Energy

The energy efficiency of a water heating system depends partly on the layout and specification of the hot water reticulation within the house. The significant influences are:

• hot water supply temperature and flow rate, and
• pipework heat losses.

Hot water supply temperature

The hot water temperature in the storage system must regularly be above 60ºC to prevent growth of the legionella bacteria. Some people set the temperature higher again to reduce the risk of running out of stored hot water. As hot water temperature increases, energy efficiency decreases.

In addition, the temperature delivered to any tap used for personal hygiene (such as washing or bathing) must be reduced by a tempering valve to no higher than 55ºC to prevent scalding (65ºC may be delivered to domestic kitchen sinks and laundries).

Hot water supply flow rate

A higher flow rate means more hot water is consumed, which means more energy is used. For homes, flow rates don’t need to be higher than:

• 0.3 litres per second or 18 litres per minute for kitchen sinks and baths
• 0.1 litres per second or 6 litres per minute for handbasins and showers
• 0.2 litres per second or 12 litres per minute for washing machines
• 0.2 litres per second or 12 litres per minute for laundry sinks.

Designers can check the installed flow rate by running the hot water into a bucket for one minute, then measuring the number of litres in the bucket. Flow rates can be reduced by specifying low-flow showerheads, and spray taps in kitchen and laundry sinks. Specifying single lever mixer taps will also save hot water.

Hot water pipework

Heat is lost in hot water pipes while the hot water is flowing, and while it is sitting in the pipe between uses. To minimise the heat loss in hot water pipes:

• limit pipe runs by locating the water heater or hot water storage in a warm area and as close as practical to the place where the majority of hot water is used (see storage cylinders for more)
• specify water pressure that gives higher water velocities in the pipes up to a maximum of 3.0 m/s
• wrap pipes with insulation.

Pipework insulation

Insulation on pipework should have thermal resistance (R-value) not less than 0.3 m².C/W (see AS/NZS 3500 Part 4 Table 8.1). Insulation materials include:

• closed cell foam with 12 mm nominal thickness
• preformed fibreglass with 12 mm nominal thickness.

Insulation should be installed on all pipework that carries hot or warm water. This includes the portion of pipe from the cylinder to the pressure relief valve or to 300 mm above the maximum standing water level in the vent pipe.

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.