Systems Evaluation at 

the Cool Energy House

J. Williamson and S. Puttagunta

Consortium for Advanced Residential Buildings

September 2013
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iii

Systems Evaluation at the Cool Energy House

Prepared for:

The National Renewable Energy Laboratory

On behalf of the U.S. Department of Energy’s Building America Program

Office of Energy Efficiency and Renewable Energy

15013 Denver West Parkway

Golden, CO 80401

NREL Contract No. DE-AC36-08GO28308

Prepared by:

J. Williamson and S. Puttagunta

Steven Winter Associates, Inc.

of the

Consortium for Advanced Residential Buildings

61 Washington Street

Norwalk, CT 06854

NREL Technical Monitor: Cheryn Metzger

Prepared under Subcontract No. KNDJ-0-40342-03

September 2013
iv

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v

Contents

List of Figures ............................................................................................................................................ vi

List of Tables ............................................................................................................................................. vii

Definitions................................................................................................................................................. viii

Executive Summary................................................................................................................................... ix

1 Introduction........................................................................................................................................... 1

1.1 Heat Pump Water Heater................................................................................................................. 2

1.2 Whole-House Dehumidifier.............................................................................................................. 4

2 Methodology ......................................................................................................................................... 8

2.1 Research Focus ............................................................................................................................... 8

2.2 Equipment Monitored....................................................................................................................... 8

2.2.1 Heat Pump Water Heater ............................................................................................................. 8

2.2.2 Whole-House Dehumidifier........................................................................................................... 9

2.2.3 Central Air-Conditioning System................................................................................................. 10

2.3 Measurements ............................................................................................................................... 10

3 Energy Modeling................................................................................................................................. 11

3.1 Predicted Performance of the Heat Pump Water Heater............................................................... 11

3.2 Predicted Performance of the Whole-House Dehumidifier............................................................ 14

4 Results From Field Monitoring.......................................................................................................... 17

4.1 Heat Pump Water Heater............................................................................................................... 17

4.2 Whole-House Dehumidifier............................................................................................................ 25

5 Conclusion .......................................................................................................................................... 33

5.1 Heat Pump Water Heater............................................................................................................... 33

5.2 Whole-House Dehumidifier............................................................................................................ 34

5.3 Next Step ....................................................................................................................................... 34

References................................................................................................................................................. 36

Appendix A. Measurements and Calculations....................................................................................... 38

A1. Heat Pump Water Heater............................................................................................................... 38

A2. Whole-House Dehumidifier ............................................................................................................ 40

Appendix B. Cooling Scenario Data Detailed Plots............................................................................... 44

B1. Cooling Scenario Outdoor Condition Comparison......................................................................... 44

B2. Cooling Scenario Indoor Air Conditions Maintenance ................................................................... 45

Appendix C. Energy Performance of the Cool Energy House.............................................................. 48
vi

List of Figures

Figure 1. Front elevation (west) of the CEH post-retrofit........................................................................ 1

Figure 2. Perceived air temperatures based on actual temperature and RH........................................ 4

Figure 5. RH box plots for zone 2, hot-humid climate for each sensor location.................................. 6

Figure 6. Cover removed from top of the monitored HPWH................................................................... 8

Figure 7. How the monitored dehumidifier works ................................................................................... 9

Figure 8. Schematic of dehumidifier ports............................................................................................... 9

Figure 9. Outdoor condensers and the AHUs for the second floor..................................................... 10

Figure 10. Annualized space conditioning costs of building with various cooling configurations. 15

Figure 11. HPWH COP versus water draw.............................................................................................. 18

Figure 12. HPWH average water draw..................................................................................................... 19

Figure 13. Comparison of various DHW draw profiles.......................................................................... 20

Figure 14. Water draw profile resulting in low daily COP (COP = 1.4)................................................. 20

Figure 15. Water draw profile resulting in high daily COP (COP = 2.6)............................................... 21

Figure 16. HPWH performance with minimal water draw (COP = 0.3)................................................. 22

Figure 17. Air temperature change through HPWH............................................................................... 22

Figure 18. Humidity ratio change through the HPWH........................................................................... 23

Figure 19. HPWH condensate removal ................................................................................................... 24

Figure 20. HPWH condensate removal performance ............................................................................ 24

Figure 22. RH frequency distribution chart............................................................................................ 27

Figure 23. Temperature frequency distribution chart ........................................................................... 27

Figure 25. Dehumidifier condensate removal versus energy performance........................................ 29

Figure 26. Humidity ratio change of airstream through dehumidifier................................................. 30

Figure 27. Temperature change of airstream through dehumidifier.................................................... 30

Figure 28. Daily heat into dehumidifier airstream ................................................................................. 31

Figure 29. Scenario comparison of outdoor RH .................................................................................... 44

Figure 30. Scenario comparison of outdoor temperatures .................................................................. 44

Figure 31. Scenario 1 (78°F, 50% RH) RH maintenance........................................................................ 45

Figure 32. Scenario 1 (78°F, 50% RH) temperature maintenance ........................................................ 45

Figure 33. Scenario 2 (75°F, dehumidifier off) RH maintenance .......................................................... 46

Figure 34. Scenario 2 (75°F, dehumidifier off) temperature maintenance .......................................... 46

Figure 35. Scenario 3 (78°F, 60% RH) RH maintenance........................................................................ 47

Figure 36. Scenario 3 (78°F, 60% RH) temperature maintenance ........................................................ 47

Figure 37. Cumulative contribution to total simulated energy savings, by measure and end use.. 49

Unless otherwise noted, all figures were created by CARB.
vii

List of Tables

Table 1. HPWH Operating Condition and Performance Study (June 23, 2012 to October 30, 2012).. x

Table 2. 2009 Residential Energy Consumption Survey Data Sample of Households With ERWHs. 3

Table 3. Performance Summary of Monitored HPWHs by Model........................................................... 3

Table 4. Inputs of Economic Analysis .................................................................................................... 11

Table 5. Lifetime Costs of Various DHW Technologies ........................................................................ 11

Table 6. Space Conditioning Utility Costs of DHW Heating Methods ................................................. 12

Table 7. Combined Water Heating and Space Conditioning Utility Costs for Various DHW Methods

.............................................................................................................................................................. 13

Table 8. Lifetime Costs of Various DHW Technologies Including Space Conditioning Impact ....... 13

Table 9. Percent Frequency of Conditions > 0.012 lbm Water/lbm Dry Air........................................... 14

Table 10. Space Conditioning Savings of 78°F/55% RH Case Over 75°F Case .................................. 15

Table 11. Combined A/C and WHD Cost Analysis................................................................................. 16

Table 12. HPWH Monitored Performance Summary (June 22, 2012 to October 31, 2012) ............... 17

Table 13. COP Regression Fitting Coefficients...................................................................................... 19

Table 14. Power Draw of HVAC System Components .......................................................................... 25

Table 15. Equivalent A/C Capacity Metrics............................................................................................. 26

Table 16. Scenario Test Summary........................................................................................................... 32

Table 17. Percent Frequency of Conditions Over 60% RH ................................................................... 34

Table 18. Existing, Proposed, and Post-Retrofit Specifications .......................................................... 48

Table 19. Simulated Energy Use Distribution ........................................................................................ 49

Unless otherwise noted, all tables were created by CARB.
viii

Definitions

A/C Air conditioner

AHU Air handler unit

BEoptTM Building Energy Optimization software 

Btu British thermal unit 

CARB Consortium for Advanced Residential Buildings 

CEH Cool Energy House 

CFM Cubic feet per minute

COP Coefficient of performance

DHW Domestic hot water

EF Energy factor

ERWH Electric resistance water heater

HERS Home Energy Rating System

HPWH Heat pump water heater

HVAC Heating, ventilation, and air conditioning

kWh Kilowatt hour

NREL National Renewable Energy Laboratory

RH Relative humidity

S/T Sensible heat to total heat ratio

SWA Steven Winter Associates, Inc. 

WHD Whole-house dehumidifier
ix

Executive Summary

Steven Winter Associates, Inc. monitored several advanced mechanical systems within a 2012 

deep energy retrofitted home in the small Orlando suburb of Windermere, Florida. This report

provides performance results of one of the home’s heat pump water heaters (HPWHs) and the 

whole-house dehumidifier (WHD) over a six-month period. In addition to assessing the energy 

performance of these systems, this study sought to quantify potential comfort improvements over 

traditional systems. This information is applicable to researchers, designers, plumbers, and 

heating, ventilation, and air conditioning contractors. Though builders and homeowners can find 

useful information within this report, the corresponding case studies are likely better references 

for these audiences. 

Heat Pump Water Heaters

Over recent years, HPWHs have grown in both market interest and product capability. These 

units harness the heat transfer benefits of the vapor compression heat pump cycle to extract heat 

from surrounding air space and supply it to water for domestic hot water (DHW) needs. In doing 

so, an HPWH can provide DHW more efficiently than a standard electric resistance tank water 

heater. Furthermore, the unit’s operational byproduct of cooling and dehumidification can be 

beneficial in hot-humid climates. 

Energy simulation analysis was performed with Building Optimization Energy Plus (BEoptE+) 

v1.3 software. Energy modeling predicts that an HPWH can annually save the Cool Energy 

House test home approximately 64% (or $113/yr) on DHW utility costs over a standard electric 

resistance tank water heater (at the local electricity rate). There is an interesting inverse effect 

with HPWHs in the hot-humid climate zone. While the cooling/dehumidification benefits and 

higher coefficients of performance of an HPWH would be more advantageous in the hot-humid 

climate, mains water temperature tends to be higher. The mains water temperature ranged from 

75°–85°F over the June through October monitoring period. This means less water heating is 

required (less than a cold-climate location), resulting in the overall water heating cost being 

minimized. Therefore, the cost benefit of the HPWH was diminished. 

The long-term monitored data show that water draw profile (both volume and frequency) has a 

strong influence on the unit’s operating efficiency. If too great a volume is drawn in a short 

period of time, the system will revert to electric resistance backup elements to provide 

supplemental heat. Table 1 displays a summary of HPWH operating and performance conditions 

that existed during the monitoring period. 
x

Table 1. HPWH Operating Condition and Performance Study

(June 23, 2012 to October 30, 2012)

Hot Water Set Point 120°F

Average Water Inlet Temperature1 82°F

Average Water Outlet Temperature1 117°F

Total COP 2.2

% Electric Resistance 2 23%

Average Hot Water Use3 48.8 gal/day 1 Average estimated with 15-min periods containing near-continuous flow 2 % electric resistance = % of total kWh consumed by resistance

3 Average of daily averages

Whole-House Dehumidifier

In hot-humid climates, it becomes a challenge to simultaneously control moisture levels (latent 

heat) and temperatures (sensible heat) within a living space. As a result, it is often beneficial to 

utilize a WHD to specifically address the building’s latent load while controlling the sensible

load with the home’s central air conditioning (A/C) unit. Once the control of each of these units 

is independently decoupled, the homeowner can achieve improved comfort and cost savings. 

BEoptE+ energy modeling predicts that a combined A/C-WHD system, set at 78°F and 55%

relative humidity (RH), will be able to provide similar indoor humidity ratio levels to an A/C 

only system at 75°F. However, the combined system will accomplish this while maintaining an 

increased moisture-comfort level and energy savings. Modeling shows that the combined system 

has the potential to support RH levels below the defined 60% comfort limit for 15.8% more of 

the year while saving 8.2% annually (or $53/yr) over the 75°F A/C only base case.

