Heated water is potentially dangerous, both from the possibility of scalding and the possibility for a hot water system explosion. Safety is therefore an essential part of a hot water system.

Find out about:

• NZ Building Code requirements
• hot water supply options
• selection considerations
• hot water supply temperature
• Legionella bacteria
• tempering hot water supply
• storage water heaters
• pipes, valves and controls.

NZ Building Code requirements

The NZ Building Code clause G12 Water supplies requires hot water systems to meet a number of objectives including to:

• provide potable hot water to outlets for consumption, food preparation, utensil washing and oral hygiene.
• prevent growth of Legionella bacteria
• protect users from scalding during personal hygiene activities
• be safe from explosion
• be efficient and avoid energy wastage.

Hot water supply options

Hot water may be:

• stored in a storage water heater (heated by electric/gas/solar/wetback), or
• continuous flow (heated by electric/gas).

Selection considerations

Three factors determine the selection of a hot water system:

• Energy source – options include electric (direct heat/heat pump/heat exchange), gas, solar (direct heat/heat exchange), wetback.
• Storage or continuous supply.
• Capacity – including delivery rate, recovery rate, actual and potential number of users, type and number of fixtures within a household.

Hot water supply temperature

Water heated to over 50°C can cause serious burns in less than a minute. At greatest risk are:

• children because of their sensitive skin
• the elderly because they have slower reaction times.

NZBC Clause G12/AS1 requires that the temperature of hot water delivered to sanitary fixtures (basins, baths, showers) does not exceed:

• 45°C for early childhood education centres, primary and secondary schools, nursing homes or similar facilities for young, sick, elderly and disabled people, institutions and hospitals
• 55°C for other buildings in New Zealand.

Hot water may be delivered to domestic kitchen sinks and laundries at higher temperatures – generally around 55°C to 65°C.

Legionella bacteria

To prevent the growth of Legionella bacteria, stored water should be heated to not less than 60°C at least once a day.

If the thermostat control is set above 60°C, hot water storage systems are not at risk of Legionella growth.

If the hot water storage system is partially heated by solar power or heat exchange system (wetbacks or solar heat transfer system), the temperature must be boosted at least 60°C or higher on a daily basis.

Continuous flow systems are not at risk as they do not store water that would allow growth to occur.

Tempering heated water

Temperature reduction is required between the hot water source and the fixtures requiring hot water to be delivered at a lower temperature – this is achieved by installing:

• a tempering valve, or
• a thermostatic mixing valve.

A tempering valve is installed in the hot water line close to the cylinder and has a cold water connection to provide a pre-set hot water temperature at fixtures. Valves are factory pre-set but are able to be adjusted to cater for specific temperature requirements.

As water may be delivered at any temperature to non-personal hygiene fixtures such as sinks and laundries, a tempering valve is not required to these fixtures. However, if a wetback water heater or other uncontrolled heat source is used, tempering the supply to all fixtures is good practice.

Most dishwasher and washing machine manufacturers may require the installation of a tempering valve for warranty purposes where the unit does not heat its own water.

Find out more

• Energy>Water heating