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Tank vs. Tankless Water Heaters: How’s a Person to Decide?
Bill Hoover William R. Hoover LLC
301 W. Thornapple Lane Grafton, WI 53024
262-388-4489 [email protected]
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Disclaimer Ø Speaker’s Credentials
Ø Speaker’s Prejudices: l Focus on cost effectiveness l Discuss primarily gas water heating l Focus on whole-home water heaters rather
than point of use water heaters.
Ø Speaker’s Goal l Provide honest, objective information about
water heating technologies. l Give the audience the background and
understanding to make appropriate equipment choices. 2
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Class Outline Ø The hot water event Ø How big is hot water? Ø How do we use hot water? Ø How are water heaters tested? Ø Storage water heater operation Ø Tankless water heater operation Ø How do you choose? Ø What does the future hold?
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Typical “Simple” Hot Water System for Single Family or Single Unit Applications
Fixture or Appliance
Drain
Fuel Source Water
Source Sewer
Water Heater
Indoor Boundary
Hot Water Cold
Water Mixed
Temperature Water
Energy
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Typical Hot Water Event
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Typical Hot Water Event
Time
Tem
pera
ture
Delivery Use Cool Down
Water Heater Temperature
Useful Hot Water Temperature
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Typical Hot Water Event
Time
Tem
pera
ture
Delivery Use Cool Down
Water Heater Temperature
Useful Hot Water Temperature
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Typical Hot Water Event
Time
Tem
pera
ture
Delivery Use Cool Down
Water Heater Temperature
Useful Hot Water Temperature
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What Phases in a Hot Water Event Does Your Water Heater
Impact?
Ø Delivery Temperature l Water heater temperature influences the temperature
of the delivered water. l Water heater does not directly influence the
temperature drop in the piping during delivery
Ø Delivery Time l If you do not have hot water when the fixture is
opened, delivery of hot water will be delayed and water will be wasted.
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How do we use Hot Water?
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What are Your Hot Water Usage Patterns?
Ø Do you regularly take long showers? Ø What is the most common duration of your hot
water use? Ø When do you think you use the most hot water? Ø Where do you use hot water most frequently? Ø Have you changed your behavior to minimize
your hot water usage?
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What do most people want from the hot water system?
1. Hot water NOW! 2. Never run out.
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02,0004,0006,0008,000
10,00012,00014,00016,00018,00020,00022,00024,00026,00028,00030,00032,00034,00036,00038,00040,000
0-0.5 0.5-1.0 1.0-1.5 1.5-2.0 2.0-2.5 2.5-3.0 3.0-3.5 3.5-4.0 4.0-4.5 4.5-5.0 5.0-5.5 5.5-6.0 6.0+
Flow Rate Bin (gpm)
Min
utes
at F
low
020406080100120140160180200220240260280300320340360380400
Min
utes
at F
low
Left Axis Right Axis
Source: NAHB Research Center, November 2002
Flow Rate Distribution of Hot Water in High Volume Home
Most hot water events are less than
0.5 gpm
Less than 600 minutes a year
are greater than 3.0 gpm
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-2,0004,0006,0008,000
10,00012,00014,00016,00018,00020,00022,00024,00026,00028,00030,00032,00034,00036,00038,00040,000
0-0.5 0.5-1.0 1.0-1.5 1.5-2.0 2.0-2.5 2.5-3.0 3.0-3.5 3.5-4.0 4.0-4.5 4.5-5.0 5.0-5.5 5.5-6.0 6.0+
Flow Rate Bin (gpm)
Min
utes
at F
low
020406080100120140160180200220240260280300320340360380400
Min
utes
at F
low
Left Axis Right Axis
Source: NAHB Research Center, November 2002
Flow Rate Distribution of Hot Water in Low Volume Home
Most hot water
events are less than 0.5 gpm
Less than 800 minutes a year
are greater than 1.5 gpm
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Time and Temperature at the Master Bath Sink
Source: National Renewable Energy Laboratory
The average draw is 30 seconds
Hot water never arrived
Hot water arrives in 45 – 60 seconds
Hot water arrives quickly
Average draw was 30 seconds
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How do we use hot water?
Ø Frequent short, low flow-rate draws Ø Occasional long draws at low
flow-rates Ø High flow-rate and high volume
draws are rare!
