ASHRAE Journal - March 2021 - 63

COLUMN RESIDENTIAL BUILDINGS

associated with water heating is
to reduce the amount of hot water
used. Reductions of hot water use
can be achieved by several methods, depending on the end use. In
some cases, providing ﬁxtures with
reduced ﬂow rates is effective-in
other cases it is not. In some cases,
reducing the total volume of hot
water for batch-ﬁll operations (e.g.,
washing machines) is beneﬁcial. In
other cases this reduction is not possible. In some cases, changing the
habits and customs of end users can
save water and energy.
Some end-use ﬁxtures and appliances serve end-use needs by providing ﬂow rates that are adequate
to achieve a desired end goal, such as
the invigorating " feel " of a shower or
a " scrubbing " action when washing
dishes. With these types of ﬁxtures
and end uses, the total volume of
hot water use is of secondary importance compared to the " velocity
impact " of the ﬂow. For these types
of needs, use of lower ﬂow ﬁxtures
that still achieve desired " velocity
impact " save water and energy while
not sacriﬁcing usability.
Other types of end-use ﬁxtures and
appliances are more dependent on
volume of hot water used to achieve
desired end uses than they are on ﬂow
rate. These types of end uses typically involve batch-ﬁll operations.
Examples include ﬁlling bathtubs,
sinks or cooking pots and using
dishwashers and clothes washers.
Typically no water or energy efﬁciency
savings are associated with providing
reduced ﬂow rates to batch-ﬁll operations, and often a decrease in user satisfaction occurs if the user must wait
longer for the batch ﬁll to complete.
If, however, end-use appliances
can be designed and operated to

function well while using reduced
amounts of hot water in their batch
ﬁll, water and energy savings can
be achieved. An example of such
savings is the reduced water use of
dishwashers and clothes washers in
response to efﬁciency standards.6

TABLE 1 Average daily hot water use per household.

User Behavior
Another important consideration
is end user behavior. Hot water use
behaviors vary with both personal
preferences and cultural norms. An
example of a behavior that impacts
hot water consumption is the use of
multiple shower heads in a shower
stall to get a more " invigorating "
shower. Another frequently seen
behavior is for users to turn on the
hot water (e.g., at a sink or shower)
and walk away while waiting for hot
water to arrive. This behavior often
results in hot water having already
reached the ﬁxture before the person returns to use the hot water,
" dumping " hot water directly down
the drain without being used.
A common " ﬁx " for the hot water
delivery delay problem is equipping
piping systems with pumps that circulate hot water through the piping
to keep hot water immediately available at ﬁxtures. Unfortunately, while
recirculation loops save water, they
can consume signiﬁcant amounts of
energy by keeping water in the loop
hot at all times.

Distributing Hot Water
Residential water heating systems
most commonly use a central water
heater with storage capacity. The
hot water needs to be transported to
the end-use ﬁxtures, and heat is lost
along the way. While ensuring enough
capacity to deliver the required
amount of hot water is a key design

END USE

PERCENT

GALLONS

Shower

41.1

17.8

Faucet

35.6

15.4

Clothes Washer

10.2

4.4

Bathtub

6.0

2.6

Dishwasher

5.1

2.2

Other

2.1

0.9

Total

100.0

43.3

issue with distribution systems,
opportunities exist to minimize the
amount of heat lost in transmission.
Reducing distribution system
energy losses can be accomplished
in several ways. First, the layout of
the plumbing system should minimize the volume of water between
the water heater and the end uses.
This objective can be achieved, for
example, by locating ﬁxtures closer
to water heaters. Pipe sizing should
also be appropriate. Many codes
require large-diameter pipes based
on outdated assumptions for water
ﬂows prior to the introduction of
modern ﬁxtures. However, it is likely
that smaller-diameter pipes can be
used to achieve desired ﬂow rates.
Oversizing hot water pipe diameters
is a common error, in the mistaken
belief that larger diameter pipes allow
hot water to arrive quicker, when the
opposite is true because of the greater
volume of water in those pipes.
Smaller pipe diameters and shorter
piping runs are being implemented
as parts of so-called " home-run " piping systems, in which a manifold near
the water heater distributes hot water
in smaller-diameter dedicated lines
to end uses instead of using a trunk
and branch system.
Another hot water system design
approach that can minimize wait
time is using more than one water
heater. Some types of water heaters

MARCH 2021

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ASHRAE Journal - March 2021

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