Appendix B to RIS 2009-03 - Australian Building Codes Board

Appendix B to RIS 2009-03 - Australian Building Codes Board

The efficiency of a water heater can be defined as Useful Energy (UE) which it transfers into

hot water divided by Delivered Energy (DE) – ie electricity, gas, LPG or other fuel.

For conventional gas and electric water heaters, UE/DE is always less than 1, but for heat

pump and solar types it is higher than 1.

The efficiency range for conventional water heaters is fairly narrow, across both types and

water heater drawoffs (Table 9). The range for heat pumps is somewhat wider. The

efficiency range for solar water heaters is by far the widest, and is sensitive to many factors

including delivery, drawoff, collector efficiency etc. For the same model of solar water

heater, the lower the hot water demand the higher the efficiency. For conventional water

heaters and heat pumps, efficiency increases gradually with delivery, all else being equal. For

solar water heaters on the other hand, efficiency increases steeply as delivery declines, and

falls as delivery increases.

Table 9 Indicative range of modelled water heater efficiencies


Highest Efficiency Lowest Efficiency (b) Range


Electric (off peak) 0.70 0.90 0.20

Electric (continuous) 0.78 0.90 0.12

Gas IWH 0.60 0.75 0.15

Gas SWH 0.55 0.78 0.23

Heat pump 2.2 3.5 1.3

Solar-electric (evacuated tubes) 6.0 1.5 4.5

Solar-gas (flat plate, small) 2.7 1.1 1.6

Solar-gas (flat plate, medium) 7.0 1.5 5.5

Solar-electric (flat plate) 10.0 1.7 8.3

(a) See Appendix 4. Efficiency varies with delivery and other factors.

Figure 10 illustrates the emissions intensity, for the 40 MJ/peak day task, in the four States

and Zones which together account for about 85% of new construction. The differences in the

emissions for each type of water heater reflect the efficiency of the water heater, the

variations in the energy requirement for the same water heating task from zone to zone, and

differences in the projected CPRS-5 greenhouse intensities (which vary from State to State)

and the default factors proposed for the BCA (which are the same in all States, so the intensity

of water heaters only vary with their Zone).

Electricity-related emissions are also included for those gas and solar-gas water heaters which

use electricity for standby energy, combustion fans or pumps. It is apparent that in all the

cases shown, conventional electric water heaters give by far the highest emissions, but the

ranking and relative differences between the other options depends on zone and State.

In 2007 the Australian Building Codes Board and the Australian Greenhouse Office

commissioned a study to develop options for specifying domestic water heaters in the BCA.

The report of the study (GWA 2007) recommended an assessment method for calculating the

greenhouse gas emissions associated with water heater operation, and discussed a range of

possible emissions intensity factors, especially for electricity. The method (in Appendix 1 of

the present RIS) is proposed as the BCA method.

Although the most cost-effective benchmark level (expressed as a maximum g CO 2 -e/MJ

added to hot water) was not determined, it was clear that if a greenhouse benchmark had any

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