Residential Ground Source Heat 

Pumps with Integrated Domestic 

Hot Water Generation: 

Performance Results from 

Long-Term Monitoring

Dave Stecher and Katherine Allison 

IBACOS, Inc.

November 2012
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iii

Residential Ground Source Heat Pumps with Integrated Domestic Hot 

Water Generation: Performance 

Results from Long-Term Monitoring

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:

Dave Stecher and Katherine Allison 

IBACOS, Inc.

2214 Liberty Avenue

Pittsburgh, PA 15222 

NREL Technical Monitor: Michael Gestwick

Prepared under Subcontract No. KNDJ-0-40341-02 

November 2012 
iv

Acknowledgments

The authors thank the U.S. Department of Energy’s Building America Program for support to 

conduct this research. They also thank Pine Mountain Builders, especially David Barnes and 

Marc Hoefert, for their assistance on site. Thanks also go to Bill Rittelmann and Chris Imm, who 

were instrumental to the test procedure development and the instrumentation installation, 

respectively.
v 

Contents

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

List of Tables .............................................................................................................................................. vi

Definitions.................................................................................................................................................. vii

Executive Summary................................................................................................................................. viii

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

2 Specifications ....................................................................................................................................... 2

3 Technical Approach ............................................................................................................................. 5

4 Analysis ................................................................................................................................................. 8

4.1 Space Conditioning Coefficient of Performance .................................................................8

4.2 Domestic Water Heating Efficiency ..................................................................................10

5 Conclusions ........................................................................................................................................ 13

6 References .......................................................................................................................................... 14
vi 

List of Figures

Figure 1. Test House 1 (left) and House 2 (right)..................................................................................... 2

Figure 2. Diagram of the GSHP and desuperheater operation in heating mode.................................. 4

Figure 3. Diagram of the GSHP and desuperheater operation in cooling mode.................................. 4

Figure 4. Diagram showing temperature and flow measurement locations on the GSHP 

and DHW systems ................................................................................................................................ 6

Figure 5. EWT and monthly average COP for both houses; heating COPs are shown as 

negative values, and cooling COPs are shown as positive values. (Data for House 2 

in April 2010 are unavailable because a data logger malfunctioned.) ............................................ 8

Figure 6. As-measured EF for three different daily water consumption volumes with rated 

efficiencies of four other water heating systems shown; all efficiencies are weighted 

with respect to source energy type .................................................................................................. 12

Unless otherwise noted, all figures and photos were created by IBACOS. 

List of Tables

Table 1. Building Specifications................................................................................................................ 3

Table 2. Seasonal Average COP................................................................................................................ 8

Table 3. Equipment Rated COPs at Given EWTs..................................................................................... 9

Unless otherwise noted, all tables were created by IBACOS. 
vii 

Definitions

COP Coefficient of performance

DHW Domestic hot water

EER Energy efficiency ratio

EF Energy factor

EWT Entering water temperature

ft Foot

gal Gallon

gpm Gallons per minute

GSHP Ground source heat pump

Hz Hertz

kW Kilowatt

L Liter

LED Light-emitting diode

SHGC Solar heat gain coefficient

XPS Extruded polystyrene
viii

Executive Summary

Ground source heat pumps (GSHPs) show great promise as a means to substantially reduce 

energy consumption in houses. Laboratory test data from manufacturers of these units indicate 

that using the soil between 6 and 600 ft below the surface of the earth as the thermal energy 

source or sink gives GSHPs the potential to obtain coefficients of performance (COPs) ranging 

from 4 to 6 for heating and cooling. 

In addition, generating domestic hot water (DHW) consumes substantial amounts of energy in a 

house. As energy for space heating and cooling decreases, reducing energy for hot water 

generation becomes the next target. Using a desuperheater to generate DHW is a potential 

strategy for reducing this energy consumption. In this strategy, the heat pump from the space 

conditioning system is used to create hot water instead of electric resistance or fossil fuel. Before 

widespread implementation can be recommended, however, it is necessary to understand the 

magnitude of the impact of the factors that affect the thermal energy output of the desuperheater, 

such as heat pump runtime. 

