Water Heaters - 7x7x7 | Design Energy Water

FACTORS IN CONSIDERING THE SUITABILITY OF DEMAND -TYPE WATER HEATERS
INTRODUCTION
Heating water accounts for up to 30 percent of the average home’s
energy budget. Conventional domestic hot water (DHW) heaters rely
on storage tanks to meet service demands, but maintaining the reserve
hot water supply can contribute to standby energy losses. Generally the
standby losses are significantly lower for larger families that use a lot of
hot water (90 gallons per day or more) and somewhat higher for smaller
families that use less hot water (50 gallons or less).
Gas-fired tankless DHW heaters (also known as demand-type DHW
heaters) have the potential to save household energy costs because
they supply hot water very rapidly by use of high-powered gas burners
which transfer heat to water running through a heat exchanger. Ondemand supply of hot water eliminates standby losses because there
are no hot water storage tanks. Unfortunately, according to an article
by Consumer Reports, the average annual energy savings for tankless
DHW heaters for typical households may not payback on a life-cycle
basis due to their high initial equipment and installation costs.
• Tankless water heaters sometimes give inconsistent water
temperatures, such as the so called “cold water sandwich”
which occurs in pipes between “old” and “new” hot water cycles.
Another shortcoming is that very small hot water demand may fail
to activate the gas burner.
• Tankless water heaters use electric controls, as such they cannot
provide hot water during power outages.
WHAT ARE HYBRID TANKLESS-STORAGE WATER HEATERS?
Tankless DHW electric water heaters are not evaluated here
because they cannot supply hot water fast enough to replace
conventional water heaters in a typical residential scenario if
groundwater temperature is low. Therefore all reference to
tankless DHW heaters below are the gas-fired burner type.
This is basically a demand-type water heater (made by A.O. Smith
and others) that features a relatively small “buffer” storage tank. The
buffer tank eliminates two annoying performance issues associated
with tankless DHW heaters. The buffer tank always has some stored
hot water to deliver right away so the user never experiences the
dreaded “cold water sandwich” while in the shower. The buffer tank
also eliminates the aforementioned problem of burner activation
failure for small water demand. This unit reaches an impressive 90%
thermal efficiency because it is designed to capture waste heat from a
secondary heat exchanger from heated flue gas which is then routed
back to the buffer tank. However the remaining aforementioned pros
and cons relative to tankless DHW heaters would also apply to the
hybrid tankless-storage DHW heaters.
PROS AND CONS OF DEMAND − TYPE (TANKLESS)
WATER HEATERS
ARE TANKLESS WATER HEATERS MORE SUITABLE FOR
INSTITUTIONAL AND COMMERCIAL APPLICATIONS?
• Tankless water heaters do not have standby energy losses and
are 22 percent more efficient than storage water heaters (on
average) as long as there is electronic ignition rather than a
standing pilot light.
• Tankless water heaters average life expectancy is 20 years
compared to 15 years for gas-fired storage models.
Many schools have domestic hot water requirements that are distinct
from typical households. Households can have scenarios that involve
multiple, simultaneous hot water demand requirements (shower,
dishwasher, clothes washer) and where the hot water appliances and
devices are in close proximity. This scenario usually favors a whole
house conventional DHW heater with a storage tank to ensure that a
relatively high volume of hot water is supplied without interruption.
• Tankless water heaters cost substantially more to purchase and
install compared to gas-fired storage water heaters.
• Tankless water heaters cannot supply enough hot water for
multiple high-volume uses at once.
DSA 7x7x7 DESIGN ENERGY WATER SUSTAINABILITY ARTICLE WATER HEATERS
In contrast, consider a small elementary school which features several
small classroom buildings with hot and cold water supplied utility sinks,
and separate portable restroom buildings with hot and cold water
faucets. Assume that the classrooms and restrooms have little space
for a dedicated conventional water heater. Here, the installation of
dedicated tankless DHW heaters might prove a viable solution given
the following situation. 1) Space is too restricted in the portables for
bulky hot water tanks, 2) Hot water demand is low volume and from a
single source and 3) Hot water demand is widely separated in several
buildings. Here the installation of several very compact point-of-use
tankless DHW heaters could prove a more feasible and cost-effective
design solution compared to piping hot water to several buildings from
a remote central DHW heater.
REFERENCES & RESOURCES
Consumer Reports www.consumerreports.org
“Tankless Water Heaters,They’re Efficient, But Not Necessarily Economical”
U.S. Department of Energy www.energy.gov
“Tankless or Demand-Type Water Heaters”
GreenSpec www.greenspec.buildinggreen.com
“A.O. Smith’s Innovative Hybrid Tankless-Storage Water Heater”
SUMMARY
Consumers should carefully consider installing tankless DHW heaters.
While the later can offer better energy efficiency without standby
energy losses, they do not perform as well as conventional systems
with storage tanks if multiple hot water demands are needed at the
same time. For most households, either the conventional water heater
or the hybrid tankless-storage DHW heater is probably a better choice.
The use of tankless DHW heaters may be preferable in settings that are
remote, have space restrictions and have smaller point-of-source hot
water demands.
DSA 7x7x7 DESIGN ENERGY WATER SUSTAINABILITY ARTICLE WATER HEATERS
Download PDF
Similar pages