Some previous research on dehumidifiers (primarily stand-alone dehumidifiers) has been done, 

but it has been mostly focused on lab testing or analysis of the annual dehumidifier use based on

estimates from initial measurements and manufacturer’s performance data. This study sought to 

provide long-term field data to validate this energy modeling. 

Long-term testing shows that the combined system, at the homeowner’s preference of 76°–78°F

and 65% RH, was able to maintain acceptable indoor summer conditions for a majority of the 

monitoring period. Indoor temperature was maintained between 74°F and 80°F for 91.5% of the 

time while RH did not exceed the 60% limit for 99.9% of the period. The dehumidifier’s average 

performance was 3.56 pints/kWh over the monitoring period. Short-term scenario testing that

compared three set point configurations (78°F and 60%, 78°F and 50%, and 75°F with no 

dehumidifier) suggest that a paired A/C-WHD system at 78°F and 60% can maintain indoor 

comfort in the most cost-effective manner. 

These results indicate that utilizing a WHD to specifically control a building’s latent load while 

supporting the sensible load with a central A/C unit can provide multiple benefits. Most 

importantly, the homeowner can see improved comfort that comes with heightened moisture 

control. Additionally, the system will maintain this comfort while saving energy and utility costs.
1

1 Introduction

In 2012, the Consortium for Advanced Residential Buildings (CARB) provided the technical 

engineering and building science support for a highly visible demonstration home in connection 

with the National Association of Home Builder’s International Builders Show. This project, 

which was unveiled at the 2012 International Builders Show in Orlando on February 9, is known 

as the deep energy retrofit Cool Energy House (CEH). The CEH began as a mid-1990s two-story 

traditional spec house of about 4,000 ft2 in the upscale Orlando, Florida suburb of Windermere. 

The homeowners, who were planning some interior renovations and a small addition, were 

recruited for this research project through their contractor, Southern Traditions. 

The initial objectives of the CEH project included reducing simulated annual source energy 

consumption by 50% compared to the existing pre-retrofit home. The project team realized from 

the start that reaching this performance goal in the context of a relatively light (not gut-rehab) 

renovation was a significant challenge, and a challenge that might not be fully accomplished. 

However, by aggressively pushing the performance level, this project was able to achieve an 

estimated 49% reduction in source energy use compared to the existing home.

Nearly all the interior wall surfaces and all the exterior brick veneer remained in place. The twostory insulated frame walls were insulated further by blowing fiberglass from the interior through 

6-in. horizontal slots in the gypsum board. The partially occupied attic space was sealed, 

finished, and encapsulated with R-30 closed-cell spray polyurethane foam at the roof deck. 

Downsized air-source heat pumps replaced the two older space conditioning systems, while most 

of the existing ducts were sealed with mastic and reused (Zoeller et al. 2013). Figure 1 shows the 

front façade of the post-retrofit home.

Figure 1. Front elevation (west) of the CEH post-retrofit
2

The 49% simulated reduction in energy consumption over the pre-existing conditions translates 

to approximately 12,687 kWh or $1,500 in anticipated annual electricity bill savings (see 

Appendix C for more information). The retrofit also provided an improvement to indoor air 

quality by supplying whole-house and local ventilation, as well as dehumidification, 

improvements to thermal comfort by air sealing, duct sealing, and reductions in solar heat gain 

through windows.

The home also features advanced mechanical systems that are the focus of this research report.

CARB monitored a heat pump water heater (HPWH) and a whole-house dehumidifier (WHD) 

over a six-month period to assess their performance in an occupied setting. These two 

technologies address two common gaps in this region—high efficiency electric water heating and 

effective humidity control in a hot-humid climate zone.

Domestic hot water (DHW) is a major source of energy consumption in almost every residential 

building. In most cases, DHW is supplied through fossil-fuel combustion units (e.g. gas, oil, and 

propane) or electric resistance water heaters (ERWHs) when fossil fuels are not available or 

desired. In recent years, HPWH technology has grown in both product capability and market

demand. The high efficiency potential of this technology makes it a desirable alternative

compared to other DHW technologies—specifically ERWHs. 

A heating, ventilation, and air conditioning (HVAC) system’s ability to maintain indoor 

environmental quality levels within human comfort preferences is one of the most important 

features of a building. In hot-humid climates, such as Orlando, this is not always a trivial task. 

Year-round high outdoor relative humidity (RH) levels create a unique situation for comfort 

control. Since moisture removal (latent heat) and temperature maintenance (sensible heat) are not 

always needed simultaneously, comfort conditions can be truly optimized only when each is

addressed by separate systems. By utilizing a WHD to address the latent load and a separate 

central air conditioner (A/C) to address the sensible load, indoor comfort should be improved.

1.1 Heat Pump Water Heater

Water heating is the third largest contributor to residential energy consumption in the United 

States, after consumption attributed to space conditioning and lighting, appliances, and 

miscellaneous electric loads. In the United States, residential water heating consumes 2.11

quadrillion Btu of site energy per year, which is 20% of total residential site energy (EIA 2005). 

The vast majority of water heaters are powered by natural gas (58.4 million households) and 

electricity (46.7 million households), but fuel oil (3.6 million households) and propane (4.2 

million households) also have sizable shares of the water heating market (EIA 2009).

Fortunately, more efficient water heaters for both major water heating fuels are becoming more 

readily adopted in the marketplace. In terms of electric heating options, HPWHs are a promising 

technology that has the potential to reduce water heater energy consumption by around 50% 

compared to traditional ERWHs. 

HPWH technology promises to significantly reduce energy consumption for DHW over 

traditional ERWHs. While ERWHs perform with energy factors (EFs) around 0.9, new HPWHs 

boast EFs upward of 2.0. High EFs are achieved by combining a vapor compression system to 

extract heat from the surrounding air at high efficiencies; electric resistance element(s) are still 
3

present to meet large demands. Looking at Table 2, swapping ERWHs to HPWHs could result in 

roughly 50% reduction in water heating energy consumption for 41.1% of all households. This 

impact is even greater in hot-humid climates in the South, where ERWHs make up more than 

half of all installed systems.

Table 2. 2009 Residential Energy Consumption Survey Data Sample of Households With ERWHs

Census Region Fraction of Households 

With ERWHs by Region

Northeast 24.5%

Midwest 29.3%

South 64.6%

West 27.8%

National 41.1%

A key feature of an HPWH unit is that it is a hybrid system. When conditions are favorable, the 

unit will operate in heat pump mode (using a vapor compression system, which extracts heat 

from the surrounding air) to efficiently provide DHW. Yet it does not require homeowners to 

adjust their behavior to conform to the heat pump’s capabilities. If a heat pump is unable to meet 

a higher water draw demand, the heater will seamlessly switch to electric resistance to provide a 

higher heating rate. Thus, hybrid mode provides the energy savings of heat pump mode (when 

possible) while being able to perform as an ERWH during periods of high DHW demand

(Shapiro et al. 2013).

CARB monitored the in-situ performance of 14 HPWHs from 2010–2012 in homes in the 

heating-dominated climates of Massachusetts and Rhode Island. Data collected during this study 

have been used to address issues facing researchers of residential water heating systems. Usage 

patterns and environmental characteristics can vary dramatically between households, and 

researchers must be able to determine the correlation between various household characteristics 

and HPWH performance. The results provide considerable insight into the sensitivity of HPWHs 

to environmental variables, such as mains water temperature, set point temperature, ambient 

temperature, ambient humidity, and water draw profiles. This monitoring was done on HPWHs 

located primarily in unconditioned basements. The results of the evaluation were encouraging 

(Table 3). Most importantly, all customers were satisfied with the supply of hot water and 

noticed a reduction in their utility bills (Shapiro et al. 2013).

Table 3. Performance Summary of Monitored HPWHs by Model

Model Capacity

(gal) Rated EF

First Hour 

Rating 

(gal/h)

Measured 

Average

COP*

COP 

Range

General Electric 50 2.35 63.0 1.82 1.5–2.1

A.O. Smith 60/80 2.33 68.0/84.0 2.12 2.1

Stiebel Eltron 80 2.51 78.6 2.32 2.0–2.6

* Coefficient of performance, the ratio of heat provided per unit of electrical energy used.
4

A tradeoff that is often overlooked is that if the HPWH is located within the home (rather than an 

attached garage), the heat pump is not collecting “free” energy to heat water, as is often touted 

for this technology. It is removing heat from the home, which may need to be replenished by the 

space conditioning system. During the cooling season, this serves as an added benefit, but during 

the heating season, this is a concern. Although many desire to integrate this cooling and 

dehumidification byproduct into the space conditioning system, it is not a simple system to 

integrate. Since a HPWH’s space conditioning impact is sporadic, due to its being a function of 

hot water demand and not room air conditions, the quantity of heating/dehumidification that can 

occur on any day is inherently variant.

This report analyzes the system performance of an HPWH installed within the thermal boundary 

of a hot-humid climate test home. The intent is to examine the extent to which geographical 

install location influences DHW heating performance. In addition, the study analyzes the effects 

that this system has on space conditioning.

1.2 Whole-House Dehumidifier

One of the most common complaints that homeowners have in a hot-humid climate is that they 

are too cold when indoors. Often the temperature set point is set lower than the desirable cooling 

level to try to increase moisture removal (latent cooling), so that the interior air is not humid or 

“muggy.” Even when a thermostat with humidity control installed, the humidity control logic is 

still based on dropping the dry-bulb temperature (sensible cooling) set point by a couple of 

degrees to have the system run more frequently. However, this method is not always effective in 

maintaining indoor RH or human comfort. By separating the systems that control the latent and 

sensible cooling, this specific complaint can be eliminated.

Figure 2 provides an illustration that gauges human comfort based on the RH of the ambient air. 

As seen from the range of perceived temperatures at a summer set point of 75°F, the perceived 

temperature can range from a maximum of 80°F at 100% RH to a minimum of 69°F at 0% RH. 

This justifies the importance of humidity control to maintain comfort levels and provides an 

insight into how comfort can be maintained by removing latent energy with a dehumidifier rather 

than a more energy-intensive central A/C unit.

Figure 2. Perceived air temperatures based on actual temperature and RH

(courtesy of Honeywell; Honeywell 2008a)
5

Indoor comfort, as defined by ASHRAE, is achieved when indoor humidity ratio is controlled 

below 0.012 lbm water/lbm dry air (ASHRAE 55-2010). For common indoor summer set points 

of 75°–80°F, these conditions are achieved while RH is below the range of 55%–65%. In 

addition to improved comfort, maintaining indoor RH at < 60% also can reduce: (1) insects like 

earwigs and millipedes; (2) occupant health issues with allergies, asthma, and diseases from 

fungal toxins; and (3) damage to building contents from corrosion or warping (Aprilaire 2009). 

For this research, an upper limit of 60% was used as the acceptable RH threshold within a 

conditioned living space. This upper RH limit was chosen to support human comfort at common 

set point temperatures, create a health-promoting environment, and maintain interior building 

conditions that support the durability of construction components.

A WHD was installed in the CEH with its own dedicated compact distribution system to better 

address latent cooling. This allows the A/C system to have a smaller capacity such that it will be 

sized to specifically address the sensible cooling needs during the summer. One added benefit of 

this configuration is that the dehumidifier also has a lower power draw than the air conditioner—

strictly considering energy used for moisture removal. Additionally, with the combined 

capability of maintaining better indoor RH, the thermostat cooling set point can often be set 

higher (~78°F), resulting in additional energy savings while maintaining (or improving) occupant 

comfort.