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How Big is Hot Water? Water heating is the 1st or 2nd largest
residential energy end-use: 15 – 30% of a house’s total energy pie. l What is number 1? Number 3? l Percentage grows as houses and appliances
get more efficient How does this compare to your:
l Cell phone bill? l Internet bill? l Cable or Satellite bill? l Starbucks bill? 18
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Annual Energy Use for Heating Water Natural Gas Electricity
Gallons Per Day 60
Gallons Per Year 21,900
Energy into Water 16.4 Million Btu
Efficiency 0.6 0.9 Cost per Unit $1.00/therm $0.10/kWh Cost per Year $275 $535 Assumes hot water is 90 degrees F above incoming cold water. Cost per
year has been rounded off.
Add about $110 per year for water and sewer (at $0.005 per gallon combined) 19
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Typical Water heating Costs Ø Total hot water cost (gas) $35/month Ø Water heating cost (gas) $25/month
Ø Total hot water cost (elec.) $60/month Ø Water heating cost (elec.) $50/month
Ø At these costs, it is difficult to justify spending much money to improve your water heater’s efficiency.
Ø But, if all 120 million water heaters in the U.S. were more efficient, the nation would save a huge amount of energy. 20
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Fuel Cost Comparison Fuel Cost Cost/M BTU Cost /M BTU
H2O* Natural Gas $1.20/Therm
Electricity $0.16/ KWHr
Propane $2.85/gal
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Fuel Cost Comparison Fuel Cost Cost/M BTU Cost /M BTU
H2O* Natural Gas $1.20/Therm $12.00
Electricity $0.16/ KWHr $46.88
Propane $2.85/gal $31.32
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Fuel Cost Comparison Fuel Cost Cost/M BTU Cost /M BTU
H2O* Natural Gas $1.20/Therm $12.00 $19.35
Electricity $0.16/ KWHr $46.88 $50.96
Propane $2.85/gal $31.32 $50.52
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* 62% efficiency assumed for natural gas and propane water heaters. 92% assumed for electric water heaters.
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Fuel Cost Comparison Fuel Cost Cost/M BTU Cost /M BTU
H2O* Cost /M BTU H2O HPWH**
Natural Gas $1.20/Therm $12.00 $19.35
Electricity $0.16/ KWHr $46.88 $50.96 $23.44
Propane $2.85/gal $31.32 $50.52
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* 62% efficiency assumed for natural gas and propane water heaters. 92% assumed for electric water heaters. ** EF = 2.0 assumed for HPWH
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Testing Water Heaters
Ø How do we test water heater performance and what does Energy Factor, EF, mean?
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Testing NAECA Water Heaters Tank
(Storage)
<4000 Btu/hr/gal
Tankless
(Instantaneous)
< 2 gallons
Natural Gas ≤ 75,000 Btu ≤ 200,000 Btu
Oil ≤ 105,000 Btu ≤ 210,000 Btu Electric
Ø Resistance
Ø Heat Pump ≤ 12 kW
≤ 24 amps ≤ 12 kW
NA
Measure of Efficiency
Energy Factor (EF) 26
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Testing NAECA Water Heaters Tank
(Storage)
<4000 Btu/hr/gal
Tankless
(Instantaneous)
< 2 gallons
Natural Gas ≤ 75,000 Btu ≤ 200,000 Btu
Oil ≤ 105,000 Btu ≤ 210,000 Btu Electric
Ø Resistance
Ø Heat Pump ≤ 12 kW
≤ 24 amps ≤ 12 kW
NA
Measure of Efficiency
Energy Factor (EF) 27
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Energy Factor Ø A test dictated by the U.S. Department of Energy
which simulates residential usage. Ø Incorporates both thermal (combustion) efficiency
and stand-by loss Ø Assuming 64.3 gallons per day, EF is determined
by drawing 10.7 gallons at the rate of 3 gpm once each hour for 6 hours and then an 18 hour stand-by period.
Ø Test is very difficult to conduct accurately since so many variables must be accurately measured.
Ø Energy Factor is the only legal way to describe the efficiency of a NAECA heater.
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Range of Energy Factors Volume (gallons) Minimum EF Maximum EF available
Natural Gas Storage Water Heater 30 0.61 0.64 40 0.59 0.70 50 0.58 0.70 75 0.53 0.59
Electric Storage Water Heater 40 0.92 0.95 50 0.90 0.95 66 0.88 0.95 80 0.86 0.95 29
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Your Mileage Will Vary… Ø If actual use deviates from the test protocol, EF becomes
a less effective measure of energy efficiency. l If very little hot water is used, stand-by losses will be under-
estimated. l If large amounts of hot water is used, stand-by losses will be over-
estimated l Test does not simulate residential use data…total hot water
consumption of about 64 gals per day is OK but most homes have many more, short draws than the EF test has.
l If you have no energy loss during stand-by, the EF value becomes the combustion efficiency.
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