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Heat Pump Water Heater Modeling

in EnergyPlus

Building America Residential Energy 

Efficiency Stakeholder Meeting

Eric Wilson

Craig Christensen

March 1, 2012

NREL/PR-5500-54318
2

Modeling 

Issues Motivation Results

Heat Pump Water Heater Modeling...
3

Gap: Existing analysis tools cannot accurately 

model HPWHs with reasonable runtime.

Photos by Kate Hudon, NREL/PIX 18675, NREL/PIX 18671, NREL/PIX 18667, 

NREL/PIX 18676, and NREL/PIX 18919
4

What have we achieved so far?

Laboratory Evaluations

14 x

Field Monitoring

Photo from Steven Winter Associates, Inc.

Photos by Kate Hudon, NREL/PIX 18675, NREL/PIX 18671, 

NREL/PIX 18667, NREL/PIX 18676, and NREL/PIX 18919
5

Closing the Gap

6 sec timestep

hourly timestep

Laboratory Evaluations

14 x

Field Monitoring

Photo from Steven Winter Associates, Inc.

Photos by Kate Hudon, NREL/PIX 18675, NREL/PIX 18671, 

NREL/PIX 18667, NREL/PIX 18676, and NREL/PIX 18919
6

Why is modeling important?

• Performance varies:

Can’t just use EF

• System interaction

o HPWH affects building 

heating and cooling

o Space conditions 

affect HPWH 

performance
7

Modeling Goals

• Manage Risks

o Accuracy

o Run time

o Occupant satisfaction

• Flexibility to explore the effects of:

o Tank location, volume, setpoint

o Hot water use patterns

• HPWH vs. all other measures

(optimization)
8

Heat Pump Water Heater Modeling...

Motivation Results

1) Tank Stratification 

2) Draw Profile

3) Performance Maps

4) Sizing

Modeling 

Issues
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1) Tank Stratification

Mixed

tank

Tmains

Existing Model

EnergyPlus HPWH mixed tank
10

1) Tank Stratification

Mixed

tank

Tmains

Existing Model

EnergyPlus HPWH mixed tank

Tmains

Stratified

tank

Realistic Performance

Stratified tank
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1) Tank Stratification

Mixed

tank

Tmains

2) warmer coil 

worse performance

1) Colder 

water supplied

Existing Model

EnergyPlus HPWH mixed tank

Tmains

Stratified

tank

Realistic Performance

Stratified tank

Implications of 

using a mixed 

tank model...
12

1) Tank Stratification

Tmains

Mixed

tank

Mixed

tank

New Model

Two tanks in Series

Tmains

Realistic Performance

Stratified tank

Stratified

tank
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2) Draw Profile

HPWH Power

Hot Water Use

Existing Model

Smooth Draw Profile

Hourly Timestep
14

2) Draw Profile

Existing Model

Smooth Draw Profile

Hourly Timestep
15

2) Draw Profile

Realistic Performance

Discrete Draw Events

6 Second Timestep

Existing Model

Smooth Draw Profile

Hourly Timestep

HPWH Energy

14 MBtu/yr

HPWH Energy

23 MBtu/yr

~4.5 kW

~1.0 kW

If these draw profiles are used for an annual simulation...
16

2) Draw Profile

Realistic Performance

Discrete Draw Events

6 Second Timestep

New Model

Hourly Averaged Discrete Draws

Hourly Timestep

~4.5 kW

~1.0 kW
17

2) Draw Profile

Realistic Performance

Discrete Draw Events

6 Second Timestep

New Model

Hourly Averaged Discrete Draws

Hourly Timestep

February

9 10 11 12 13 14 15

0.00

0.02

0.04

0.06

0.08

0.10

Total Hot Water Flow Rate (cfm)

Hourly Time Series

 

~4.5 kW

~1.0 kW
18

2) Draw Profile

Realistic Performance

Discrete Draw Events

6 Second Timestep

New Model

Hourly Averaged Discrete Draws

Hourly Timestep

HPWH Energy

~23 MBtu/yr

HPWH Energy

23 MBtu/yr

~4.5 kW 

(element)

~1.0 kW 

(heat pump)

With the new model draw profile, annual results match:
19

3) Performance Maps

Existing Model

Constant Performance?

Constant Performance?
20

(constant 67°F, 50% RH air)

Mfgr 1

Mfgr 2

Mfgr 3

Realistic Performance

Manufacturer Specific

3) Performance Maps

Existing Model

Constant Performance?