The climate pattern in a hot-humid climate, such as Orlando, Florida, typically shows conditions 

in which the morning outdoor RH will often sustain moisture levels upward of 80% throughout 

the entire year. However, the outdoor dry-bulb temperature exceeds comfortable levels only

during summer months. Even though latent energy removal will be needed year-round, sensible 

cooling is needed only from roughly May through October. This brings forth the need for a 

cooling system in which moisture removal can be accomplished without decreasing indoor air 

temperature. Hence, an independent WHD in conjunction with a central A/C system can be more 

effective in maintaining indoor comfort than an A/C alone. Even though the central A/C unit 

may have the mechanical capability of controlling latent load, it is unlikely that it will be able to 

do so without inducing uncomfortably low temperatures in the space. 

The latent (moisture) load in homes is traditionally addressed by the central A/C unit, but how 

well these systems will actually control humidity levels varies drastically. One major issue with 

traditional A/C systems is the common industry practice of oversizing system capacity. With an 

A/C unit, the greatest moisture removal will occur once the unit has reached steady-state 

operating conditions. Steady-state performance generally begins 5–15 min after the initial 

compressor startup (Shirey et al. 2006). Hence, if the unit is oversized and can drop the indoor 

temperature below the set point very quickly (without spending much time in steady state), the 

latent cooling effects will be minimal since the evaporator coil has not achieved the coil 

temperatures where dehumidification performance is optimized.

Additionally, the initial period of a system’s startup provides a condition where moisture can reenter the airstream as warm air is blown across the not fully chilled evaporator coil (Katipamula

and O’Neal 1991). Likewise, this re-evaporation effect can also occur with homeowners who 

chose to alternate between cooling and continuous fan-only mode. As a result, HVAC practices 
6

of oversizing system capacities and homeowner preferences of A/C settings can strongly 

influence the effectiveness of the A/C dehumidification process.

Unfortunately, there is an insufficient amount of measured data available on actual indoor 

humidity levels in U.S. households. Steven Winter Associates, Inc. (SWA) collected one full 

year of indoor temperature and humidity data for a sample of 60 homes across three different 

climate regions—the hot and humid Southeast, the cold Northeast, and the marine Northwest

(Arena et al. 2010). Figure 3 shows RH box plots in various interior locations throughout a home 

for the hot-humid climate study homes. All whiskers represent 1.5 times the inter quartile range, 

whereas, inter quartile range is the 75th percentile minus the 25th percentile. 

Figure 3. RH box plots for zone 2, hot-humid climate for each sensor location

(Arena et al. 2010)

Though the mean RH was within acceptable indoor levels, there were numerous cases of RH

levels exceeding 60% for significant portions of the time. Additionally, RH levels of up to 96%

were measured inside of the home’s conditioned space (bathroom 96%, bedroom 86%, and 

living room 84%). This suggests that a central A/C unit alone does not always maintain adequate 

RH levels within the living space.

Min 21.0 21.0 22.0 9.0 1.0

Max 84.0 86.0 96.0 93.0 100.0

Mean 49.0 51.2 53.2 49.9 70.8

Median 49.0 51.0 53.0 51.0 75.0

% Outliers 0.51% 2.64% 1.43% 0.00% 0.09%

0

10

20

30

40

50

60

70

80

90

100

Living Room Bedroom Bathroom Attic Outdoor

Relative Humidity (% )
7

In 2005, a report was published that describes research done by Building Science Corporation on 

residential dehumidification in hot-humid climates (Rudd et al. 2012).This study monitored the 

indoor temperature and RH, as well as energy use of various dehumidifiers and ventilation 

systems. However, the report does not directly address the impact that the dehumidifier and 

ventilation units have in reducing cooling requirements of the building’s central A/C system. 

As follow-up research, Building Science Corporation performed a study in 2012 that evaluated 

the construction of new homes in the hot-humid climate of New Orleans (Osser and Kerrigan 

2012). This study provides some evaluation of the supplemental use of a dehumidifier in a 

cooling system in hot-humid climates. However, since supplemental dehumidification could not 

be modeled in BEopt or Energy Gauge USA software at the time of the research, it is not 

included in the report. Rather than long-term monitoring with various cooling scenario set points, 

this study analyzes the annual dehumidifier use with estimates from initial measurements and 

manufacturer’s performance data.

Further supporting the need for supplemental dehumidification, the National Renewable Energy 

Laboratory (NREL) performed energy modeling to analyze indoor RH in various home types. 

This study concluded that supplemental dehumidification is needed to maintain interior RH

levels < 60% in a hot-humid climate home. (Fang et al. 2011) Again, this 60% RH threshold is 

based on providing suitable comfort and health to occupants as well preserving the home and the 

contents within it. Additionally, in 2011, NREL performed laboratory research on the 

performance of six dehumidifiers (Winkler et al. 2011). This study analyzed the lab-controlled 

operational performance of units at an array of conditions to determine performance curves. As 

this study does provide extensive results on the in-laboratory performance, it does not directly 

evaluate the performance of the units in an occupied setting.

Aprilaire, a manufacturer of dehumidifiers, performed Transient System Simulation Tool 

modeling on a 2,000-ft2 home in Miami, Florida, to demonstrate the benefit of an A/C and 

dehumidification system (Aprilaire 2009). When comparing a conventional A/C set at 75°F 

versus an A/C set at 78°F and dehumidification system set at 59% RH, the latter system 

maintained indoor RH below 60% for the entire year and resulted in 18% annual energy savings.

The conventional A/C system resulted in 1,641 hours of RH >60% or 18.7% of the time.

In an alternative case, the conventional A/C was set to a 72°F set point as homeowners may 

lower the thermostat setting further in an attempt to reduce the moisture levels. In this scenario,

the dual-system approach was even more beneficial. Though the occurrence of RH >60% was 

reduced to 280 hours for the conventional A/C or 3.2% of the year, the energy consumption was 

greatly increased. The dual-system approach resulted in a 44% annual energy savings. This 

modeling suggests significant cost savings that homeowners could achieve while improving the 

overall comfort of their homes.

Overall, this report addresses each of the previously mentioned topics while detailing the benefits 

of decoupling the humidity and temperature control such that a more comfortable living 

environment may be provided while minimizing electric consumption. It examines the system 

performance of the combined A/C and WHD to analyze energy and costs savings, as well as 

improved comfort conditions.
8

2 Methodology

2.1 Research Focus

The primary questions addressed by this research were: 

• What is the expected efficiency of an HPWH located within the conditioned volume of 

hot-humid climate homes? 

• What types of space conditioning implications are associated with utilizing a HPWH in 

the conditioned space of the home?

• Can a more energy-efficient home be created by separating the mechanical systems that 

address sensible and latent cooling? 

• Can indoor comfort levels be improved by primarily addressing sensible load with a 

central A/C and latent load with a WHD?

2.2 Equipment Monitored

There were two main equipment sets that were remotely monitored in order to provide the data 

for this study: (1) an HPWH; and (2) a WHD. In addition, the air handler units’ (AHU) runtime 

and indoor/outdoor conditions (temperature and RH) were monitored with Onset’s HOBO data 

loggers. 

2.2.1 Heat Pump Water Heater

Figure 4 shows the HPWH that was monitored in this study. This unit is a General Electric 

GeoSpring 50-gal hybrid water heater (first generation) and is located in the unvented attic of the 

CEH. The hot water that is provided by this unit is used in the upstairs bathrooms of the home.

There is an additional HPWH that services the lower level water fixtures that was not monitored

due to logistical reasons.

Figure 4. Cover removed from top of the monitored HPWH
9

2.2.2 Whole-House Dehumidifier

A dehumidifier utilizes refrigeration (vapor-compression cycle) to cool the inlet air below its 

dew point so that condensate forms on the evaporator coil and drains to the condensate pan. The 

air first passes by the evaporator coil to remove moisture and cool the inlet air, then flows over 

the condenser coil which reheats the air. The net result is warmer, drier outlet air to be supplied 

back to the home. Figure 5 provides terminology for the various components of the dehumidifier.

Figure 5. How the monitored dehumidifier works

The WHD that was monitored in this study was a Honeywell TrueDRY DH150 dehumidifier 

(with a daily condensate removal capacity of 150 pints/day @ 80°F/60% RH inlet air and an 

energy factor of 3.56 L/kWh). The sizing of this unit followed Honeywell’s recommendation 

based on the square footage of the home. 

This unit is located in the unvented attic of the CEH. This dehumidifier also provides wholehouse supply-only ventilation. A 6-in. duct brings in outdoor air (~69 CFM), mixes with air from 

a 10-in. return duct (~330 CFM) from the stairwell of the home, is dehumidified as needed, and 

is supplied back to the home via a 10-in. supply duct (~399 CFM). For the CEH duct layout, the 

10-in. supply duct feeds three branches going to the master bedroom, play room, and family 

room. Figure 6 shows a simplified schematic of the dehumidifier port configuration. 

Figure 6. Schematic of dehumidifier ports

Outdoor air 

inlet

Indoor air 

return

Indoor air 

supply

Evaporator

Condenser

Fan

Condensate pan

Outdoor air 

inlet

Indoor air 

return

Indoor air 

supply
10

Control of this unit was specified to be Honeywell’s TrueIAQ digital control, which can display 

indoor and outdoor temperatures and RH, the desired humidity setting, maintenance/service 

reminders, and can control whole-house ventilation to meet ASHRAE 62.2-2010 requirements. 

Unfortunately, the system was installed with a basic dial humidistat, so whole-house ventilation 

is provided only when RH levels in the home exceed the desired humidistat set point.

2.2.3 Central Air-Conditioning System

Ideally, CARB would have fully monitored both A/C units to be able to have a direct comparison 

between the dehumidifier and the A/C system, but the equipment layout and timing of this 

project did not allow for these A/Cs to be fully monitored. Only the runtimes of the two AHUs

were able to be monitored. The entire central A/C system consists of two units that provide a 

combined cooling capacity of 4 tons (2 tons per unit). Figure 7 shows the home’s outdoor 

condenser units (left image) and one of the indoor AHUs (right image).

Figure 7. Outdoor condensers and the AHUs for the second floor

2.3 Measurements

The monitoring of the mechanical equipment (HPWH, WHD, and central A/C) was carried out 

over a two-stage process. This process included gathering initial one-time readings at the start of 

data monitoring and continuously collecting various measurements in increments of 10 s—while 

outputting sums and averages in 15-min intervals—throughout the entire testing period.

Although house temperature and RH were measured within the return of the WHD, Onset 

HOBO remote temperature/RH sensors were installed to quantify these conditions in the central 

stairwell. An additional temperature/RH sensor was located outdoors. See Appendix A of this 

report for the detailed list of measurements taken and calculations that were performed for this 

field monitoring.
11

3 Energy Modeling

Hourly energy simulation was performed with NREL’s BEopt EnergyPlus v.1.3 software 

(BEoptE+) to estimate annual energy usage, utility bill costs, indoor conditions, and general 

system performance for various control configurations of the mechanical systems. As individual 

mechanical systems are being evaluated, it was assumed that the full cost of each system would 

be paid in cash rather than financed. Installation costs were selected from the National 

Residential Efficiency Measures Database (NREL 2012). The economic values used in the 

energy simulations are summarized in Table 4. 

