ASHRAE Journal - March 2021 - 64

COLUMN RESIDENTIAL BUILDINGS

are optimized for installation near points of use, such
as small storage water heaters (e.g., 8 L to 22 L [2 gallons
to 6 gallons]) and tankless water heaters designed to be
located under sinks. A combination of multiple large and
small water heaters in a single residence is becoming more
commonplace as designers begin to recognize potential
performance and cost benefits. These actions decrease the
amount of hot water sitting in pipes between uses, minimizing heat loss to the surroundings. These energy savings
often exceed the added heat loss from additional water
heaters installed near the end use.7 These steps have the
added benefit of reducing wait time for hot water and of
reducing initial costs of piping. Finally, insulating the distribution system will cut down on heat losses.

Making Hot Water

That brings us to the final part of the system, the
water heater. For many years, consumers primarily had
one of two options: a storage tank fired by fossil fuel
(natural gas, propane or oil) or one heated with electric
resistance elements. Approximately half the U.S. uses
electricity to heat water, while the other half uses combustion water heaters.8 Recent years, however, have
brought a wider range of water heating options.
On the electric side, heat pump water heaters (HPWH)
have emerged as a viable method of heating water. These
units, which use a refrigeration cycle to move heat from
the surroundings or outdoors to heat the water, achieve
significantly higher energy efficiencies than resistance
water heaters. The efficiency rating metric in the U.S.
and Canada, the uniform energy factor (UEF), measures
the efficiency under a standard draw pattern and set of
environmental conditions.
The best 189 L (50 gallon) electric resistance water
heater achieves a UEF of 0.93, whereas a HPWH is listed
with a UEF as high as 3.55.9 Hence, the HPWH will tend
to use approximately one-quarter as much energy as
the resistance water heater, albeit with the penalty of a
higher first cost. In the U.S., electric water heaters that
are sold with storage volumes above 208 L (55 gallons)
are required to have UEFs greater than 1.86, which can
currently be met only through the use of HPWHs.*9
For gas water heaters, several technologies have
emerged that reduce energy consumption. First, condensing water heaters offer higher combustion efficiency.
While standard natural draft (atmospheric) combustion

water heaters with a storage volume of 189 L (50 gallons)
achieve UEFs of approximately 0.60, condensing water
heaters can achieve UEFs approaching 0.90.
A technology for the future is the gas-fired heat pump
water heater. While not yet commercially available,
significant research and development has occurred in
recent years. Coefficients of performance in field tests
have exceeded 1.5,10 which would decrease energy use
by approximately 60% compared to conventional atmospherically vented products and by about 40% compared
to condensing water heaters.
For both electric and gas, another increasingly popular option has been tankless water heaters. While storage water heaters maintain a volume of hot water for
use, tankless water heaters only heat water when it is
needed, passing the incoming mains water through
heat exchangers. Several advantages exist for tankless
water heaters. First, they take up less space than storage
water heaters of equal capability, which allows them to
be more easily located closer to end uses. From an efficiency viewpoint these units do not lose heat continuously to the environment (storage losses) because they
are only kept hot when water is flowing.
Tankless water heaters require much larger heat input
rates compared to storage water heaters since there
is no stored energy waiting to be delivered. For fossil fuel-fired units, firing rates tend to be above 29 kW
(100,000 Btu/h), whereas gas storage water heaters typically fire at about 12 kW (40,000 Btu/h). At high enough
flow rates, required heating rates could be so high to
necessitate changes to inlet gas piping or exhaust flues,
an issue that would increase labor costs for installation.
Electrically powered tankless water heaters are also
available. Versions are available from those with heating rates large enough to serve " whole-house " hot
water loads, down to small units intended for single
sink applications. For whole-house applications with
relatively cold incoming water at the main, a high current draw may be required to raise the temperature to
a usable level. These units, however, could be installed
in a distributed manner under sinks to provide more
localized heating, reducing water and energy waste due
to lengthy hot water distribution piping. In general,
the first costs of tankless units tend to be higher than
conventional storage water heaters of similar delivery
capability.

*Minimum efficiency standards for water heaters are based upon storage volume and delivery capacity as measured by the first hour rating.
64

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ASHRAE Journal - March 2021

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