In this study, the IBACOS team instrumented two unoccupied houses built by Pine Mountain 

Builders in Pine Mountain, Georgia. The project was designed to document the installed 

operational space conditioning and water heating efficiency of the houses’ GSHP systems. The 

monitoring period began in January 2010 and continued through April 2011. During this period, 

interior thermostat set points were maintained at constant values for heating and cooling, and hot 

water was consumed according to the occupancy profile schedule used in the U.S. Department of 

Energy’s Building America Program.1

This report discusses the instrumentation methods and field operation characteristics of the two 

GSHPs in the test houses, including a comparison between the manufacturer’s listed COP values 

and the COPs calculated from field data. Note that both GSHPs are made by the same 

manufacturer. The measured efficiency of the desuperheater-assisted DHW system is compared 

to efficiencies of other DHW systems available on the market.

 

1 See http://www1.eere.energy.gov/buildings/residential/ba_index.html for more information.
1 

1 Introduction

Ground source heat pumps (GSHPs) show great promise as a way to substantially reduce energy 

consumption in houses. GSHPs use the soil at depths ranging from 6 to 600 ft below the earth’s 

surface as the thermal energy source or sink. In the years preceding this study, observers noted 

an annual increase of 12% in the number of GSHP installations in the United States, primarily in 

the northern Midwest and along the East Coast. Most of these installations were closed-loop 

systems with vertical ground wells (Lund et al. 2004). 

As part of its work under the U.S. Department of Energy’s Building America Program (see 

footnote 1), IBACOS collaborated with Pine Mountain Builders in 2008 to build two houses in 

Pine Mountain, Georgia. As part of the upgrades necessary to meet the 50% source energy 

reduction level with respect to the 2008 Building America benchmark definition (Hendron 

2008), each house uses a GSHP for its heating and cooling needs. As energy used for heating and 

cooling decreases, the amount of energy used for potable hot water can become the largest single 

point of household energy consumption. As a result, reducing energy for hot water production 

becomes a priority. Options are limited, though, in all-electric houses such as these. A potential 

strategy for reducing the energy required to generate hot water is using a desuperheater (or 

supplemental heat exchanger) on the GSHP. This strategy, which was incorporated into the test 

houses, creates hot water in conjunction with an electric resistance tank water heater. Research 

performed by Merrigan and Parker (1990) indicates measured efficiency levels of 110% (thermal 

energy output divided by site electrical energy input as a percentage) for desuperheaters installed 

on air source heat pumps in Florida during the 1980s. 

Because the test houses in Georgia were intended to be unoccupied through the course of the 

study, there was the potential to eliminate variability resulting from user behavior. The two goals 

of the project were to document the space conditioning efficiency and the water heating 

efficiency of the GSHP systems installed in the test houses.
2 

2 Specifications

The two test homes, which are located next door to one another, share the same solar orientation. 

Both have four bedrooms, four and a half baths, and roughly the same usable interior area. House 

1 has one story; House 2 has two stories and slightly more usable interior floor space because of 

a loft on the second floor (see Figure 1 and Table 1). 

Figure 1. Test House 1 (left) and House 2 (right)
3 

Table 1. Building Specifications

Building Envelope

Geometry 2,024 ft2

, 1-story, or 2,946 ft2, 2-story (both have 4 bedrooms and 

4.5 bathrooms)

Roof Unvented attic, spray foam on underside of roof sheathing, R-30

Walls 2 x 6 wood framing @ 16-in. on center open cell spray foam R-22 

cavity

Foundation 12- to 18-in. tall monolithic slab with 1-in. XPS (R-5) slab edge 

insulation

Windows U-value = 0.38, SHGC = 0.35, wood frame

Mechanical Systems

Heating 4.2 COP GSHP

Cooling 20.1 EER GSHP: House 1, 2.6 tons; House 2, 3.8 tons

DHW

40- or 85-gal (151.4- or 322-L) electric tank, 0.94 or 0.92 nominal 

EF, two 4.5-kW (1.28-ton) nonsimultaneous operation electric 

resistance elements in each tank, desuperheater

Ducts In conditioned space

Leakage 2% to outside

Ventilation Passive into return with ventilation controller

Appliances All-electric ENERGY STAR appliances

Lighting 100% energy efficient fixtures—fluorescent and light-emitting diode 

(LED)

Source: See IBACOS (2010).