Table 4. Inputs of Economic Analysis

Economic Variables Modeling Inputs

Electricity Rate* $0.11/kWh + $8.00 monthly charge

Natural Gas Rate* $1.32/therm + $8.00 monthly charge

* State average for Florida

3.1 Predicted Performance of the Heat Pump Water Heater

The costs of various DHW options were evaluated over their respective projected life

expectancies. Table 5 displays a breakdown of lifetime costs that the energy simulation software 

uses to determine the finances of each system. It is important to note that the HPWH is modeled 

with a shorter lifetime and with a significantly higher initial cost than other tank units. The 

shorter estimated lifetime cost of the HPWH is due to uncertainties that develop because the

technology is relatively new on the market and the unit’s slightly more complicated system 

components than common ERWHs—with the main component difference being its compressor.

The higher initial cost is because these HPWHs use a newer technology and HPWHs may 

require two technicians to install due to the added weight of the heat pump components and the 

additional routing of a condensate drain line. Note: Installed cost includes purchase price and 

installation cost. 

Table 5. Lifetime Costs of Various DHW Technologies

Unit Installed

Cost

Annual 

Utility 

Cost

Lifetime 

(years)

Lifetime 

Cost

Cost per Year 

(Lifetime 

Cost/Lifetime)

Electric Tank (0.92 EF) $708 $177 13 $3,015 $232

HPWH (2.35 EF) $2,100 $64 10 $2,737 $274

Gas Tank (0.59 EF) $797 $138 13 $2,589 $199

Gas Tankless (0.82 EF1) $1,160 $81 20 $2,774 $139

1 A one-time energy factor derate of 8% is included due to cycling inefficiencies (Davis Energy Group 2006).

It is likely that an HPWH will also be able to reliably supply hot water to a home for 13 years, 

like other tank water heaters. If the HPWH does last this long, the technology becomes more 

financially competitive with traditional tank water heaters. A 13-year HPWH life expectancy 

would give the unit a lifetime cost/year metric of $225/yr. At this rate, the HPWH can be a costeffective alternative to traditional ERWHs. However, to be the least-cost option based on this 
12

cost analysis, the first cost for the HPWH would have to be $753 or less, which is not likely even 

once the technology is market mature.

Annual lifetime costs show that a gas tankless water heater offers the greatest financial benefit 

for the homeowner. However, these financial benefits come with a few caveats in performance. 

With a higher EF rating and no standby losses, it would seem that there is no reason not to install 

a tankless gas water heater (assuming natural gas is available to the home). Even though it is true 

that tankless water heaters can be beneficial, there are some potential concerns:

• Tankless water heaters eliminate the standby loss of tank water heaters, but they do not 

provide hot water any quicker to the faucet. In fact, they may take 5–15 s longer as the 

unit senses the call for hot water and turns on the burner.

• Tankless water heaters typically require a minimum flow rate of 0.5 gpm to fire, based on 

their ability to modulate burner capacity. This limitation is designed to prevent continual 

operation in the case of a leak or during the event of a faucet not being fully shut off. 

Additionally, a minimum allowable flow rate helps to control the risks of the heat 

exchanger overheating. With low-flow fixtures/aerators being more standard watersaving features, this flow rate may not always be met. 

• If the water source has a high mineral content, scaling of the low-mass heat exchanger 

can occur. The heating coils need to be flushed with a descaling solution per 

manufacturer’s specifications, but most homeowners are not aware of this requirement.

• Tankless water heaters require venting for combustion air and to get rid of hot 

combustion exhausts. This venting introduces an additional cost over electric water 

heaters. Additional care also must be taken to ensure that carbon monoxide from the 

exhaust does not leak into the living space

• The gas line usually needs to be upsized from conventional 1⁄2-in. diameter piping to 

3⁄4-in. diameter piping because of the increased capacity required to heat the water more 

quickly.

Energy simulation was used to determine potential space conditioning benefits that can be 

achieved by providing DHW heating through an HPWH. When operating in heat pump mode, air 

that passes through the HPWH will be both cooled and dehumidified. Table 6 shows the 

estimated influence that an HPWH has on various segments of space conditioning. Since 

Orlando is a cooling-dominated climate, the greatest space conditioning benefits are realized 

through cooling (both sensible and latent). Most directly, it will provide a “free” added benefit in 

cooling months by both cooling and dehumidifying. Still the overall impact on space 

conditioning based on current energy models is minimal.

Table 6. Space Conditioning Utility Costs of DHW Heating Methods

DHW Heating Method Cooling ($) Heating ($) AHU Fan ($) Total ($)

Electric Tank 570 115 53 738

HPWH 562 118 52 732

Gas Tank 575 111 54 740

Gas Tankless 568 116 53 737
13

Finally, the utility costs of each DHW heating method were analyzed as the combined cost that

resulted from water heating and space conditioning utility costs (Table 7). This table indicates 

that the HPWH is lower than each of the other DHW options. As a result, the HPWH provides 

the lowest total operating cost alternative for supplying hot water to a home under these 

assumptions.

Table 7. Combined Water Heating and Space Conditioning

Utility Costs for Various DHW Methods

DHW Heating Method Cost

($)

Electric Tank 915

HPWH 795

Gas Tank 878

Gas Tankless 818

Table 8 once again looks at the cost of various DHW options evaluated over their respective 

projected life expectancy, but in this case, the space conditioning impact was included. The 

space conditioning with the electric tank cost was considered the base case for assessing the 

space conditioning impact of the alternative heating methods. It is unlikely that an HPWH will 

ever compete with a gas tankless water heater, but if comparing to traditional ERWHs, this 

technology has promise once the HPWH market matures. 

Table 8. Lifetime Costs of Various DHW Technologies Including Space Conditioning Impact

Unit

Installed 

Cost

($)

Annual 

DHW 

Cost

($)

Annual Space 

Conditioning 

Impact

($)

Lifetime 

(years)

Lifetime 

Cost

($)

Lifetime 

Cost/Year

($)

Electric 

Tank

(0.92 EF)

708 177.45 – 13 3,015 $232

HPWH

(2.35 EF) 2,100 63.68 –6.05 10/13 2,676/

2,849

268/

219

Gas Tank

(0.59 EF) 797 137.86 2.20 13 2,618 201

Gas 

Tankless

(0.82 EF*)

1,160 80.69 –0.59 20 2,762 138

* A one-time EF derate of 8% is included due to cycling inefficiencies (Davis Energy Group 2006).

Currently many utilities are providing incentives of $400–$1,000 to homeowners who install 

HPWHs. Additionally, a federal tax credit of $300 is available. With a ~$1,000 incentive/credit

included in the costs analysis, the lifetime cost per year reduces to $168 or $142 for a 10- or 13-

yr life expectancy, respectively. Again, this analysis does not include fuel price escalation, so the 

future cost of fuels will have a major impact on whether this technology is an ideal solution.
14

There is an interesting inverse effect with HPWHs in the hot-humid climate zone. While the 

cooling/dehumidification benefits and higher COPs of an HPWH would be more advantageous 

in the hot-humid climate, mains water temperature tends to be higher. The mains water 

temperature ranged from 75°–85°F over the monitoring period. This means less water heating is

required than in a cold climate, resulting in the overall water heating cost being minimized. 

Therefore, the cost benefit of the HPWH is slightly diminished.

3.2 Predicted Performance of the Whole-House Dehumidifier

Table 9 presents the total percentage of time that various cooling configurations are predicted to 

allow indoor conditions to exceed ASHRAE-recommended humidity ratio levels. This table

shows the simulated percent frequency that this humidity ratio threshold is exceeded throughout 

both the entire year and just the summer months of June through August. It is useful to analyze 

each configuration as both annual and summer only cases, as the dehumidifier will be likely to 

run year round while the A/C will typically run only in sensible-cooling demand months. The

hourly results from the energy simulation suggest that a WHD and A/C control configuration of 

55% RH and 78°F, respectively, can adequately manage indoor humidity ratio levels to 

acceptable conditions for the entire year. Alternatively, to provide comparable (1.44% worse)

conditions with an A/C only configuration, the unit’s set point would need to be maintained 

below at least 75°F.

Table 9. Percent Frequency of Conditions > 0.012 lbm Water/lbm Dry Air

Control Scenario > 0.012 lbm Water/lbm

Dry Air > 60% RH

A/C 

(°F)

Dehumidifier 

(%)

Entire Year

(%)

6/1–9/1

(%)

Entire Year

(%)

6/1–9/1

(%)

78 65 17.03 19.59 14.98 17.21

78 60 13.85 15.42 5.48 3.58

78 55 0.00 0.00 0.09 0.00

78 50 0.00 0.00 0.03 0.00

78 off 17.85 19.59 15.98 17.66

75 off 1.44 0.00 15.87 18.25

72 off 0.56 0.00 19.95 31.79

70 off 0.56 0.00 23.40 44.79

Even with indoor air humidity ratios being maintained below the recommended threshold, it is 

also essential that RH is controlled below ~60%. Table 9 shows energy simulation results of RH 

maintenance for each cooling configuration during both the entire year and the summer months 

of June through August. An A/C and WHD at 78°F and 55% RH has the capability of providing 

sub-60% RH conditions for > 99.91% of the entire year. In contrast, even though an exclusive 

A/C configuration can control the humidity ratios to acceptable ASHRAE levels, it is not a 

viable option for satisfactory 60% RH control. A primary reason for this occurrence is that the 

RH of air will naturally increase as the air temperature decreases—colder air can hold less 

moisture resulting in higher RH (even with the same humidity ratio). 
15

Figure 8 shows the annual energy costs of each space conditioning configuration. For this 

analysis, space conditioning is categorized between cooling, heating, and HVAC fan/pump use. 

In general, each of the acceptable A/C only configurations (that are able to maintained ASHRAE 

acceptable humidity ratio levels) will generate moderately higher utility costs than their 

equivalently comfortable combined system counterparts. Table 10 shows the percentage and total 

cost savings that can be achieved by providing cooling with the combined A/C and WHD system 

set to 78°F/55% RH system over just the A/C system set to 75°F. More impressively, each of 

these monetary savings will come as an added benefit to the improved comfort and heightened 

living conditions that the dehumidifier provides. Note: Columns with titles containing 

temperature and RH represent combined A/C-WHD configurations and columns titled with 

temperature only represent A/C only configurations.

Figure 8. Annualized space conditioning costs of building with various cooling configurations

Table 10. Space Conditioning Savings of 78°F/55% RH Case Over 75°F Case

Space Conditioning Savings

(%)

Savings

($)

Cooling/Dehumidification 6.7 37

Heating 10.2 5

HVAC Fan 21.4 11

Total 8.2 53

• Cooling utility bill savings are a result of the A/C unit not operating as often due to the 

WHD handling the majority of the latent load. 

Adequate Humidity Ratio

Adequate Relative Humidity
16

• Heating utility bill savings result from the dehumidification sensible heat byproduct 

offsetting some of the heating load requirements.

• HVAC fan utility bill savings are a result of the minimized operation of the AHU related 

to the reduced cooling and heating requirements.

It is useful to consider these utility costs in a metric that accounts for the moisture-related 

comfort that they provide. The 78°F and 55% RH arrangement will support acceptable RH levels 

for 8,752 h/yr while the A/C only case at 75°F will only support acceptable RH conditions for 

7,360 h/yr. With this information, a metric that describes the annual building energy cost per 

number of comfortable hours per year can be derived for each. The combined A/C system with a 

WHD is predicted to provide comfortable moisture conditions at the rate of $0.21/h, while the 

A/C system is predicted to carry a rate of only $0.23/h. This suggests that the combined A/C and 

WHD setup can provide far more moisture-adequate hours per year at a lower annual cost.

