Designing homes to conserve energy and use it efficiently, from sources that cause least environmental harm.

Solar water heating storage cylinders

Storage cylinders should be designed to meet household hot water needs, maximise the amount of water heated by solar radiation, and protect safety.

On this page:

  • Cylinder location
  • Cylinder size
  • Control of temperature
  • Temperature and pressure relief valves

Solar water heating cylinders must:

  • be able to handle expected temperatures stainless steel and copper cylinders are frequently used
  • have sufficient capacity to meet both peak loads and overall daily demand without requiring excessive use of a booster heating system.

For general Building Code requirements relating to water heating, see hot water supply in the Water section of this site. For general requirements relating to storage cylinders, see storage cylinders. Clause G12/AS2 provides an acceptable solution for solar water heating systems.

Cylinder location

The cylinder location depends on whether heat is transferred by thermo-siphon or pump and whether an open loop (direct) or closed loop (indirect) system is used (see Heat transfer systems for information on these system types).

For thermo-siphon systems, relative heights of the cylinder and the panel must be carefully designed to allow a natural thermo-siphon to occur.

Some open loop solar water heating systems incorporate a solar hot water cylinder, while others can be used with a remote or existing hot water storage cylinder. The storage cylinder may be on the roof just above the solar panels or separate, e.g. inside the roof space or fixed to the exterior of the building.

The weight of the water must be considered when sizing framing for roof-mounted or ceiling space cylinders, and the cylinder must have earthquake restraints.

In buildings with low pitched roofs or limited attic space and in retrofit situations, the cylinder may need to mounted outside on the roof, which means there may be increased heat losses.

Allow for maintenance access to the cylinder and other components of the water heating system.

Cylinder size

The storage cylinder should be larger than for a single energy source cylinder such as gas or electric, so that when there is good solar gain, the cylinder can store the heat that is available.

Control of temperature

The boosting element in a solar water heating cylinder needs to be installed and configured to:

  • maximise the proportion of water heated by solar energy, and
  • ensure that water is heated to control Legionella.

Maximising the proportion of water heated by solar energy

It is important that the booster system does not heat water when there is solar energy available. Any solar systems should incorporate a timer to turn the element off in the mornings. In a 2006-08 BRANZ study of solar water heating systems, only seven of 35 systems had a timer.

The position of the element within the system is also important. If there is an element at the top of the cylinder, only the water above the element is heated when solar energy is low. In the 2006-08 BRANZ study found there were three cylinders with elements at the top and bottom and two of these systems were found to perform well.

Controlling Legionella

Domestic hot water cylinders should reach a temperature of at least 60C daily to kill any Legionella organisms present. Acceptable Solution G12/AS2 Solar water heaters requires that systems are capable of heating the water to 60C either:

  • continuously, where the element is in the bottom 55% of the cylinder, or
  • once a day, where the element is in the bottom 20% of the cylinder, or
  • weekly, where all of the water within the system is heated for at least 1 hour (and the temperature is measured in the bottom 20% of the cylinder).

While many solar water heating systems can theoretically reach 60C in the cylinder, in practice, it requires careful system configuration and owner operation to achieve this. To prevent scalding, the temperature will need to be tempered before the water reaches outlets. See storage cylinders for detail.

In the 2006-08 BRANZ study of solar water heaters, many of the operators of solar water heating systems were not aware of the 60C anti-Legionella temperature boost recommendation and were not running their systems with that in mind.

Temperature and pressure relief valves

Over-temperature pressure relief valves are essential on solar water heating systems. Water heated by solar radiation can reach very high temperatures, and if there is an event that stops water circulating (such as an electricity failure that stops a pump), the temperature and the pressure can rise quickly within the collector and pipes. While new systems have been tested to confirm they are physically strong enough to contain very high pressures, this system strength may be reduced in older systems, and the very high pressures should be avoided.

In the 2006-08 BRANZ study of solar water heating systems, 10 of 31 systems had no apparent over-temperature relief system. Others depended on roof-top air admittance valves, while others relied on the temperature/pressure relief (TPR) valve on the storage system itself. BRANZ recommends the installation of over-temperature pressure relief valves on all solar water heating systems.

See hot water supply for more about valves and controls.

Updated: 25 June 2013