Wasting Hot Water Wastes Energy - Con Edison

CON EDISON WEB-BASED MIDDLE SCHOOL ACTIVITY
Wasting Hot Water
Wastes Energy
Overview
In this activity, your students will consider the cost of energy in water heating, an area
they may not realize accounts for a great deal of energy consumption. Wasting hot water
does not just waste water, it wastes energy and money as well. They will also consider
the impact of scale. While a few pennies or a touch of pollution here and there may not
seem like very much, they add up quickly when viewed from the perspective of societal
use over time.
After an initial class discussion, students will complete the activity at home with the
help of their families. They will measure the flow rates of faucets, showers, and baths to
determine the amount of hot water they are using, and then they will calculate the
approximate cost of the energy needed to heat that water. You should give students more
than one night to complete the activity out of consideration for other family obligations.
When they bring their data to the classroom, you will oversee follow-up calculations
and discussions.
Objectives
By completing this activity, students will:
• Understand that heating water requires energy.
• Understand the relationship between hot water use and energy use.
• Understand that Btus (British Thermal Units) and calories are measures of
the energy required to raise the temperature of water.
• Understand the roles of insulation and water conservation in saving energy.
• Understand that inefficiencies in heaters and furnaces increase the consumption
of fuel.
• Realize that saving water and energy also saves money.
Time Requirements
15 minutes to assign and discuss the take-home activity
Take-home component for students
30 minutes to time and measure the flow rate of the bath and shower
30 minutes for completing the worksheet calculations
Classroom Follow-up
30 – 60 minutes
1.

Materials
• Watch with second hand
• Gallon jug with the top opening enlarged
• Student Worksheet
Procedure
Initial Discussion
1. Ask your students to think of the costs related to bathing.
The cost of water, sewage, water heating, piping infrastructure, and the cost
of washing and drying linens.
2. Ask students how they could calculate the cost of the hot water used for bathing.
The energy required to heat water comes from electricity, natural gas, or fuel
oil. Their energy content is measured in Btu’s (British Thermal Units) or calories.
A Btu is the amount of energy needed to raise the temperature of one
pound of water by one degree Fahrenheit, and a calorie is the amount of energy
needed to raise the temperature of one gram of water by one degree Celsius. If
they knew the energy content of the fuel, the temperature rise of the water (such
as 50˚F incoming water and 100˚F shower water), and the efficiency of the
water heating system, they could calculate the exact cost of the water heating.
For the purpose of this activity, water heating costs about 1¢ per gallon of total
flow (hot water and cold water combined).
Assign the Take-home Activity
3.Demonstrate to your students how to measure the flow rate of a bath or shower,
and explain that their answer will be expressed in terms of gallons per minute
(GPM).
a. Enlarge the opening of a plastic one-gallon jug (such as a milk container) with
scissors so you can fill it from a showerhead without
spilling too much. (Students should ask an adult for
help.) Demonstrate how to fill the jug by filling it
from a sink in your school. (You may want to compare
the flow rates of a low-flow sink, such as one in a
rest room, with a high- flow sink, such as one in the
janitor's closet.)
b. Measure how long it takes (in seconds) to fill the jug.
c. Divide the length of time needed to fill the jug into
60 and convert to a decimal. The result of this calculation
will be the flow rate in gallons per minute (GPM).
60 ÷ time required to draw one gallon of water = GPM
For example, if filling the jug took 20 seconds, the flow rate
is 3 GMP (60/20).
d. Demonstrate to the class how to calculate the amount of water used in
a 10-minute shower.
GPM X 10 = gallons in 10-minute shower.
2.

e. As a rough rule of thumb, energy used for water heating costs about $.01 (1¢)
per gallon, so a ten-minute shower in a shower with a 3 GPM showerhead
costs about $.30.
4.Assign your class the task of completing the worksheet. They will measure the
flow rate of their shower and tub, time the length of their showers (or the length of
time the water runs as the tub fills), and calculate the cost of water heating for
bathing for themselves, their entire household, their class, and the city.
Classroom Follow-up
6.Discuss the water heating implications of more efficient hot water use.
Extender
1. If possible, have students determine:
• The type of energy used for their hot water: natural gas, oil, or electricity.
• Whether or not their water heater and hot water pipes are insulated.
(Students who live in a multi-family building may be able to find this
information by interviewing the building superintendent.)
2. Burning fuel for any purpose contributes to environmental pollution. Conserving
fuel, on the other hand, contributes to environmental protection. The pollution may
take the form of carbon dioxide, sulfur, or other chemicals and particulates that contribute
to disease and poor air quality. Have your students conduct web-based
research on the relationship between burning hydrocarbon-based fuels and pollution
and report their findings to the class. You may want to begin with organizations such
as EPA (http://www.epa.gov/ebtpages/airairpollutioneffects.html) and the National
Safety Council (http://www.nsc.org/ehc/mobile/airpollu.htm).
Background Science
Water Heating
In many parts of the world, running water in the home is a luxury in and of itself.
Hot water is even more luxurious if you think about how hard it used to be to
warm water in the days when the main source of heat was a fireplace. Today, most
hot water heaters use oil, gas, or electricity. In general, heating water with electricity
is the most expensive method. Some hot water systems use the furnace,
while others have a separate hot water tank.
Btus and Calories
Scientists measure the amount of energy required to raise the temperature of water
in Btus or calories. A Btu is a British Thermal Unit, and one Btu is the amount of
energy required to raise 1 pound of water by 1 degree Fahrenheit. A gallon of
water weighs approximately 8.33 pounds, so raising the temperature of a gallon of
water 1˚F uses 8.33 Btus. Calories measure heat on the metric scale. One calorie
is the amount of energy required to raise the temperature of 1 gram of water 1
3.

