Section 5: Planning a solar hot water system

Split and close-coupled systems explained

Close-coupled systems

For close-coupled (non-split) systems, the circulation of the water through the panels and tank occurs naturally as the heated water rises by a process called thermosiphon. With no pump required, these systems are simpler than split systems, with less maintenance required.

However, the storage tank needs to be above the collector panels, and the roof must be able to support the full weight of the tank.

Close-coupled systems have the advantage that all components are close together, reducing heat loss from pipe runs. However, with the tank on the roof, maintenance can be more difficult.

Split systems

Split systems use a pump to circulate water from a ground-mounted tank through the roof-mounted panels. A temperature differential controller gathers information from temperature sensors at the top of the panels and at the bottom of the tank, and tells the pump to operate when there is heat to be gained from the collector.

The advantages of a split system include much less weight on the roof and easier tank maintenance. However, a split system has added complexity due to the pump and controller, as well as longer pipe runs between the collectors and tank which can lead to heat loss.

Collector types

There are two common types of solar hot water collector panels: flat-plate and evacuated-tube. Which type of collector you use depends on personal preference and your climate. Evacuated tubes tend to work better in the cooler months in colder climates, but in warmer climates, either type of collector will perform well. Evacuated tubes can also be up to 163% more efficient.

Boosting

Virtually all hot water systems on the market incorporate some form of boosting.

Electric elements are the most common boosting option. They fit in well with cheaper night rate tariffs. If you have a solar panel (PV) system installed then this might help reduce energy costs associated with boosting.

Frost protection

The system should be fitted with a freeze protection system if frosts are a possibility in your area. Water expands as it turns into ice and if there is no ability to absorb this expansion or prevent it happening, then the result will be ruptured pipes in the collectors. Repairing collectors can be difficult and replacement is expensive.

All manufacturers offer frost protection on at least some models.

Temperature control

Solar hot water systems are effectively connected to an uncontrolled heat source (the sun) and so the temperature in the tank may become much higher than in a regular non-solar water heater.

In hot weather, with minimal hot water use, tanks could actually reach boiling point. A number of mechanisms can be used to prevent this from happening, include pressure and temperature relief (PTR) valves, heat dumps, circulation control increasing the collector slope at insulation time and tempering valves.

Installation considerations for solar hot water systems

Installation methods will vary depending on the type of system chosen, the house roof angle and structure and the location and orientation of the home.

Pipe length

When considering plumbing the pipe runs should be kept as short as possible by locating the system central to all usage points; this will minimise heat loss from pipes.

Panel siting and collector angle

To get the most out of a solar hot water system the collectors should be tilted towards the sun for optimal performance, which means mounting them on a north-facing roof. The main requirement is that there are no long periods of shading.

Tilt angle should be set to optimise solar gain for winter in Victoria, where possible, as this is when sunlight hours are reduced and demand for hot water can be greater.

Non-north-facing options

A system will still perform well enough if a west-facing roof is the only option, provided there are no shading issues. Such a system may perform up to 20 per cent less efficiently than a north-facing system but should still produce good quantities of hot water. Oversizing the collector will help.

Special frames are also available that allow a collector to face north, even if mounted on an east, west or even shallow south-facing roof. However, they may cost more than simply oversizing the collector.

Roof strength

Roof strength is another important consideration for the installation of close-coupled systems. Every litre of water weighs one kilogram, which means that a 300 litre tank will weigh as much as 420 kilograms when the weight of the materials is taken into account. Needless to say, the roof must be inspected and its carrying capacity assessed.

The system should span as many rafters as possible, and these may need to be braced from inside.

Roof strength is not a concern with split systems as only the collector panels are mounted on the roof and their weight and area represent very light loadings.

Insulate the pipes

All water pipes should be insulated to reduce heat loss and to minimise or eliminate frost damage. Australian Standard 3500.4 defines minimum levels of pipe insulation for hot water systems. Be aware that this is an area where installers might cut corners. Some types of exposed pipe insulation can also degrade rapidly when exposed to sunlight. It is a good idea to ask the vendor to be explicit about the level of insulation to be used and whether it is UV-rated.