cylinder.

Supplementary heating from wetbacks can be particularly effective in areas with low sunlight hours in winter reducing the need for alternative booster heating. Some wetback systems can supply hot water when the power is off.

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 2.5m2 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.

While in theory there are differences about the efficiency of various collectors, New Zealand research has shown that the set-up and quality of the installation significantly influences the overall efficiency of the system.

Turning solar for water heating on the Build magazine website has further information.

The Performance of solar water heaters in New Zealand [PDF 2.8 MB] on the BRANZ website contains the full report.

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 degrees from horizontal and in Dunedin about 46 degrees. 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. Your roof and framing will need to be strong enough to support the weight of the cylinder if it is on the roof. Some strengthening may be needed, and this may require a building consent – check with your local council.

'Open loop' and 'closed loop'

In some systems, water is heated directly as it flows through the solar panel pipes. These are known as 'open loop' systems, and may need to meet additional building consent conditions.

With other systems, the water is heated indirectly via a fluid. This fluid is usually a mixture of water and glycol, which passes through the solar panels and absorbs the heat, before being transferred to 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.

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
  • once it is heated by the sun, it rises back up into the cylinder
  • there is no pump, so no reliance on electricity.

Systems that use pumps to circulate the water or fluid are often called 'active' systems. In this type of 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 have the cylinder on the roof.
  • the pump must be used in conjunction with a controller to ensure the pump operates only when necessary
  • the need for a pump means this system is reliant on electricity.

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 specialist solar water heating cylinder, or in some cases you might be able to use a conventional hot water cylinder – check with a professional installer.

The key difference is that specialist cylinders are specially designed to maximise the use of solar energy. If you choose a specialist cylinder, the system is likely to perform a lot better and will also better deal with very high water temperatures (ie above 80oC) which will occasionally be produced by the solar collectors.

If you choose to use a conventional cylinder, it will need to be able to cope with high temperatures, and 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 good performance for a household of three or more people. 100 litres capacity per person is recommended. 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 your existing cylinder is nearing the end of its expected lifespan, it’s worth considering installing a new system. If you are installing a new system, check the expected lifespan of the cylinder before you decide on system.

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 which could require a building consent.

In colder parts of New Zealand, the heat loss from the hot water cylinders located on roofs or outside the conditioned area of the house can be high, reducing their efficiency. It’s a trade-off between not needing a pump (and the system working even in a power cut) and winter-time heat loss.

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 and for systems where the cylinder is located on the roof.

High temperatures

All pipes and cylinders will need to be able to withstand temperatures above 100°C. Many existing cylinders may not be able to withstand temperatures this high – talk to your plumber or solar installer.

Installing a solar water heating system

Who can install a solar water heating system?

All solar water heating systems must be installed by a qualified plumber who is experienced installing solar water heating.

The installer will also make sure that the system does not compromise the waterproofing of the roof.

You may also need an electrician to connect an electric booster system and timer. Alternatively, your plumber may be qualified to do this.

What are the legal requirements?

You will need a building consent, possibly as part of a larger building or renovation project and for strengthening your roof or floor for large cylinders. Before you apply for the building consent your installer will need to consider these points:

  • The panels have to be firmly fixed so they don't blow away in strong winds.
  • The roof must be able to support the weight of the solar panels and possibly the cylinder.

Applying for building consents on MBIE’s Building Performance website has more information.

Dealing with suppliers

Always use an industry-accredited supplier and installer and ask what they provide in the way of post-installation service or warranties. See the Solar Association of New Zealand website for a list of accredited suppliers and installers

As part of their quote, installers should include an assessment of your house, roof orientation and any structural support requirements for the cylinder. 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).

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 that only has collector panels on the roof, this is unlikely to be a concern.

Regardless of which type of system you choose, the installation will need to comply with the New Zealand Building Code and is likely to need a building consent from your local council.

Check the Local Councils website for council contact details

Obtain quotes and information from several industry-accredited suppliers or installers to ensure you get the best system at a competitive price.

Making the most of solar water heating

The benefits you get from solar water heating are very dependent on how well your system is set up and managed.

A solar hot water system normally produces more hot water in summer because the sun shines for longer and is higher in the sky. It obviously can't heat water at night. Sometimes there will be plenty of hot water and sometimes not enough. This is where the supplementary booster system will ensure you have enough hot water for your needs.

To make the most of your system you should use:

  • water efficient taps, showerheads, and appliances
  • hot water efficiently, especially in winter
  • a timer or programmable control to make sure your booster system isn't heating water you won't need. For example, use the time to make sure your supplementary heating isn't heating after early morning use, if you are not home during the day.

Maintaining a solar water heating system

A solar water heating system typically requires little maintenance but, for maximum performance and energy savings, some regular 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). As the anode is very long (check the specs of your particular hot water cylinder), you need to ensure that the cupboard in which it is installed allows good access for easy replacement.

Collector panels that are out of sight should occasionally be visually inspected for leaks, which may occur in the 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 only usually a problem 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 collector panels, 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, or according to the manufacturer's instructions.

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Note that this document is published by the Ministry of Business, Innovation and Employment Chief Executive as Guidance under Section 175 of the Building Act 2004. This is a guide only and, if used, does not relieve any person of the obligation to consider any matter to which the information relates according to the circumstances of the particular case.