As seen in Table 11, the A/C system likely cannot be downsized significantly enough to offset 

the added cost of the WHD. By optimizing the A/Cs for sensible cooling, it would be possible to 

reduce each 2-ton heat pump to a 1.5-ton heat pump. The initial cost difference was estimated to 

be $1,652 ($2,600 for the dehumidifier and –$948 for downsizing the two heat pumps). Based on 

an 11-yr life expectancy of the dehumidifier, the lifetime cost is 60% higher for the combined 

A/C-WHD configuration. This cost analysis doesn’t account for the added comfort associated 

with complete (temperature and RH) space conditioning control provided be the combined A/CWHD configuration.

Table 11. Combined A/C and WHD Cost Analysis 

Cooling Configuration Initial Cost 

($)

Lifetime 

(yr)

Additional A/C

($/yr)

Lifetime Cost 

($)

A/C only (75°F) – 11 54 589 

A/C-WHD (78°F/55%) 1,652 11 – 1,652
17

4 Results From Field Monitoring

4.1 Heat Pump Water Heater

Long-term monitoring was performed to assess the on-site performance of the HPWH unit in an 

occupied setting. Table 12 shows a data summary of the performance results that the unit 

achieved between June 22, 2012 and October 31, 2012. It is important to consider that this data 

summary provides results for only one of the two HPWHs that supplied DHW needs to the house

(though it was the primary water heater for the bathrooms). As seen from the data summary, the 

unit effectively provides DHW via the heat pump for the majority of the needs of the house 

while only deferring 23% of energy use (not thermal fraction) toward the electric resistance 

heating elements.

Table 12. HPWH Monitored Performance Summary

(June 22, 2012 to October 31, 2012)

Hot Water Set Point 120°F

Average Water Inlet Temperature1 82°F

Average Water Outlet Temperature1 117°F

Total COP 2.2

% Electric Resistance2 23%

Average Hot Water Use 48.8 gal/day

Average Inlet Air Temperature 74°F

Average Inlet Relative Humidity 55% 1 Average estimated with 15 min. periods containing near-continuous flow. 2 % electric resistance = % of total kWh consumed by resistance.

Figure 9 shows the daily COP of the HPWH plotted against the measured water draw for the 

day’s period. As seen from the plot, there are a number of days where a large electric resistance 

load was present and as a consequence, the unit operated with a lower COP value. The system 

switched over to electric resistance heating due to large volumes of hot water being drawn over a 

short period of time. Still the efficiency for days when the electric resistance percentage 

exceeded 4% was higher than a standard ERWH. Additionally, standby losses from the tank and 

piping also have an effect on the HPWH unit’s performance. This impact is especially 

pronounced at low daily hot water consumption (< 20 gal/day).
18

Figure 9. HPWH COP versus water draw

The unit operated with an average COP of 2.2, while achieving a maximum COP of 3.0 over the 

course of the monitoring period. In comparison, the manufacturer-rated EF for this HPWH is 

2.35. This difference is not surprising because the EF is calculated under lab conditions with an 

unrealistic draw profile, while our monitored data are specific to the conditions and loads 

observed. However, on average, this HPWH operated at a 144% higher efficiency than the rate 

efficiency of a comparable ERWH (with a COP of 0.9) despite the fact that the hot water loads 

are smaller than the EF test loads.

To quantify the performance of the heat pump based on hot water demand, the daily COP of the 

unit was plotted against the total water drawn for that day. To develop a performance model for 

instances where the unit was running in heat pump mode, days with > 4% electric resistance total 

energy consumption were excluded from the regression. A performance curve for the HPWH 

was fitted using the exponential form of Equation 1. 

( ) bV COP = a 1 − e (1)

The fitting coefficient a represents the COP of the tank without thermal losses (COPadiabatic). The 

fitting coefficient b represents the losses of the tank as a function of water draw. The variable V

corresponds to the total volume of water drawn for one day. The fitting coefficients that

described the performance of this General Electric HPWH unit are displayed in Table 13.

days with electric resistance 

energy >4% of total use
19

Table 13. COP Regression Fitting Coefficients

Fitting Coefficients

a 2.73

b –0.038

The hot water draw profile is a primary factor on the unit’s performance. The average draw 

profile for this unit over the monitoring period is provided in Figure 10. The draw profile is 

compiled as the average water drawn during each 15-min time period starting at 12:00 a.m. each 

day. Since there is a often a significant water draw at midnight, daily COP values were 

calculated starting at 3:00 a.m. and going to 3:00 a.m. the next day to avoid major recovery

periods extending into the next day averages.

Figure 10. HPWH average water draw

The water draw profile of this occupied home is shifted more to the evening hours. Figure 11

provides a comparison of the draw profile of this occupied home, the Building America 

normalized draw profile (for a water heater servicing four bedrooms and three bathrooms with 

weather data from the Orlando International Airport location), and the draw profile used for the 

EF rating test. Variations in draw profiles are the hardest attribute of HPWHs to model as they 

are highly dependent on individual homeowner’s behavior. Even though these hybrid water 

heaters provide hot water regardless of homeowners’ usage patterns, the efficiency of these units 

is greatly impacted based on the total quantity and frequency of hot water draws.
20

Figure 11. Comparison of various DHW draw profiles

The HPWH unit will call upon electric resistance heat when the heat pump mode cannot supply 

heat to the water at a fast enough rate to replenish the hot water being drawn from the tank. As a 

result, when large quantities of water are drawn in short period of time, the heat pump cannot 

transfer enough heat to the tank to support the high demand. Figure 12 shows the water draw 

profile in which 97.7 gal were drawn over the course of the day and the unit performed with a 

COP of only 1.4. This low COP was largely influenced by the high electric resistance energy use 

between the hours of 9:30 p.m. and 10:30 p.m. The HPWH required the use of the electric 

resistance elements in order to recover from the series of hot water draws that occurred shortly 

before (8:45 p.m. to 10:15 p.m.). Note: Hot water draws are shown as gal/15-min periods.

Figure 12. Water draw profile resulting in low daily COP (COP = 1.4)

0

2

4

6

8

10

12

Water Draw [gal.]

Time of Day

Orlando Profile (48.8 gal) BA Benchmark (37.6 gal) EF draw (64.3 gal)
21

If the draw profile follows a more distributed pattern of use throughout the day, rather than high 

concentrations of use in short periods of time, it is likely that the HPWH will be more able to 

support the entire DHW load with heat pump operation. As a result of avoiding the use of 

electric resistance elements, a higher COP can be achieved. The unit will not need to revert to 

electric resistance as a supplementary heat source; it will be able to comfortably replenish heat to 

the tank with the vapor compression cycle alone. Figure 13 displays the water draw profile and 

energy consumption for an alternative day in which the total water consumption was only 1.3 gal 

less than the previous “low COP draw profile case.” However, due to the spaced out water draw 

pattern, the HPWH was able to operate with a daily COP of 2.6—which is more than 85% more 

efficient than the previous case’s daily COP of 1.4. Even though roughly the same daily amount 

of hot water was used, the heat pump was able to replenish the tank’s hot water without utilizing 

the electric resistance elements. As a result, all of the heat transfer to the tank was accomplished 

through the heat pump and more efficient performance operation was achieved. This comparison 

speaks to the significant impact of load patterns as well as the sensitivity of the HPWH’s control 

logic.

Figure 13. Water draw profile resulting in high daily COP (COP = 2.6)

Figure 14 displays the water draw profile and energy consumption from a low total water usage 

day. On this day, the homeowner used only 3.3 gal of hot water over the course of the entire day.

The day contained a series of very small draws, with none of them exceeding more than 1 gal 

over the course of a 1-h period. As a result, the unit was able to operate with a daily COP of only

0.3. As seen from Figure 14, the heat pump cycle runs for a period of time even though there is 

essentially insignificant amount of hot water drawn over the course of the day. This pattern 

indicates that the heat pump is running to replenish heat that is lost into the surrounding space

through the tank walls, referred to as standby losses. 
22

Figure 14. HPWH performance with minimal water draw (COP = 0.3)

An HPWH also affects the temperature and humidity of the ambient air surrounding the unit. As 

the ambient airstream passes through the HPWH at a rate of roughly 175–200 CFM, the air 

temperature is decreased and moisture is extracted through condensation. Figure 15 displays the 

change in temperature that occurred during typical HPWH operation. On average, the airsteam 

temperature was decreased from ~75°F to ~60°F. In a hot-humid climate, such as Orlando, this 

will serve as an added benefit for a majority of the year. 

Figure 15. Air temperature change through HPWH

0

10

20

30

40

50

60

70

80

90

6/22 7/2 7/12 7/22 8/1 8/11 8/21

Temperature [°F]

Air Temp Decrease Air In Temp Air Out Temp
23

In addition to temperature change, humidity properties of the airstream are also affected. 

Figure 16 shows the change in humidity ratio that occurred as air passed through the heat pump. 

As moisture is extracted in the form of condensate on the HPWH evaporator coil, the humidity 

ratio of the airstream decreases from ~0.011 to ~0.009 lbm water/lbm dry air. Even with moisture 

being removed, the RH of the airstream increases due to the decrease in air temperature (from 

~60% to ~90%). As air temperature decreases, it possesses less of an ability to hold moisture. 

Although the outlet airstream has a higher RH, once this air mixes with the surrounding air, the 

lower moisture content of this air will result in dehumidification of the surrounding space. 

Figure 16. Humidity ratio change through the HPWH

The impacts of the HPWH on the space conditioning systems depend heavily on climate, home 

configuration, HPWH location, and the space conditioning systems used. Analysis of the air

enthalpy change through the HPWH showed that cooling capacity averaged 5,540 Btu/h (with a 

maximum of 7,696 Btu/h) when the unit was operating continuously over a 15-min logging 

period. This cooling capacity is sufficient to impact the room in which the HPWH is located.

Many manufacturers are offering ducting kits (typically less than 10 ft combined between return 

and supply ductwork) to allow designers to better integrate HPWHs into the control of the space 

conditioning load of the home.

Figure 17 displays a bar graph of the volume of the condensate removed for daily periods. 

During the case of maximum moisture removal, the HPWH is responsible for extracting more 

than 4.5 pints/day (or ~4.9 kBtu of latent heat removal). However, this number is somewhat 

deceptive toward relaying the HPWHs dehumidification capabilities since the unit is operating in 

a home that is already being dehumidified by a WHD. 

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.020

6/22 7/2 7/12 7/22 8/1 8/11 8/21

Humidity Ratio [lb water/lb dry air]

Air HR Decrease Air In HR Air Out HR
24

Figure 17. HPWH condensate removal

Since the HPWH is operating only while there is a demand to replenish hot water, a “pints/day” 

metric provides only small insight into the latent performance of the unit. Figure 18 shows a 

scatterplot that describes the “pints/kWh” removal rate of the unit on a daily basis. The best fit 

line resulting from a linear regression fit (though the coefficient of determination is not strong) 

reveals that this unit was operating at an average removal rate of 1.17 pints/kWh.

