Review of Proposed Measures to
Reduce ‘Water Wastage’ in
Residential Heated Water Supply
Prepared for the
Australian Building Codes Board
GEORGE WILKENFELD AND ASSOCIATES Pty Ltd
ENERGY AND WATER POLICY AND PLANNING CONSULTANTS
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.
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
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
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)
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
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
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