ASHRAE Journal - March 2021 - 62

RESIDENTIAL BUILDINGS
Column

How Residential Water
Heating is Changing
BY WILLIAM M. HEALY, PH.D., MEMBER ASHRAE; CARL C. HILLER, PH.D., P.E., FELLOW/LIFE MEMBER ASHRAE; JAMES LUTZ, P.E., MEMBER ASHRAE

Water heating system energy use in U.S. residences is second only to space conditioning.1 Recent advances are changing the way water heating service is provided. This
column describes hot water demand in residences, discusses new options for providing hot water and explains ASHRAE's role in the water heating industry.
Hot Water Demands
Key considerations for a building's water heating
system are obtaining the needed amount of hot water
rapidly and efficiently, while maintaining safe, reliable, long-life operation at reasonable installed and
operating costs. A key phrase in the previous sentence
is " water heating system, " meaning not only the water
heater, but also the hot water end uses, the distribution
system and user behaviors. All four play important roles
in delivering expected hot water service and providing
opportunities for performance improvements.
On average, homes in North America use approximately 164 L (43 gallons) of hot water per day, but that
number is highly variable depending on the number of
occupants, types of appliances present, day of the week
and occupant behavior. The largest hot water end uses
in residences are showers, faucets, clothes washers,
dishwashers and bathtubs. Table 1 provides average daily
hot water use per household from a recent field study.2
For human contact applications, hot water is mixed
with cold water to provide the desired temperature,
typically between 38°C (100°F) and 43°C (110°F).
Dishwashers and clothes washers tend to use higher
temperature water (49°C [120°F])-and even higher
when required to sanitize (60°C [140°F]). Some
62

ASHRAE JOURNAL

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MARCH 2021

appliances incorporate separate booster heaters to
achieve these higher temperatures, so hot water is
often provided to end uses at a temperature of approximately 49°C (120°F). Hot water temperatures are an
oft-debated topic. Water should not emerge from the
faucet so hot it causes scalding, but it should be heated
to a sufficiently high level to minimize growth of pathogens. This issue is worthy of a complete column, and
interested readers are referred to ASHRAE Standard
188-2018,3 ASHRAE Guideline 12-20204 or a recent publication by the National Academy of Sciences.5
Water temperature entering homes varies by geographic region and time of year, but a year's average value
tends to be near the average annual air temperature.
Thus, energy is needed to raise water temperature from
approximately 16°C (60°F) to 49°C (120°F) or more. Due
to water's high heat capacity, large amounts of energy are
needed to achieve this temperature increase.

End-Use Fixtures and Appliances
One key way to decrease energy consumption
William M. Healy, Ph.D., is a group leader at the National Institute of Standards and
Technology in Gaithersburg, Md. Carl C. Hiller, Ph.D., P.E., is president of Applied Energy
Technology, in Davis, Calif. James Lutz, P.E., is president of Hot Water Research, in Oakland,
Calif. All are former chairs of TC 6.6, Service Water Heating Systems. Healy is a member of
the Residential Buildings Committee. Hiller is the TC 6.6 Handbook Subcommittee chair.


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ASHRAE Journal - March 2021

Table of Contents for the Digital Edition of ASHRAE Journal - March 2021

Contents
ASHRAE Journal - March 2021 - Intro
ASHRAE Journal - March 2021 - Cover1
ASHRAE Journal - March 2021 - Cover2
ASHRAE Journal - March 2021 - 1
ASHRAE Journal - March 2021 - Contents
ASHRAE Journal - March 2021 - 3
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