Water
HeatingAbout
Water Heating
In the United States, water
heating accounted for 9.4% of all residential energy consumed in the
in the year 2000. Virtually
every residential unit has a means of heating water for internal
use. Central storage heaters, either electrical or gas-fired,
are the norm. These water
heaters are usually specified by capacity, ranging from 19 to 80
gallons. Size selection
is based on the number of occupants in the house.
In 2000, 9.16 million new units were sold – 53.5% of them
were gas, the 46.5% were electric.
Construction
of a gas-fired water heater is shown below:
The supply of gas is regulated by the
gas valve and thermostat control. The thermostat has a dial to allow
the homeowner to set the water temperature. The valve shuts the burner
on and off as signaled by the thermostat. Above the burner is the
bottom of the tank and an exhaust flue for the escape of combustion
gases. As the burner heats the bottom of the tank, conduction causes
the temperature of the water in the tank to rise. Heat transfer also
takes place between the flue and the water in the tank. Convection
currents within the tank distribute heat throughout the tank. When the
temperature of the water exceeds the set point, the thermostat closes
the gas valve.
Cold water enters the tank from the
bottom as heated water exits from the top. As a safety precaution, a
pressure and temperature relief valve is included on every water
heater.
Also included in the water heater is
a "sacrificial" anode rod. Its purpose is to inhibit rust on
the inside of the tank. Every so often it must be checked and
replaced.
Between the tank and the outer shell
of the water heater is a layer of fiberglass or foam insulation. The
thicker the layer, the more resistant the unit will be to heat loss
from the water in the tank to the air outside the unit. Today's water
heaters are typically R-6.7. In 2004, DOE requirements will mandate
R-20 in all new units sold.
Construction of an electrical unit is
shown below.
Electric water heaters have two
thermostats - one to regulate the temperature at the top of the tank
and one for the bottom of the tank. Connected to each thermostat is an
electrical heating element. The bottom heating element does most of
the work as incoming cold water enters the tank at the bottom. As the
water surrounding the bottom heating element increases in temperature,
it rises to the top of the tank, from where the outgoing hot water is
drawn.
Since no combustion takes place in an
electric water heater, there is no need for a flue or exhaust piping.
R values of 11.5 are typical.
Service requirements
The expected life of a water heater
is 20 years. When a heater fails, it is usually due to leaks in the
tank. If the cost of repairing the unit exceeds the cost of a new
unit, the homeowner usually decides to replace the unit.
Energy Consumption
The amount of energy consumed by the
residential water heater is a function of the hot water usage, the
temperature of the incoming water, the temperature setting, the size
of the tank, and the amount of insulation around the tank.
While few homes meter the actual
gallons of hot water consumed, it is possible to estimate daily or
weekly usage through the applications of hot water throughout the
home. Typical uses and consumption rates of hot water:
Hand washing - 1 gallons per wash
Showering - 2.5 gallons per minute w/ low flow shower head, 5.0
gallons otherwise
Dishwasher - 14 gallons for full cycle, 10 gallons for short cycle
Laundry - 20 gallons for hot water setting, 10-12 gallons for warm
wash, cold rinse
Each water heater sold in the U.S. is
required to have an Energy Guide label. The label indicates the amount
of energy consumed by the unit annually by a typical household. It
also shows a range of energy consumption for all water heaters of
comparable size.
For further comparision, Energy
Factor (EF) is also marked on the Energy Guide label. This measurement
indicates how efficiently a unit converts gas or electricity into hot
water. It takes into account heat losses from the tank itself.
For gas water heaters with an R value
of 6.7, the EF is typically in the mid-50's, meaning that slightly
less than half of the heat value is lost. Most of the heat goes up the
flue - the water in the tank simply cannot absorb the heat from the
combustion gases quickly enough. Never touch the flue while a gas
water heater is operating - you will burn your hand.
Electric water heaters with an R
value of 11.5, typically have EF values in the high 80's.
The table below shows minimum current
and future EF's as required by the US Dept of Energy. In January 2004,
EF's for new gas water heaters sold will increase by five percentage
points. For electric water heater's, EF's will increase by four
points.
Minimum EF's for Water Heaters
Sold in U.S.
Tank
Size
|
Since
April 1991
|
Starting
Jan 2004
|
(Gallons)
|
Gas
|
Electric
|
Gas
|
Electric
|
|
|
|
|
|
20
|
0.58
|
0.90
|
0.63
|
0.94
|
30
|
0.56
|
0.89
|
0.61
|
0.93
|
40
|
0.54
|
0.88
|
0.59
|
0.92
|
50
|
0.53
|
0.86
|
0.58
|
0.90
|
60
|
0.51
|
0.85
|
0.56
|
0.89
|
70
|
0.49
|
0.84
|
0.54
|
0.88
|
80
|
0.47
|
0.82
|
0.52
|
0.86
|
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Copyright © 2003 HEM Technologies, LLC. All rights reserved.
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