Figure 18. HPWH condensate removal performance 
25

4.2 Whole-House Dehumidifier

Initial power measurements for each of the mechanical systems, which are responsible for 

controlling the sensible and latent cooling load in the house, are shown in Table 14. In this test 

home, the WHD draws approximately 830 W during normal operation, while the two central A/C 

systems draw a combined load of 3,330 W. As seen from the power consumption values, the 

home’s A/C system uses more than four times as much power as the WHD, so minimizing the 

A/C system operation as much as possible could be advantageous in terms of energy savings.

Table 14. Power Draw of HVAC System Components

Mechanical System Power 

(W)

WHD 830

Condenser Unit 1 1,460

Condenser Unit 2 1,460

AHU 1 173

AHU 2 237

As seen from Table 14, there is a difference in power consumption between the two AHUs. The 

higher power draw in AHU 2 can be attributed to greater static pressures that exist in this 

system’s duct configuration. As air resistance in the ducts increases, the AHU’s electronically 

commutated motor compensates by increasing power consumption to maintain the desired 

airflow rate. 

In order to quantify the latent heat removal efficiency of each of these units, it is useful to look at 

their performance when running at maximum capacity. The energy performance quantifies the 

volume of condensate that is removed from the airstream per kWh of dehumidifier power 

consumed by the fan and compressor. The product capacity denotes the maximum amount of 

condensate that the unit can remove (pints/day) while running continuously for 24 h. According 

to the manufacturer’s product specification sheet, the Honeywell DH150 WHD has a capacity of 

150 pints/day and an energy factor of 3.56 L/kWh (or 7.51 pints/kWh). These metrics are 

determined under lab conditions with a constant 80°F, 60% RH supply airstream. 

Even though standard A/C units are often used for moisture removal purposes, manufacturers do 

not regularly publish their rated latent energy performance (pints/kWh) and moisture removal 

capacity (pints/day). Rather, the manufacturer will publish a table of sensible-to-total capacity 

(S/T) ratios that describe A/C performance under various conditions. The expanded performance 

table for the Lennox AHU and condensing unit that were used in this study were used for this 

analysis.

In order to compare A/C latent energy removal rates with published dehumidifier latent rates, the 

potential A/C moisture removal performance was calculated at similar conditions to the WHD’s

performance data. The equivalent capacity metrics for the two 2-ton A/C systems combined are 

displayed in Table 15. For these calculations, the S/T ratio was selected under conditions of

outdoor dry bulb of 85°F, AHU flow of ~720 CFM, entering dry bulb of 80°F, and entering wet 

bulb of 67°F. 
26

The energy performance (pints/kWh) was determined by calculating total latent capacity (at an 

S/T of 0.74) and assuming the average heat of vaporization (at ~75°F) to be 1,050 Btu/lbm. 

Power consumption values from initial HVAC measurements were used to determine kWh usage 

of the equipment over a continuous run scenario of a 1-h period. The condensate capacity is 

calculated as the volume of condensate that could potentially be removed if the system were

running at maximum capacity for 24 h.

Table 15. Equivalent A/C Capacity Metrics

Equivalent A/C Capacity Metrics

Total Capacity 48,000 Btu/h

S/T ratio 0.74

Condensate Capacity 274 pints/day

Energy Performance 3.42 pints/kWh

As seen from calculated values, the A/C system’s condensate capacity of 274 pints/day is almost 

double the dehumidifier’s capacity of 150 pints/day. Even though the A/C may theoretically be 

able to extract a greater volume of condensate, it will be removed at a less efficient rate. The 

unit’s calculated EF of 3.42 pints/kWh is significantly lower than the WHD’s published value of 

7.51 pints/kWh. As a result, the A/C carries a condensate removal energy performance that is 

54% lower than the WHD’s when operating at capacity. It is important to note that these energy 

metrics will be much smaller for daily use as typical outdoor conditions will not generally 

require the system to run at maximum capacity for entire-day spans. Overall, these results 

suggest that specifically controlling the latent cooling load with a WHD and addressing the

sensible cooling load with a central A/C unit can maximize cooling system efficiency.

Long-term performance monitoring of these systems was also carried out in order to examine 

their operation in an installed environment. For a monitoring period between mid-June and midNovember, the home’s indoor set points were maintained with the A/C thermostat at 76°–78°F 

and the dehumidifier set to 65% RH. Indoor temperature and RH levels were monitored in an 

upstairs living room for the entirety of this period. Figure 19 displays the frequency distribution 

of RH occurrences. The RH within the home was maintained at levels < 60% for 99.86% of the 

monitored period. 
27

Figure 19. RH frequency distribution chart

Maintaining adequate temperature levels within the building environment is equally important 

for achieving human comfort. Figure 20 shows the frequency distribution of temperature levels 

maintained within the second-floor living room of the house. The ASHRAE summer comfort 

zones suggest human comfort at summer conditions can be achieved at higher temperatures as 

long as RH is maintained at a lower level. While keeping adequate indoor RH levels (< 60%), 

the home’s indoor temperature was kept within the comfort zone (~74°–80°F) for 91.5% of the 

data monitoring period.

Figure 20. Temperature frequency distribution chart
28

It is important to note that a majority of the occurrences where indoor temperature was beyond 

the comfort design range was during times when the outdoor temperature was colder than A/C 

set point requirements. During the monitoring period, outdoor temperature dipped to as cold as 

45°F on some nights. Therefore, since the A/C would not be called on at this temperature, the 

sub-74°F indoor conditions should not be directly attributed to the A/C running more frequently 

than needed in order to maintain RH. Therefore, the home was kept within the comfort zone 

temperature range for 99.3% of the cooling portion of the data monitoring period. Overall, these 

temperature data support the idea that the presence of a dehumidifier allows the A/C to be fixed 

at a higher set point. As a result, the A/C is no longer required to run more often (in order to 

address latent loads) than required to achieve the desired indoor temperature. 

The total volume of condensate removed was monitored to assess the moisture-removal 

effectiveness of the dehumidifier. The maximum condensate volume removed over the course of 

the monitoring period was 96 pints in one day. This high condensate removal occurred during a 

period of atypical conditions with long periods of heavy rainfall. If an A/C had been solely 

responsible for removing this amount of moisture, it would have needed to run for roughly 35% 

of the day to pull the same amount of moisture out of the air. Unless sensible cooling was also 

required, this could have resulted in the space overcooling.

On November 12, 2012, a water alarm sensor that SWA installed as a precautionary measure was 

tripped, suggesting that water was leaking onto the wood platform that the dehumidifier and 

drain pan sat on. The condensate pipe connection to the dehumidifier was leaking and 

condensate was dripping into the drain pan. This was immediately corrected by the contractor. 

During the data analysis, it was estimated that the leak may have started as early as August 24, 

2012 and continued to worsen over time until our water alarm was triggered. Unfortunately, we 

were measuring the condensate only through the condensate pipe and did not plumb the drain 

pan to our condensate measurement device.

Figure 21 shows three distinct time periods of our monitoring. The red data points show that 

even though the dehumidifier is running, we were not capturing the condensate that was being 

produced. This was confirmed by evaluating the enthalpy differential during operation based on 

airside measurements. The enthalpy differential remained fairly consistent over these three 

periods, so the condensate data from August 24, 2012 through November 5, 2012 were ignored. 

The manufacturer rated this dehumidifier with a moisture removal rate of 7.51 pints/kWh (at 

80°F and 60% RH inlet air). Figure 21 shows actual moisture removal performance metrics 

achieved throughout the monitoring period. A linear regression fit of the pre-August 24 places 

the average performance slope at 4.95 pints/kWh. This is lower than the manufacturer’s rated 

performance, but as the rated performance is at a single condition, it is not surprising.
29

Figure 21. Dehumidifier condensate removal versus energy performance

The immediate goal of the dehumidifier is to extract moisture from the air and remove it from 

the confines of the building envelope. The RH of indoor air can be an effective indicator of 

moisture-related comfort conditions. Figure 22 shows that the dehumidifier reduces humidity 

ratio as the mixture of fresh outdoor air and building return air passes across its evaporator coil. 

As condensate is removed from the airstream, the humidity ratio of the airstream decreases from 

~0.011 to ~0.008 lbm water/lbm dry air. The RH of the airstream is reduced from ~55% to ~20%. 

However, this RH decrease is attributed to the temperature increase that occurs as heat is 

transferred from the condenser coil. Essentially, the dehumidifier is converting latent heat energy

contained in the moist inlet stream to condensate and sensible heat in the warmer-dryer outlet 

airstream. Figure 23 displays the effect that the dehumidification process has on the temperature 

of the airstream. On average, the mixed inlet air, with a temperature of ~75°F, exits the unit at an 

increased temperature of ~95°F.
30

Figure 22. Humidity ratio change of airstream through dehumidifier

Figure 23. Temperature change of airstream through dehumidifier

From November through April, half of the year, this temperature increase can serve as a small

added benefit to the moisture removal process. Heat that may have been otherwise required to be 

supplied by an additional source (furnace, heat pump, etc.) is acquired as a “free” byproduct of 

the WHD’s operation. However, during summer months, the central A/C system will need to 

address this sensible heat gain.

Sensible, latent, and total heat changes that occurred as air passed through the dehumidifier were

calculated for the entire monitoring period. Heat removal values were calculated using the 
31

differential enthalpy changes that resulted as a function of temperature increase and RH

decrease. Figure 24 shows the daily totalized heat transfer into and out of the airstream over the 

monitoring period through August 24. It is important to note that these calculations serve only as 

estimates of the true heat transfer effects that occur in the dehumidification process. The sum of 

sensible heat gain over the entire period equates to approximately 25% of the total sensible 

cooling load predicted by energy simulations. The estimates seem fairly reasonable with the heat 

of vaporization accounting for roughly 50% of the sensible heat gain. 

Figure 24. Daily heat into dehumidifier airstream

Additionally, short-term scenario testing was performed during November and December 2012 

to evaluate how well various set point configurations of the combined A/C and WHD system 

fared. Three short-term testing cases, each 4–10 days in total length, were run and the system’s 

performance and home’s comfort conditions were continuously monitored. Table 16 shows 

various control configurations of A/C and WHD that could be used to provide comfortable 

conditions within a home. This table shows the total monitoring time of each scenario (days), 

total energy used by the A/C system and WHD (kWh), average outdoor conditions during the 

period (°F and % RH), and the daily energy to provide cooling and dehumidification to the 

building (kWh/day). It is important to note that the cooling energy rate is not normalized per 

outdoor temperature conditions and serves only as an estimate of cooling energy use per day. In 

general, WHD and AHU runtime is often highly dependent on factors other than outdoor 

temperature and humidity. Indoor temperature and RH levels are influenced strongly by solar 

heat gains and internal heat gains (occupants, cooking, lights, appliances, etc.). Therefore, 

weather normalization for a cooling-dominated climate does not always provide an apples-to-
32

apples comparison between varying outdoor conditions. As a result, it was not performed for this 

section of the analysis.