degree Celsius. (Food Calories are correlated with heat calories: they measure the
amount of energy contained in food which your body can then burn off – or store
as fat, and a food Calorie is 1,000 times larger than an energy calorie.) Thus, the
higher the temperature of the hot water system, the more energy it takes to heat
that water. Likewise, the more hot water you use, the more energy you consume.
Fuels are measured and rated according to their energy content.
1 kilowatt-hour of electricity = 3,412 Btus or 860,369 calories
1 cubic foot of natural gas = 1,000 Btus or 252,164 calories
1 gallon of #2 fuel oil = 138,500 Btus or 34,924,714 calories
Efficiency
There is more to the energy cost of hot water than the type of fuel used. No hot water
system is 100% efficient. Some heaters can be as high as about 90% efficient, but most
are about 50% to 60% efficient. Efficiency describes the percentage of energy from the
fuel that actually winds up in the water.
Calculating the Real Cost of Hot Water
To figure out how much it really costs to heat the water for a shower, you would
need to know:
• The type of energy used
• The energy content of the energy source
• The efficiency of the water heater (assume 55% for gas and oil heaters and
90% for electric)
• The amount of water being heated
• The temperature difference of the water (Temp of incoming water – Temp of
heated water)
(You do not need to know the ratio of hot water to cold water since your calculation
takes into account the actual temperature of the heated water.)
Suggestions for students to explore
1. What other activities in the house use hot water?
• Dishwasher
• Clothes washer
• Shaving, washing hands, etc.
• Cleaning bathroom and sink surfaces.
2. How can water-saving devices like low-flow showerheads also save energy?
4.

3. What actions can students and their families take to save hot water in the home?
• Turn off hot water in sinks when not needed (while doing dishes, shaving, etc).
• Use cold water when hot water is not needed.
• Take shorter showers.
• Use dishwashers and washing machines only with full loads. (They use the same
amount of water regardless of how full they are.
• Use low-flow faucets and showerheads.
4. What actions could families or building superintendents take to improve the energy
efficiency of a hot water system?
Review
1. What forms of energy are used to heat water in most homes and buildings?
Natural gas, fuel oil, or electricity.
2. Explain what flow rate is and how you can measure it.
Flow rate is the amount of water that flows per unit of time. You can measure it
by timing how long it takes to fill a gallon jug with water from a faucet or
shower and then divide that figure into 60 (seconds) to get the flow rate (in gallons)
per minute.
3. What is the difference between a Btu and a calorie?
A calorie is a metric measure, and a Btu is a standard measure, so if a Btu were
like a foot or a pound, a calorie would be like a meter or a gram. A Btu is the
amount of energy required to raise one pound of water by one degree
Fahrenheit, while a calorie is amount of energy required to raise one gram of
water by one degree Celsius.
4. What would you need to know to determine the energy used in heating water for a
shower or bath?
You need to know how much water is heated, how many degrees the water was
heated, the amount of energy in the fuel, and the efficiency of the water heater.
5. Name at least three benefits of conserving hot water.
Saving energy, saving water, saving money. Indirect benefits include slower
depletion of natural resources and less environmental damage.
5.

Worksheet Name: ___________________________
Wasting Hot Water Wastes Energy: Measuring Flow Rate
Procedure
1. With the help of an adult, enlarge the opening in a one-gallon plastic milk jug.
Make the hole large enough to fill from the shower or the tub without spilling.
The container should still hold one gallon.
2. Turn on the water in the fixture you want to measure (bath, shower, sink).
Run the water at the same pressure you normally use.
Fixture you are measuring: ____________________
3. Use a watch with a second hand to time how long it takes to fill the jug with
one gallon of water. Record your measurement.
Flow = One gallon per __________ seconds.
4. Calculate gallons per minute (GPM) by dividing your answer from #2 into 60.
60 ÷ seconds needed for one gallon = _____ GPM
5. Time how long you run the water for a few activities, such as bathing,
showering, dishwashing, or washing.
Activity ____________________
Minutes of water use __________
Gallons of water ______________ (Length of time the water runs X GPM)
Activity ____________________
Minutes of water use __________
Gallons of water ______________ (Length of time the water runs X GPM)
Activity ____________________
Minutes of water use __________
Gallons of water _____________ (Length of time the water runs X GPM)
6.

Analysis
1. Average water heating costs approximately 1¢ per gallon of water from the tap.
What is the water heating cost of one of the activities you measured in #4 on page 3?
Gallons used X $.01 = $______
2. Estimate the number of times you and your family use water for that purpose each
year.
3. Calculate your household’s annual cost for that activity. (# gallons per year X 1¢).
$________
4. If all of the households represented in your class used that amount of water each
year, how much would they be spending on water heating alone?
Water heating cost for one year for ____ people = $________.
5. There are 8,000,000 people in New York City representing approximately 3.2 million
households. How much money would they spend on this activity each year if
each household used hot water at the same rate you and your household do?
$___________
6. Develop a plan for saving energy used for water heating.
I will save energy used for water heating by
________________________________________________________________
________________________________________________________________
________________________________________________________________
______________________________________________________________
Carrying out this plan will save my family $_____________________ per year.
It would save all the households in my class $___________________ per year.
It would save all the households in New York City $_______________per year.
7.