Energy

Hot water storage systems can be used with energy-efficient heating sources such as solar, air-to-water heat pumps or they can use gas or electricity as the primary energy source. A disadvantage is that they can run out of hot water.

Find out about:

• statutory requirements
• cylinder size
• cylinder location
• cylinder insulation
• specific requirements for gas storage water heaters.

Statutory requirements

NZS 4305 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.

Under NZBC G12 Water supplies, a storage cylinder must be heated to at least 60ºC daily to remove the risk of microbial contamination in the water.

Note: Cylinder specifications and protection from Legionella bacteria for solar water heating differ from those for gas and electric water heating.

Cylinder size

The hot water storage cylinder must be large enough to provide for a household’s peak hot water demand, but more water will be heated than needed if the cylinder is too large. The appropriate size depends primarily on the number of people in the household. Typical hot water usage is in the order of 40–60 litres per day per person.

Significant standing losses occur from the cylinder and during the transfer to the point of use. For greater efficiency, hot water storage cylinders should be short and broad rather than tall and slim, as this reduces the surface area. Cylinder diameter is typically 450 mm or 560 mm.

Taller cylinders may be better where heat is exchanged to and from other sources (such as a solar collector or wetback) to the cylinder. For example, a wetback may draw cold water from the bottom of the cylinder and return it as hot water to the top of the cylinder. A taller cylinder will allow a greater temperature difference to be maintained and therefore improves the effectiveness of the wetback.

Cylinder location

Minimise heat loss through the hot water pipes by locating the cylinder as close as possible to the outlets where the majority of hot water is used.

The cylinder will lose more heat in a cold location (e.g. when located outside the insulated building envelope) than a warm one (such as in the middle of the house or in a well-insulated space). Locating the cylinder in a cupboard will help to retain heat. Modern gas storage cylinders are often designed for installation outside the building envelope.

Other design factors to consider include:

• hot water system pressure – low or mains pressure
• provision of sufficient space for the cylinder
• access to replace the cylinder
• plan spaces where hot water will be used to be in close proximity – if an isolated hot water outlet is required, or hot water demand will be low, a continuous flow water heater may be a better option
• placement in relation to solar collectors or solid fuel burners where these are used for water heating.

Cylinder insulation

Maximise energy efficiency by:

• wrapping the cylinder with additional insulation
• insulating the hot water pipework.

Electric storage water heaters installed since 2002 must have ‘A’ grade insulation. These have a layer of insulation of around 50mm between the water jacket and the outside of the cylinder. Cylinders that were installed prior should have insulation in the form of a cylinder wrap added. Any cylinders that are warm to the touch would benefit from having a cylinder wrap added.

Available cylinder wraps include:

• wool with cotton exterior
• wool with foil exterior
• fibreglass with foil exterior.

Specific requirements for gas storage water heaters

Gas storage water heaters must have:

• adequate ventilation of the cylinder
• a flue to remove exhaust gases.

They must be:

• serviced annually
• flushed out regularly to remove water sediment at the bottom of the cylinder
• checked to ensure that vents are not blocked.

More information

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