Table 16. Scenario Test Summary

Test Scenario 78°F, 

50% 

75°F, Dehumidifier 

Off

78°F 

60%

Total Time (days) 8.0 4.0 10.3

Dehumidifier Energy (kWh) 72.80 0.00 10.59

AHU #1 Energy (kWh) 1.85 1.24 4.06

AHU #2 Energy (kWh) 4.88 3.87 1.79

Condenser #1 Energy (kWh)

1 15.57 10.49 34.29

Condenser #2 Energy (kWh)

1 30.05 23.85 11.05

Average Outdoor Air Temperature (

o

F) 60.88 63.02 67.93

Average RH (%) 73.31 77.45 81.75

Dehumidifier Energy Rate (kWh/day) 9.10 0.00 1.02

A/C System Energy Rate (kWh/day)

2 6.54 9.76 4.95

Cooling Energy Rate (kWh/day)

3 15.60 9.76 5.97 1 Assumed condenser power draw of 1.46 kW while AHU was operating 2 Combined AHU energy use and condenser energy use 3 Combined A/C system energy use and dehumidifier energy use

Cooling energy rates for each of the scenarios support the same configuration ranking that was 

generated in the BEoptE+ hourly simulation models. Both scenario testing results and energy 

modeling simulations rank the scenarios in the following order (from most energy consumptive 

to least energy consumptive): (1) 78°F and 50% RH; (2) 75°F and dehumidifier off; and (3) 78°F

and 60% RH. Even though the model and scenario tests are in agreement, it is important to 

recognize that the energy modeling is simulating usage over the course of an entire year, while 

the scenario test only accounts for a short time period during November/December 2012. Thus, it 

is important to mention that A/C energy use would be greater during summer months when a 

higher sensible cooling demand is needed. However, these scenario test results effectively 

demonstrate that a WHD can be effective and efficient in controlling latent loads during swing 

seasons in hot-humid climates. Appendix B of this report contains detailed plots of the indoor 

and outdoor data conditions during each of the scenario tests. 

Occupant evaluations are generally the best indicators of true comfort, since comfort is truly a 

subjective metric. Following several months of living with the combined A/C and WHD system, 

the homeowner provided an unsolicited evaluation of the comfort improvement that took place in 

the home. The homeowner stated, “We are happy with the systems and surprised that we can set 

the thermostat so much higher than before and still be very comfortable.” This statement 

supports the notion that a home that addresses sensible cooling loads with an A/C and primary 

latent cooling loads with a WHD can maintain or improve indoor comfort levels while saving 

energy.
33

5 Conclusion

The results of this energy modeling and field monitoring study show support for the use of 

HPWHs as well as combined A/C and WHD cooling systems in hot-humid climates. The HPWH

was able to achieve high COPs while providing some space conditioning benefits to the home.

Additionally, by adding a WHD to the home’s cooling system, the home was able to achieve 

improved indoor comfort while saving energy.

5.1 Heat Pump Water Heater

What is the expected efficiency of an HPWH located within the conditioned volume of hothumid climate homes? 

The HPWH operated at an average COP of 2.2 over the monitoring period. Performance trends 

indicated that the efficiency of the unit was primarily a function of the volume of DHW drawn 

and the distribution pattern of each draw. The optimal draw pattern (for the highest COP) 

includes draws that are spaced just within the heating rate limit of the heat pump, such that the 

heat pump can replenish all hot water to the tank before the next significant water draw. These 

performance conclusions will vary with differences in climate (cold water inlet temperature 

effects and evaporator inlet air), unit location, and hot water loads.

There is an interesting inverse effect with HPWHs in the hot-humid climate zone. While the 

cooling/dehumidification benefits and higher COPs of an HPWH would be more advantageous 

in the hot-humid climate, mains water temperature tends to be higher. The mains water 

temperature ranged from 75°–85°F over the monitoring period. This means less water heating is 

required, resulting in the overall water heating cost being minimized. Therefore, the cost benefit 

of the HPWH is diminished.

What types of space conditioning implications are associated with utilizing an HPWH in 

the conditioned space of the home?

The impact of a HPWH on the space conditioning systems rely heavily on climate, home 

configuration, HPWH location, and the space conditioning systems used. In hot-humid climates, 

the HPWH’s cooling and dehumidification byproduct is desired for a large portion of the year. 

During operation, the HPWH affected the airstream in the following ways: (1) decreases 

temperature from 75°F to 60°F; (2) increases RH from 60% to 90%; and (3) decreases humidity

ratio from 0.011 to 0.009 lbm vapor /lbm dry air. Analysis of the air enthalpy change through the 

HPWH showed that cooling capacity averaged 5,540 Btu/h (with a maximum of 7,696 Btu/h) 

when the unit was operating continuously over a 15-min logging period. For this test home, the 

unit operated with an average heat removal rate of 16,266 Btu/day. Based on power consumption 

of this home’s A/C system running at capacity (4 ton) and local utility rates, this heat removal 

could save the homeowner ~$45/yr. This cooling capacity is sufficient to impact the room in 

which the HPWH is located. In this case, there wasn’t a significant impact on comfort as the 

HPWH was located in the unvented attic.
34

5.2 Whole-House Dehumidifier

Can indoor comfort levels be improved by primarily addressing sensible load with a 

central A/C and latent load with a WHD?

Modeling results from the energy simulation (Table 17) show that a combined A/C paired with a 

WHD cooling system at 78°F/55% RH can maintain more comfortable humidity levels within 

the building than a standard A/C only at 75°F. These results were also confirmed with limited 

data from short-term scenario testing.

Table 17. Percent Frequency of Conditions Over 60% RH

A/C 

(°F)

Dehumidifier 

(%)

Entire Year

(%)

June–

September

(%)

78 55 0.09 0.00

75 off 15.87 18.25

Additionally, long-term monitoring of the coupled system showed that RH within the home was 

maintained at levels < 60 % RH for 99.86% of the monitoring period. As human comfort is truly 

a subjective measure, it is important to note that the homeowner was satisfied with the comfort 

levels in the home and took notice of the fact that comfort could be achieved at higher A/C set

point temperatures when the dehumidifier was running. 

Can a more efficient home be created by separating the mechanical systems that address 

sensible and latent cooling? 

Modeling predicts that a home with separated mechanical systems for sensible and latent cooling 

can see an 8.2% annual decrease in space conditioning (heating, cooling, and dehumidification) 

electrical energy costs. Additionally, manufacturer rating specifications of the dehumidifier and 

A/C unit reveal that WHD can remove moisture from the air at a 54% more efficient rate when 

running at capacity under similar conditions (7.51 vs 3.42 pints/kWh). Actual monitored data 

resulted in an average performance at 4.95 pints/kWh, which is lower than the manufacturer’s 

rated performance for the dehumidifier, but is expected due to the varying operating conditions 

over the monitoring period (versus the single point conditions of the rated performance). Shortterm testing results demonstrated that the combined A/C and WHD configuration at 78°F/60% 

RH is able to use less energy per day than a standard A/C setup at 75°F.

5.3 Next Step

This study indicates favorable results for both of these mechanical systems; however, further indepth research is needed to develop a more robust understanding of their operational 

performance. More research on the space conditioning effects of HPWHs is being pursued by 

SWA to access how these units can affect indoor conditions in various climate regions and space 

configurations. Even though the space conditioning impact of the HPWH in the CEH appears to 

be relatively minimal, homeowners may realize a more pronounced effect in tighter homes or in 

buildings that are located in heating-dominate climates.
35

Additional experimental studies are also needed on WHD-A/C arrangements to further validate 

the effectiveness of this type of combined cooling system and to access other set point 

configurations that can yield acceptable indoor environmental conditions. In addition, two

manufacturers have recently released whole-house dehumidifiers that warrant investigation, as 

they eliminate the sensible heat gain issue typical of dehumidifiers and in the case of one of the 

units, actually provides some sensible cooling.
36

References

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Savings.” Research Products Corporation. Accessed February 6, 2012. 

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Arena, L.; Mantha, P.; Karagiozis, A. (2010). “Monitoring of Internal Moisture Loads in 

Residential Buildings.” U.S. Department of Housing and Urban Development. Office of Policy 

Development and Research. Accessed January 27, 2012. 

www.huduser.org/portal/publications/oth/internalmoistureload.html

ASHRAE. (2010). ANSI/ASHRAE Standard 55-2010:Thermal Environmental Conditions for 

Human Occupancy. 

ASHRAE (2012). Errata Sheet For ANSI/ASHRAE Standard 55-2010: Thermal Environmental 

Conditions for Human Occupancy. Figure 5.2.1.1. Assessed November 9th, 2012. 

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Heaters.” Accessed March 4, 2013. www.energy.ca.gov/title24/2008standards/ 

prerulemaking/documents/2006-05-18_workshop/2006-05-11_GAS_WATER.PDF

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Information Administration. Accessed October 31, 2012: 

www.eia.gov/consumption/residential/data/2005/.

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Information Administration. Accessed October 31, 2012.

www.eia.gov/consumption/residential/data/2009/

Fang, X.; Winkler, J.; Christensen, D. (2011); “Using EnergyPlus to Perform Dehumidification 

Analysis on Building America Homes.” HVAC&R Research Vol. 17(3), 2011. DOI: 

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Florida Climate Center. (2012). Orlando, FL: Relative Humidity. Accessed September 17, 2012.

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Honeywell. (2008a). “TrueDRY DH150 Dehumidification System: Professional Installation 

Guide.” Golden Valley, MN: Honeywell International Inc. Accessed December 12, 2012. 

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pdf

Katipamula, S.; O’Neal, D.L. (1991). “Transcient Dehumidification Characteristics of a Heat 

Pump in Cooling Mode.” ASME Transactions Vol. 113, 264–271.
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Lennox. (2012). “XC17 Expanded Rating Tables.” Lennox Industries Inc. Accessed December 

12, 2012.

http://tech.lennoxintl.com/C03e7o14l/G8VmneZC/ehb_xc17_rtgs_1206_210585R_004.pdf

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Renewable Energy Laboratory. Accessed October 30, 2012.

www.nrel.gov/ap/retrofits/about.CFM

Osser, R.; Kerrigan, P. (2012). Performance Evaluation of a Hot-Humid Climate Community. 

Golden, CO: National Renewable Energy Laboratory. Accessed January 30, 2013.

www.nrel.gov/docs/fy12osti/53183.pdf

Rudd, A.F.; Lstiburek, J.W.; Eng, P.; Ueno, K. (2012). Residential Dehumidification Systems 

Research for Hot-Humid Climates. Golden, CO: National Renewable Energy Laboratory. 

Accessed January 30, 2013.

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Shapiro, C.; Puttagunta, S. (2013). “Field Performance of Heat Pump Water Heaters in the 

Northeast.” To be published 2013. Golden, CO: National Renewable Energy Laboratory.

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New and Existing Homes. Golden, CO: National Renewable Energy Laboratory. Accessed

October 30, 2012. http://apps1.eere.energy.gov/buildings/publications/ 

pdfs/building_america/measure_guide_hpwh.pdf

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Air-Conditioning Equipment at Part-Load Conditions.” Florida Solar Energy Center. 2006 Jan. 

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Dehumidifiers. Golden, CO: National Renewable Energy Laboratory. Accessed January 30,

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38

Appendix A. Measurements and Calculations

A1. Heat Pump Water Heater

The following one-time measurements were made for the HPWH:

• Air Inlet (CFM), this was done by using a duct blaster as a powered capture hood.

• Power Draw (W)

• Current (A)

• Voltage (Vac)

• Power Factor.

The following HPWH parameters were measured every 10 seconds:

• Inlet Water Temperature (°F)

• Outlet Water Temperature (°F)

• Inlet Air Temperature (°F)

• Inlet Air Relative Humidity (%)

• Outlet Air Temperature (°F)

• Outlet Air Relative Humidity (%)

• Hot Water Flow (gals)

• Condensate (pints)

• Compressor Energy Consumption (Wh)

• Heating Element Energy Consumption (Wh)

• Entire System Energy Consumption (Wh).

