Water use, sustainability, and efficiency by choosing quality systems and materials, and providing environmentally friendly solutions.
Hot water supply
Hot water supply must be adequate to meet building users’ needs while also keeping them safe. It should also support efficient use of both energy and water.
On this page:
- Code requirements
- Controlling temperature
- Controlling pressure
- Water heating options
- Other considerations
In designing a water heating system, the key decisions will include the source of energy for water heating, whether to use a storage cylinder or continuous flow system, system layout, and system capacity (including delivery rate, recovery rate, actual and potential number of users, type and number of fixtures within a household).
The system must be designed to meet safety requirements, which largely concern controlling temperature and pressure to ensure there is minimal risk of scalding or of a storage cylinder exploding.
A well-designed system will also minimise energy and water use, for example by using an efficient heating source, ensuring the pipe runs are relatively short, and by using efficient fixtures and appliances.
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, which can cause serious health problems
- protect users from scalding during personal hygiene activities
- be safe from explosion
- be efficient and avoid energy wastage.
NZS 4305:1996 Energy efficiency – domestic type hot water systems sets the energy efficiency requirements for hot water storage cylinders including:
- maximum standing heat loss (kWh per day) for electric hot water cylinders of different sizes
- maximum gas consumption rate and minimum thermal efficiency for gas hot water cylinders.
Water heated to more than 50°C can cause serious burns in less than a minute. This creates risk, particularly for children (who have sensitive skin) and the elderly (who have slower reaction times).
Under Acceptable Solution G12/AS1, in most buildings hot water delivered to sanitary fixtures such as basins, baths and showers should not exceed 55°C (lower temperatures are required for some buildings such as hospitals, schools, and care facilities).
Hot water may be delivered to domestic kitchen sinks and laundries at higher temperatures – generally around 55°C to 65°C.
If a storage cylinder is used, the water must be heated to over 60°C at least once a day to prevent the growth of Legionella bacteria. The water must then be tempered to reduce the temperature before it is delivered to outlets. See storage cylinders for more detail.
Continuous flow systems are not at risk as they do not store water that would allow growth to occur.
In buildings with mains pressure water supply, a system of valves and controls will be required to regulate water pressure and temperature. Many manufacturers and importers of plumbing valves, tapware and appliances require 500kPa pressure maximums for warranty purposes, with the limiting valves protected by line strainers. See Controlling pressure in storage cylinders and Valves and controls for more detail.
Water heating options
Probably the key decisions to be made are which energy source to use, and whether to use a storage system or a continuous flow one.
In continuous flow systems, water can be heated using electricity or gas. In storage systems, the water can be heated using electricity, gas, solar energy or a wetback.
Each energy source has its advantages and disadvantages, as do storage and continuous supply systems. In general, solar, heat pump and wetback systems are more energy efficient than traditional electric and gas systems. Continuous flow systems can be efficiently used in some circumstances, such as to boost a solar system, or to feed an outlet that is some distance from the main hot water supply.
See water heating in the energy section of this site for more detail on these options.
The government’s Energywise website has an online tool here to help homeowners and their advisors select a water heating system.
As well as specifying an efficient energy source, water and energy efficiency can be enhanced by:
- designing the system to minimise pipe runs – for example, by locating the storage cylinder close to the kitchen, laundry and bathroom
- specifying low-flow fixtures
- specifying appliances that use water and energy efficiently
- installing a continuous-flow hot water system, particularly for remote outlets, so that hot water does not need to be stored
- insulating the pipes to reduce heating costs (but this has less impact than reducing pipe lengths)
Updated: 20 May 2020