in Residential Heated Water Supply Systems - Australian Building ...

in Residential Heated Water Supply Systems - Australian Building ...


Review of Proposed Measures to

Reduce ‘Water Wastage’ in

Residential Heated Water Supply


Prepared for the

Australian Building Codes Board

March 2012



POB 934 Newtown NSW 2042 Australia

Tel/Fax Sydney (+61 2) 9565 2041

Please note that the findings of this report do not necessarily

reflect the views of the ABCB or its committees.

Executive Summary

This report establishes typical water and energy losses that occur in the water heating

installations of new (and upgraded) buildings, from the interaction of the plumbing

layout and user behaviour. The report also quantifies the impacts of several options to

reduce both water and energy losses, including:

• Limiting the volume of the water in hot water supply pipes;

• Installing a secondary water heater to serve supply points that exceed this limit;


• Providing a means of recirculating or storing sub-temperature heated water.

The findings are based on an initial ‘efficient’ layout, which minimises pipe lengths

and locates water heaters as close as possible to the main draw, in line with AS/NZS

3500 and with general plumbing contractor practice. However, somewhat larger pipe

diameters are used in modelling than the minimum preferred sizes in AS/NZS 3500.

This means that the modelled losses are probably in the mid range of what actually

occurs. Actual plumbing layouts could lead to higher losses (if their layout is less

efficient) or lower losses, if they use minimum preferred (or even smaller) pipe sizes.

The plumbing layout is only one factor in determining losses. User behaviour also

needs to be modelled. This is subject to significant uncertainty since there is very little

available survey data on the frequency, duration and spacing of hot water draws,

which all have a major bearing on the losses. Different assumptions about user

behaviour will produce different loss estimates, even for the same number of

occupants in the same house with the same plumbing layout. The report documents

the modelling assumptions, and tests the sensitivity of the results to changes in the

key parameters.

The estimated water losses range from about 9% to 16% of the total water supplied

for indoor uses, and the energy losses range from about 13% to 29% of the energy

supplied by the water heater. The absolute and percentage losses vary somewhat

according to climate, mainly due to variations in assumed cold water supply

temperatures. The loss percentages are relatively insensitive to the number of

occupants; the greatest variation is between house designs.

The total economic costs of the energy and water losses from hot water use in new

houses are calculated using the same criteria as in the Regulation Impact Statement

Energy Efficiency Requirements of the Building Code of Australia (ABCB 2009). It is

assumed that the lifetime of the plumbing system is 40 years, and the net present

value (NPV) of energy and water losses over that period is calculated at a discount

rate of 7%.

The weighted average NPV of losses related to a 40 year period of hot water

‘wastage’ is estimated at $1,584 per house for houses built in Australia in 2012

(Figure S1). About 39% of this is energy cost, 52% is the cost of heated water

discarded while hot water dead runs are cleared, and the rest is the cost of mixed-in

cold water lost while hot water dead runs are cleared. Average costs vary by State and


Territory according to both quantity of water and energy lost, and local energy and

water tariffs.

The highest average costs are for houses built in Victoria (NPV $2,067) and the

lowest average costs are for the Northern Territory ($892). Taking into account all

houses likely to be built in 2012, the total NPV of the wastage costs to their occupants

over a 40 year period is projected to be about $176 million (Table S1).

Table S1 Average NPV of 40 year water and energy charges to cover hot water

losses, houses built 2012 ($M)

NPV Water


NPV Energy


Total NPV



of new


NSW $ 26.7 $ 20.5 $ 47.2 26.6%

Vic. $ 35.5 $ 15.8 $ 51.3 29.9%

Qld $ 32.0 $ 14.1 $ 46.1 26.5%

SA $ 4.0 $ 2.8 $ 6.8 3.8%

WA $ 7.5 $ 12.3 $ 19.7 10.5%

Tas. $ 0.4 $ 0.9 $ 1.3 0.7%

NT $ 0.6 $ 0.4 $ 0.9 0.5%

ACT $ 1.5 $ 1.1 $ 2.6 1.5%

Aust. (7% discount) $ 108.1 $ 68.0 $ 176.1100.0%

Aust. (3% discount) $ 195.83 $ 123.5 $ 319.3

Aust. (11% discount) $ 70.40 $ 44.0 $ 114.4

Figure S1 Average NPV of 40 year water and energy charges to cover hot water

losses, houses built 2012


Net Present Value of Losses over 40 years





Cold water

Heated water



NSW Vic Qld SA WA Tas NT ACT Aust


The NPV of the total water and energy losses sets the limit on the level of lossreducing

investment or expenditure that would be cost effective. For example, it

would be cost effective to avoid a NPV cost of $1,584 through:

• A once only capital cost of $1,584 (e.g. a more costly plumbing layout); or

• A recurrent annual expenditure of $119 over a 40 year period (e.g. the additional

energy required to operate a circulation pump); or

• A capital expenditure of $396 every 10 years (e.g. replacement of a circulation

pump or a second water heater).

There are several loss-reducing design and equipment options available at the time of

first construction or substantial renovation. Some could also be installed at other

points during the building’s life, but at substantially higher cost.

However, no measure can be guaranteed to be effective and cost-effective in every

instance because energy and water losses in use depend largely on the number of

occupants and how their hot water using behaviour interacts with the hardware of the

house. Of the house designs analysed, all have multiple bathrooms and toilets with

hand basins. Decisions such as whether to use showers or baths, and whether residents

group their hot water use or space it out over the day, will influence losses and hence

cost savings.

These uncertainties mean that the monetary benefits of particular measures are highly

uncertain. This makes it difficult to advocate a purely regulatory approach to

measures which reduce water and energy loss.

The measures most likely to reduce lifetime losses cost effectively are smaller

diameter pipes (assuming they are adequate for satisfactory flows under the pressure

conditions where the house is built) and hot water recirculators or thermal diverters.

There also appears to be considerable benefits in developing and promoting mixer

valves where the neutral or central position calls for cold only rather than mixed

water, as demonstrated at present.

It is not clear that a second water heater is cost effective in narrow monetary terms

even under the most favourable conditions – where a long hot pipe run is eliminated,

in a state where energy and water costs are relatively high. However, benefits to

occupants in terms of shorter waiting times for hot water have not been given a

monetary value in any of the options. In fact, this consideration is likely to be of

greater importance to the actual decision makers (owner, builder and/or plumber).The

interests of plumbing contractors and occupants are likely to further coincide in that if

the contractor minimises material use, pipe runs and volumes, dead run volumes are

also likely to be lower.

Insulation of hot supply pipes appears to be a less important factor in water loss

energy than might be thought, although it remains a major factor in determining water

heater standing heat loss energy through conduction.

Even without regulation there may be scope for better information and training for

builders and plumbing contractors, as well as the public, on the benefits of water loss

reducing design elements and measures. As the optimum approach to minimising


waste will depend on the particular plan and layout, it would be useful to develop and

publicise methods of assessment in a form that building and plumbing contractors can

conveniently use.


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