Energy

BRANZ research has found that correct installation is the single most important factor in determining the efficiency and performance of solar water heating systems.

On this page:

• Panel tilt angles
• Standing losses
• Back circulation
• Information for users
• Installers as quality controllers
• Safety – adequate fastening
• Durability – avoiding corrosion

From 2006 to 2008, BRANZ conducted a study on the performance of solar water heaters and heat pumps in Auckland, Wellington, Christchurch and Dunedin. On average, the solar water heating systems supplied 38% of the household’s heating needs.

Correct installation was found to be the most important factor in determining the performance and efficiency of solar water heating systems.

Installation had a bigger effect on performance than the specific technology used (for example, whether evacuated tube or flat plate collectors were used, and whether a pump or thermo-siphon effect was used to circulate the heating fluid). Installation also had a bigger effect than region or climate.

The results are available in BRANZ Study Report 184 and BRANZ Study Report 188. The study pre-dated Acceptable Solution G12/AS2.

The recommendations on this page reflect the results of that study. Also see solar water heating storage cylinders for recommendations regarding cylinder and booster systems.

Panel tilt angles

Most of the systems studied had their collectors installed at the same angle as the roof. None had a tilt angle greater than or equal to the site latitude (which is recommended to maximise solar gain). For the systems where it was possible to measure winter performance, more than half provided less than 10% of the winter water heating needs. Generally, those with steeper angles on their collector panels provided more water heating in winter.

Recommendation:

• Install solar collectors at an angle at least equal to the site latitude.

Standing losses

Standing losses (when energy is lost through the collector, pipework and walls of the storage cylinder) formed a sizable part of the energy balance of the studied systems. If standing loss is reduced, more of the solar energy can go into replacing the heat drawn off by hot water users in the house.

Many of the systems with high standing losses were thermo-siphon systems, with large cylinders installed outside. Increased thermal insulation of those cylinders may help.

Six of the systems included retrofitted cylinders with B grade insulation.

Recommendation:

• Where retrofitting solar water heating to an existing cylinder, ensure that it is an A grade cylinder (which frequently has 50 mm of polyurethane foam for insulation). If it isn’t an A grade cylinder, wrap it.
• In a thermo-siphon system, ensure that there is good insulation around the outdoor cylinder.

Back circulation

One system in the study had very high heat losses at night, as the hot water was able to circulate back through the open-loop system to the flat plate collector.

Recommendation:

• Incorporate piping arrangements to reduce the chance of back circulation at night.

Information for users

The study found many users were not aware of how to get the best out of their solar water heating system. While 80% were supplied with written instructions, 21% found the manual too difficult to follow. They were not sure how to monitor performance or how to get help if they thought the system was not working well. The majority were not shown the different parts of the system, or how to operate or manage the controls. Most were not told whether the system had a timer or not. Only eight were asked if the settings were correct for their household.

Recommendations:

• Install system displays in a prominent location in a living space of the house so occupants can monitor them.
• Give guidance to users about the expected performance of their system so they can actively monitor it.
• Provide guidance to the users so they can operate the timers to minimise booster system use.
• Include an audible alarm with the control system to tell users when there is a fault in the system.

Installers as quality controllers

Participants in the study had had little contact with the installer after the actual installation. Occupiers were left without the information or expertise to assess the performance of their system. A review of the system that was back-circulating water at night may have found that fault much earlier.

Recommendation:

• Review the system after installation to ensure it is working to specification.

Safety – adequate fastening

Roof-based collector panels (and tanks if installed alongside) need to be securely fastened to the building structure. This is usually done by screwing through the roof to the timber purlins beneath. Similarly, all feed and return pipes need to be firmly secured.

Acceptable Solution G12/AS2 recommends 10 mm coach scr/in the study used that kind of fastening, instead opting for Tek screws (self-drilling, self-tapping wood screws designed to secure metal roofing to timber framing).

The compatibility of the metals in frames, brackets, fastenings and roof surfaces is important, to avoid corrosion.

Durability – avoiding corrosion

Materials used for solar water heating systems can be affected by elements in their environment and need to be protected from corrosion, as well as prevented from causing damage to the roof itself. Damage can occur as a result of exposure to UV radiation, sea air, rain and heated water and as a result of inappropriate combinations of materials.

The two types of damage to guard against are:

• metal corrosion to the frames of collector panels, brackets and fasteners, and the roof surface
• polymer corrosion to plastics, rubbers, paints and sealants such as the gaskets and seals on collectors and lagging on pipework.

Metal corrosion can occur in a variety of ways:

• Galvanic corrosion is where dissimilar metals coupled together will corrode rapidly when wet, especially in the presence of sea salt. In solar water heating systems, this is likely to occur between any of stainless steel, copper, steel, aluminium and zinc (galvanised steel). The damage can be prevented by avoiding poor combinations of fasteners, framing, brackets and roof surfaces or using electrically isolating washers.
• Galvanic corrosion can also occur when copper dissolved in water (such as the water out of a hot water cylinder overflow or header pipe) runs over a galvanised steel roof. This causes rapid corrosion of the zinc, obvious as highly localised rusting, and should be avoided by redirecting flow from copper pipes away from the metal roof.
• Waste metal such as swarf from drilling left lying on a roof will cause corrosion. It is important to ensure the surface is left clean following installation.
• Using unsealed timber framing to support collector panels can increase risk of corrosion, as the wood will retain water against the roof or other metal surfaces, prolonging any galvanic reaction.
• If collector panels have the same pitch as the roof, they can accumulate salts and dirt underneath that form a highly corrosive electrolyte, breaking down protective corrosion products and causing rapid degradation. These areas need to be able to be washed down with fresh water regularly. New Zealand metal roofing manufacturers recommend at least 100 mm clearance between collectors and metal roof cladding, and cleaning every 3 months in severe environments.
• An inert catchment effect can occur. The rainwater runoff from the glass covers of the flat plate collectors is pure, and the zinc of an unpainted galvanised roof under a collector will dissolve more readily in pure water than in water that already has some zinc dissolved into it.

Exposure zones for New Zealand are identified in NZS 3604:2011 Timber-framed buildings, section 4.

Polymer corrosion can also occur in a variety of ways:

• Ultraviolet (UV) radiation can break the carbon backbone of polymers such as the closed cell foam commonly used to lag hot water pipes, causing loss of structural integrity, flaking and chalking. Closed-cell foam pipe lagging needs to be painted with acrylic roof paints or wrapped in PVC tape.
• Heat can also break the carbon backbone of polymers and break off molecules from the carbon chain. Polyvinylchloride (PVC) will become brittle and, when wet, can create hydrochloric acid, which will corrode galvanised steel. Solar water heating systems combine heat from the sun with very hot water in pipes causing seals to become brittle and some plastics may melt. Lagging materials must be able to cope with the very high pipe temperatures that may be encountered.
• Rainwater can also increase corrosion by washing away the broken down material, and exposing fresh areas.

^{Updated: 25 June 2013.}