Hot water storage cylinders are currently used in about 95% of New Zealand homes. Most have an electric element or gas heater inside a thermally insulated cylinder.

NZS 4305 Energy efficiency – domestic type hot water systems sets the energy efficiency requirements for hot water storage cylinders and, in particular, the:

• maximum standing heat loss (Kw/hr 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 a storage cylinder must be heated to at least 60 ºC daily to remove the risk of microbial contamination in the water.

Note: cylinder specifications for solar water heating differ from those for gas and electric water heating. Also see solar water heating: storage cylinders.

Cylinder size

The hot water storage cylinder needs to be big enough to provide for a household’s peak hot water demand. Significant standing losses occur from both the cylinder and during the transfer to the point of use. For storage cylinders heated by electricity or gas, energy will be wasted if the cylinder is too large as more water will be heated than is needed and standing losses will be greater. Energy may also be wasted if the cylinder is too small, as hot water will run out and householders may increase the water temperature.

Efficient hot water storage cylinders are typically shorter and broader, rather than tall and slim, as this reduces the surface area. Typical cylinders have appropriate energy efficient diameters of 450 mm or 560 mm. Taller cylinders however may be better where heat exchanger coils are to be incorporated into the cylinder – for example on commercially available cylinder includes two heat exchanger coils – one to receive heat from a wetback and the second to provide heat to a heated floor system – the domestic hot water is the heat exchange medium.

The appropriate size depends primarily on the number of people in the household.  Typical hot water usage is in the order of 40 and 60 litres per day per person

Where water usage is likely to be high (e.g. the design incorporates spas or appliances that do not heat their own water) select the next size up.

Reducing the available storage is recommended where off peak electricity rates are not utilised for water heating.

Recommended minimum sizes from www.energysmart.com.au are:

  

Cylinder location

Heat loss through the hot water pipework should be minimised by locating the hot water 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.

Other design factors to consider include:

• provision of sufficient space to locate the cylinder in
• hot water system pressure – low or mains pressure
• location with respect to a wetback or solar panel (For example, the cylinder must be directly above a wetback when using a thermo-siphon effect, or up high in the building for low pressure systems with an open vent pipe. Where a mains pressure cylinder is connected to a wetback, the supply of heat must be via an open vented coil within the cylinder.)
• access to replace the cylinder
• in new designs, grouping spaces where hot water is used (Where an isolated hot water outlet is required, or hot water demand will be low, a continuous flow water heater may be a better option.)
• modern gas storage cylinders are often designed for installation outside the building envelope.

Cylinder insulation

As well as determining the appropriate location and temperature, you can maximise energy efficiency by specifying:

• wrapping of an electric cylinder with additional insulation (if existing insulation is inadequate)
• insulation to hot water pipework.

New electric storage water heaters installed since 2002 must have ‘A’ grade insulation. However, insulation should be added to cylinders that are rated ‘C’ grade or ‘D’ grade, made from copper, or installed prior to 1977.

Available cylinder wraps (primarily used for electric hot water storage cylinders) include:

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

The wrap will add about 50 mm to the diameter of the cylinder.

Water pressure in the cylinder

Hot water in a closed cylinder is pressurised. To avoid the risk of a possible explosion, hot water storage cylinders must have:

• an open vent pipe that terminates above the roofline (for low pressure systems) or a pressure-relief valve (for mains pressure systems)
• a thermostat controlling the water temperature
• an over-temperature cut-out switch.

Specific requirements for gas storage water heaters

To ensure ongoing safety and energy efficiency of gas storage water heaters, designers must allow for:

• adequate ventilation of indoor cylinders into and out of the building space where the cylinder is located
• a flue to remove exhaust gases.

Designers must also advise homeowners of the need for gas storage water heaters to be:

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

For your clients

To help your clients make decisions about water heating options, and to help them find ways of reducing their hot water bills, refer them to www.smarterhomes.org.nz/energy/water-heating and www.smarterhomes.org.nz/energy/solar-water-heating.