Constant Performance?
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3) Performance Maps

New Model

Generic Normalized Performance Map

(constant 67°F, 50% RH air)

Realistic Performance

Manufacturer Specific

Mfgr 1

Mfgr 2

Mfgr 3

(equals 1 at rating point)
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3) Performance Maps

New Model

(constant 67°F, 50% RH air)

Realistic Performance

Manufacturer Specific

Mfgr 1

Mfgr 2

Mfgr 3

Use rated values to get 

specific performance map:
23

4) HPWH Sizing • No existing method for 

HPWH sizing

Conventional Electric WH Sizing:

Existing Model

0

10

20

30

40

50

60

70

12345

Tank Volume (gal)

# Bedrooms

Range = f(# baths)
24

4) HPWH Sizing

• No existing method for 

HPWH sizing

Conventional Electric WH Sizing:

Existing Model

0

10

20

30

40

50

60

70

1 2 3 4 5

Tank Volume (gal)

# Bedrooms

Reduced capacity to meet DHW 

demand (vs. conventional WHs)

Function of:

o Climate

o # bedrooms

o Tank Volume

o Setpoint Temperature

April

Realistic Performance

Range = f(# baths)
25

4) HPWH Sizing

New Model

“Percent Unmet Showers”

Realistic Performance

Reduced capacity to meet DHW 

demand (vs. conventional WHs)

Function of:

o Climate

o # bedrooms

o Tank Volume

o Setpoint Temperature

Option Display Variable

April

Percent Unmet Showers

• Function of climate, # bedrooms, tank volume, setpoint

• Estimate of shower time < 110°F

• Helps users select acceptable options

• Sensitive to hot water use patterns so will vary from 

one set of occupants to the next
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Modeling 

Issues Motivation Results

Heat Pump Water Heater Modeling...
27

Model Calibration

Calibrated to TRNSYS results ±5%

12 cases: Chicago, Houston;

1, 3, 5 BR;

50, 80 gal
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Model Calibration

Daily Draw Volume (gal/day)

Daily System COP

Chicago, 50 gal

Unfinished Basement
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Model Calibration

Daily Draw Volume (gal/day)

Daily System COP

Chicago, 50 gal

Unfinished Basement
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Model Calibration

Daily Draw Volume (gal/day)

Daily System COP

Houston, 50 gal

Garage
31

Model Calibration

Daily Draw Volume (gal/day)

Daily System COP

Houston, 50 gal

Garage
32

BEoptE+ Modeling Results

HPWH vs. Electric Resistance 

With Cooling Impact (Air-Source HP)

Model: A-B

Setpoint: 120-140°F

Location: Conditioned, Unconditioned
33

BEoptE+ Optimization Results

Space Conditioning = Air-Source Heat Pump

Maximum Energy Savings Lowest Life-Cycle Cost

1 BR 3 BR 5 BR 1 BR 3 BR 5 BR

Orlando HP HP HP Elec Elec/HP Elec/HP

Austin HP HP HP Elec/HP Elec/HP Elec/HP

Sacramento HP HP HP Elec/HP Elec/HP Elec/HP

Seattle HP HP HP Elec Elec Elec

Boston HP HP HP Elec/HP Elec/HP Elec/HP

Maximum Energy Savings Lowest Life-Cycle Cost

1 BR 3 BR 5 BR 1 BR 3 BR 5 BR

Orlando Gas/HP Gas/HP Gas/HP Gas Gas Gas

Austin Gas/HP Gas/HP Gas/HP Gas Gas Gas

Sacramento Gas Gas Gas Gas Gas Gas

Seattle Gas Gas Gas Gas Gas Gas

Boston Gas Gas Gas Gas Gas Gas

Space Conditioning = Gas Furnace

(“Gas” includes: standard, premium, tankless, or tankless condensing.)

(“HPWH” includes various models/volumes, locations, and setpoints)

(state average utility rates,

Nat’l avg Source/Site ratio)
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Conclusions

HPWH model for EnergyPlus/BEopt

• Market Benefits:

o Integrated with existing tool

o Fast

o Flexible

o Accurate
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Conclusions

Lessons Learned

• Rated EF for HPWHs not good indicator of performance

• Don’t use “smooth” BA HSP draw profile for HPWH testing

• Control logic matters

• Sizing HPWHs for adequate HW delivery: many factors

Remaining Issues

• Better draw profiles for HPWH modeling/testing

• Performance in enclosed spaces
For more information:

Laboratory Testing Report: 

Sparn, B.; Hudon, K.; Christensen, D. (2011). Laboratory Performance Evaluation of 

Residential Integrated Heat Pump Water Heaters. 77 pp.; NREL Report No. TP-5500-

52635.

TRNSYS Modeling Results: 

Maguire, J. (2011). A Parametric Analysis of Residential Water Heaters. Master’s 

Thesis. Boulder, CO: University of Colorado.