Solar water heating goes under the spotlight

This Issue This is a part of the Water heating and use feature

By - , Build 99

A new research programme being undertaken by BRANZ is looking at how well solar water heating works in practice in New Zealand.

Roof-mounted solar water heating system.

Solar water heating systems are becoming increasingly popular, with installations now at an all-time high. According to census figures, the number of homes with solar water heating systems has increased from just 624 in 1976 to 35,000 last year. Of those, 3,500 were installed between July 2005 and June 2006.

Interest in solar water heating last peaked following the oil shocks of the 1970s. Interest-free loans were then available and at one time there were 27 manufacturers producing systems for the New Zealand market. However many units performed poorly, giving the emerging industry a bad name, and by 1990 there were only five companies still involved in marketing solar water heating.

Roof-mounted solar water heating system.

Rising energy costs and growing environmental concerns have led to renewed interest in solar water heating systems. Many initiatives are now underway to increase their use. Modern systems are generally good quality and many of the standards relating to solar water have been revised recently and improved.

The Energy Efficiency Conservation Authority (EECA) has been working to support the increased use of solar water heaters. This includes helping develop standards, commissioning research and producing publications on solar water heating. EECA has also set up a financing scheme to help with the upfront costs.

Others involved in promoting solar water heating include the Solar Industries Association, Solar Action and the Australia and New Zealand Solar Energy Society.

To make it easier to get a building consent, a new Acceptable Solution specifically for solar water heating is being developed for the New Zealand Building Code Clause G12 Water supplies.

New BRANZ research project

So how well do solar water heaters work in practice in New Zealand homes? That’s the question being investigated by a new research project undertaken by BRANZ for Building Research and EECA. It will look at how much energy the systems save, whether people are happy with them, the best ways of maintaining and installing them and what effect they have on the buildings themselves.

A total of 36 solar water heaters have been installed in Auckland, Wellington, Christchurch and Dunedin, with monitoring equipment to measure their performance over one year. A number of heat pump water heaters are also being monitored. The householders will be surveyed about their experiences.

During the installation visit, a physical inspection of the water heating system and relevant aspects of the building was undertaken. A report is now being prepared which will provide some important first results.

The inspections looked at: the operation and performance of the system (such as the degree of external shading and the use of timers), structural factors (such as the weight of roof-mounted systems) and plumbing issues (such as whether pumps or other fittings can be easily isolated for servicing).

Early lessons

Each installation was unique and provided individual lessons. There were few new houses in the sample, which meant most of the installers had to add solar collectors and pipework to the existing hot water system, many of which were installed to the original hot water cylinder.

Preliminary findings suggest that when working with an existing system it is important to consider the overall operation of the system. For example, one system made use of an existing hot water cylinder, with the new roof-mounted thermosiphon solar system being installed as a ‘preheater’. The hot water from the solar system was fed into the inlet of the existing cylinder. As is common practice, the cold feed from the tempering valve was taken from the pipework leading into the main cylinder. However, with the solar preheater in place this water was no longer cold and the tempering valve would not operate correctly. The installer in this case should have modified the pipework for the tempering valve, providing an alternate cold water connection.

Questions for the future

The study will also look at what happens to solar water heating systems over time. Will they require maintenance? Will they continue to perform to the same level? What effect will they have on the building?

Anecdotal evidence suggests that pipe insulation is likely to crack after just 1 or 2 years unless it is properly weatherproofed by, for example, being painted. This will eventually make it less effective. Solar water heating systems can also have an effect on the building. Roof-mounted cylinders can be heavy and care must be taken to make sure the units do not damage the roof structure.

In addition, solar water heating systems can produce very hot water, and it is important to have adequate relief systems. The detailing of relief drains seen in the sample has highlighted some poor practices, with many relief valves discharging onto the roofing surface and in some cases causing corrosion.

Solar water heating systems are complex. The BRANZ study will help determine the best way of installing them to make sure they provide trouble-free performance for a number of years.

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Articles are correct at the time of publication but may have since become outdated.

Roof-mounted solar water heating system.

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