An additional ambient air temp/RH sensor was located at 5 ft downstream of the HPWH air 

outlet. Based upon the measured parameters, the following data were calculated for each 15-

minute logging period:

• Average Water Inlet Temperature (°F)

• Average Water Outlet Temperature (°F)

• Minimum Water Inlet Temperature (°F)

• Maximum Water Outlet Temperature (°F)

• Average Inlet Air Temperature (°F)
39

• Average Inlet Air Relative Humidity (%)

• Average Outlet Air Temperature (°F)

• Average Outlet Air Relative Humidity (%)

• Total Domestic Hot Water Usage (gal)

• Total Latent Moisture Removal (pints)

• Domestic Hot Water Energy (Btu)

• Total Compressor Only Energy Consumption (Wh)

• Total Upper Heating Element Energy Consumption (Wh)

• Total Lower Heating Element Energy Consumption (Wh)

• Total System Energy Consumption (Wh), not including condensate pump

• Total Heat Pump Energy (Wh)

• Total Standby Energy Consumption (Wh)

• Sensible Heat Added (Btu)

• Latent Heat Removed (Btu).

Note that the inlet and outlet temperature and RH measurements were analyzed only when the 

unit is operating; so these are conditional temperature and RH values, that is, conditional upon 

the unit operating. Conditional averages were also calculated for the inlet and outlet water 

temperatures. In addition to daily sums, averages, minima, and/or maxima of the 15-min data, 

the following data were also calculated on a daily basis:

• Coefficient of Performance

• Pints Removed 

• Latent Cooling (Btu/h)

The COP has been defined as the net heat delivered to the hot water system divided by the total 

electrical energy consumed over a period of time:

W Btu Wh

Q

net energy input

useful heating energy COP

DHW

DHW

× 3.413 / = =

where: 

COP = coefficient of performance (dimensionless)
40

QDHW = useful heat energy (Btu)

WDHW = energy consumed by the HPWH (Wh)

The water heating energy (QDHW) was calculated in the datalogger every ten seconds using 

measured data. These energy values will be summed and recorded at 15 minute intervals.

= (∆ × × × ρ) •QDHW TT −T V Cp Out In

where:

TOut TIn ∆T − = TOut minus TIn (°F)

•

V = hot water volumetric flow rate (ft3

/h)

Cp = specific heat of water (Btu/lbm ⋅°F)

ρ = density of water (lbm/ft3

)

A2. Whole-House Dehumidifier

During the time of initial component inspection the following one-time measurements were 

made for the whole-house dehumidifier:

• Power Draw (W)

• Current (A)

• Voltage (Vac)

• Power Factor

• Outdoor Air Inlet (CFM), measured with a hot-wire anemometer

• Indoor Air Return (CFM), measured with a hot-wire anemometer

• Indoor Air Supply (CFM), measured with a hot-wire anemometer.

The following dehumidifier parameters will be measured every 10 s and saved as average or 

summed values at 10-min time steps. 

• Outdoor Air Inlet Temperature (°F)

• Outdoor Air Inlet Relative Humidity (%)

• Indoor Air Supply Temperature (°F)
41

• Indoor Air Supply Relative Humidity (%)

• Indoor Air Return Temperature (°F)

• Indoor Air Relative Humidity (%)

• Energy Consumption of Unit (Wh)

• Condensate (pints)

Based upon the measured parameters, the following data was calculated for each 10-15 minute 

logging period:

• Average/Min/Max Air Temperatures (°F)

• Average/Min/Max Air Relative Humidities (%)

• Total Latent Moisture Removal (pints)

• Total System Energy Consumption (Wh)

• Total Standby Energy Consumption (Wh)

In addition to daily sums, averages, minima, and/or maxima of the data, the following data were 

also calculated on a daily basis:

• Pints Removed

• Latent Cooling (Btu/hr)

Two methods were used to measure the dehumidification performance of the dehumidification 

system: (1) measuring condensed water exiting the dehumidifier; and (2) calculating the system 

latent capacity using the psychrometric properties of air entering/leaving the unit and the 

measured air flow rate. 

The first method uses the measured condensate flow multiplied by the latent heat of

condensation for water (1,065 Btu/lb) to calculate the latent cooling rate. 

Q Cond Btu lb hr cond = ×1,065 / × 60 min/

where:

Qstandbyloss = Latent cooling capacity based upon condensate

Cond = Measured condensate (lb/min)
42

The psychrometric method uses the measured humidity of air entering and leaving the coil along 

with the measured air flow rate across the coil to calculate the dehumidification (latent cooling) 

rate. It was calculated as follows:

The saturation pressure over liquid water pws (psia) was found as a function of dry bulb 

temperature T (°R).

ln pws C /T C C T C T C T C13 lnT 3

12

2 = 8 + 9 + 10 + 11 + + ,

where

C8 = -1.0440397 × 104,

C9 = -1.1294650 × 101,

C10 = -2.7022355 × 10-2

,

C11 = 1.2890360 × 10-5,

C12 = -2.4780681 × 10-9

, and

C13 = 6.5459673 × 100.

The partial pressure of water vapor pw (psia) was found as a function of the saturation 

pressure over liquid water and relative humidityφ (fraction).

pw = pwsφ .

The humidity ratio W was calculated as a function of the partial pressure of water vapor 

and atmospheric pressure p (14.696 psia).

w

w

p p

p W − = 0.621945 .

The specific enthalpy of dry air hda (Btu/lbda) was approximated as a function of dry bulb 

temperature t (°F).

h t da = 0.240 .

The specific enthalpy of saturated water vapor hg (Btu/lbw) was similarly approximated as 

a function of dry bulb temperature.

h t g = 1061+ 0.44 .

The specific enthalpy of moist air h (Btu/lbda) was calculated as a function of the specific 

enthalpy of dry air, humidity ratio, and the specific enthalpy of water vapor.
43

da Whg h = h + .

The total cooling capacity Qtotal

 (Btu/h) was calculated as a function of flow rate V

(CFM), supply enthalpy ply hsup (Btu/lbda), return enthalpy return h (Btu/lbda), and the 

density of dry air ρ da (0.076474 lbda/ft3).

( ) hour

minutes Qtotal = Vρ da hsup ply − hreturn × 60   .

The latent cooling capacity Qlatent

 (Btu/h) was calculated similarly to the total cooling 

capacity, but just uses the supply and return enthalpies of saturated water vapor at the 

measured dry bulb temperature.

( ) hour

minutes , 60

sup = , − g return ×ply Qlatent Vρ da hg h   .

Finally the moisture removal efficiency of the dehumidifier was calculated. This 

dehumidification EF is the daily amount of condensate removed (commonly reported in liters) 

divided by the daily energy consumption (kWh). 

EF Vcond PDH / = 

where:

•

V = Volumetric flow rate (L/day)

PDH = Energy consumption (kWh/day)
44

Appendix B. Cooling Scenario Data Detailed Plots

B1. Cooling Scenario Outdoor Condition Comparison

Figure 25. Scenario comparison of outdoor RH

Figure 26. Scenario comparison of outdoor temperatures 
45

B2. Cooling Scenario Indoor Air Conditions Maintenance

Figure 27. Scenario 1 (78°F, 50% RH) RH maintenance

Figure 28. Scenario 1 (78°F, 50% RH) temperature maintenance
46

Figure 29. Scenario 2 (75°F, dehumidifier off) RH maintenance

Figure 30. Scenario 2 (75°F, dehumidifier off) temperature maintenance
47

Figure 31. Scenario 3 (78°F, 60% RH) RH maintenance

Figure 32. Scenario 3 (78°F, 60% RH) temperature maintenance
48

Appendix C. Energy Performance of the Cool Energy House

Energy efficiency measures implemented in the CEH are provided in Table 18. The desire to 

preserve existing brick façade and minimize impact to interior finishes figured prominently in 

CARB’s recommendations. The first column represents building components in the existing 

home based on findings from the energy audit. 

Table 18. Existing, Proposed, and Post-Retrofit Specifications

Component Existing Post-Retrofit

Foundation 

Assembly Uninsulated Same as existing

Above Grade 

Wall 

Assembly

1st floor: R-19, Grade III, 2 × 6 16-in.

o.c.

2nd floor: R-13, Grade III, 2 × 4 16-in.

o.c.

1st floor: R-21, Grade I, 

2 × 6 16-in. o.c.

2nd floor: R-15, Grade I, 

2 × 4 16-in. o.c.

Ceiling/Attic 

Assembly

R-19 blown-in fiberglass, 2 × 8 at 

ceiling, vented attic

R-30 at roof, 5-in. closed-cell 

spray foam, unvented attic 

Window 

Glazing

Rear (east): single pane, U = 0.869, 

SHGC = 0.619 Elsewhere: double pane, 

U = 0.447, SHGC = 0.547

wood, aluminum

All: low-e, double pane,

U = 0.28, SHGC = 0.21

vinyl

Building 

Infiltration Measured 6 ACH50 Measured 4.8 ACH501

Space 

Conditioning 

System

Two 2.5 ton ~SEER 10, 

HSPF* 6.2 heat pumps

Two 2 ton SEER 17.5,

HSPF 9.5 heat pumps,

whole-house dehumidifier 

(150 pints/day; 2.02 L/kWh)

Ductwork R-4 insulation, 

measured 7% leakage to outside

Ducts in conditioned space, 

sealed with mastic

Whole House 

Ventilation None

Whole-house supply-only 

ventilation controlled by 

humidistat (part of WHD)

Local 

Ventilation Spot vent only Spot vent with delay off timers

Water 

Heating

Two ERWHs, 

50-gal, EF = 0.91

Two HPWHs, 

50 gal, EF = 2.35

Lighting 14% fluorescent lighting 75% high efficacy lighting

Roofing 

Material

dark asphalt shingles, 

abs = 0.92, e = 0.91 Same as existing

Appliances ENERGY STAR refrigerator, 

standard dishwasher

New ENERGY STAR 

refrigerator and dishwasher, 

induction stove

* HSPF = heating season performance factor 1 Proposed 4.0 ACH 50 2 Proposed ASHRAE 62.2 ventilation rates
49

Table 19 shows the BEoptE+ v1.2 simulated energy consumption by end use for the existing and 

post-retrofit Cool Energy House.

Table 19. Simulated Energy Use Distribution

Existing Post-Retrofit Savings

Loads

Site 

Energy 

(kWh)

Source 

Energy 

(MMBtu)

Site Energy 

(kWh)

Source 

Energy 

(MMBtu)

Percent 

(%)

Cooling 10,295 118 3,337 38 68

Heating 1,148 13 292 3 75

Hot Water 2,116 24 699 8 67

HVAC Fan 2,524 29 340 4 87

Lights 3,813 44 2,512 29 34

Appliances 1,257 14 1,220 14 3%

Ventilation 33 0 98 1 –198

MELs 4,622 53 4,622 53 0

Total 25,807 296 13,120 151 49

Figure 33 shows the cumulative energy savings by measure and their individual impacts on the 

whole-house energy savings. The new air source heat pumps contributed the most; the unvented 

“conditioned” attic and HPWHs came in second and third, respectively. Improvements to 

lighting and windows contributed a little more than 5% each to the total 49% savings.

Figure 33. Cumulative contribution to total simulated energy savings, by measure and end use
DOE/GO-102013-3942 ▪ September 2013

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