Notes: XPS, extruded polystyrene; SHGC, solar heat gain coefficient; COP, coefficient of performance; EER,

energy efficiency ratio; DHW, domestic hot water; EF, energy factor; LED, light-emitting diode

As part of the upgrades necessary to achieve 50% energy savings with respect to the 2008 

Building America benchmark definition (Hendron 2008), both houses use dual-capacity GSHPs

for space heating and cooling. Although made by the same manufacturer, the units have different 

output capacities. The GSHP used in House 1 is rated at 9.1 kW (2.6 tons), and the GSHP in 

House 2 is rated at 13.4 kW (3.8 tons). The ground heat exchanger consists of a closed loop in

two (House 1) or three (House 2) vertical bores, each approximately 180 ft deep, using two 1-in.-

diameter polyethylene pipes with a U-bend at the bottom end of each bore hole circulating a 15% 

methanol and 85% water mixture (IBACOS 2008). 

In addition, each house features a desuperheater connected to an electric tank type water heater

rated at 0.94 nominal EF and 151-L (40-gal) capacity in House 1 and 0.92 nominal EF and 322-L 

(85-gal) capacity in House 2. The desuperheater transfers excess heat from the GSHP refrigerant 

loop before the condenser into water in the water heater tank, improving the efficiency of the

DHW system. This process occurs only when the thermostat is calling for space heating or 

cooling; desuperheater operation is not controlled based on hot water needs. Desuperheater 

operations in heating and cooling modes are shown in Figure 2 and Figure 3, respectively. If the 

DHW tank temperature falls below its set point, the electric resistance elements come online. 

Preliminary analysis indicated that the desuperheater could offset up to 80% of the DHW 

electrical demand in the summer. 
4 

Figure 2. Diagram of the GSHP and desuperheater operation in heating mode

Figure 3. Diagram of the GSHP and desuperheater operation in cooling mode
5 

3 Technical Approach

The first goal of this study was to compare the field-measured COPs with the manufacturer’s 

listed values for the two GSHPs. The second goal was to compare the measured efficiency of the 

desuperheater-assisted DHW system to other market-available DHW systems. 

Using equations 1, 2, and 3, the study team determined the COP of each GSHP by calculating 

thermal energy (TEGroundLoop). To do so, the team used temperature measurements and the 

volumetric flow rate of the fluid in the ground loop and measured the electric energy of the 

compressor (EECompressor), the ground loop pump (EEGroundLoopPump), and the air handler unit fan 

(EEAirHandlerFan). The COP values calculated based on Equation 1 are negative for heating and 

positive for cooling. 

Compressor GroundLoopPump AirHandlerFan

GroundLoop Compressor

GSHP EE EE EE

TE TE

COP

+ +

− = , (1)

TECompressor = 30 *EE. Compressor , (2)

( ) GroundLoop GroundLoopOut GroundLoopIn GroundLoop Water CP TE = T − ×Q T ρ ×× , (3)

where

TEGroundLoop is the thermal energy sent to (positive value) or extracted from (negative 

value) the ground loop wells. 

TECompressor is the thermal energy given off by the compressor.

EECompressor is the site electrical consumption of the GSHP compressor.

0.3 is the assumed fraction of electrical energy input to the compressor that is lost to the 

surroundings as heat.

EEGroundLoopPump is the site electrical consumption of the ground loop pump. 

EEAirHandlerFan is the site electrical consumption of the air handler unit fan. 

TGroundLoopOut is the temperature of fluid leaving the GSHP and traveling toward the 

ground loop well. 

TGroundLoopIn is the temperature of fluid entering the GSHP (known in manufacturers’ 

literature as entering water temperature, or EWT) returning from the ground loop well. 

QGroundLoop is the volume of water flowing through the ground loop. 

ρWater is the density of water. 

CP is the specific heat of water.
6 

To conduct thermal energy metering on the ground loop, the team mounted a high-temperature, 

chemical-resistant flow meter on the loop line returning to the GSHP unit from the ground loop. 

Type T immersion thermocouple probes (Figure 4) measured the temperature of both the 

outgoing and incoming fluids. Thermal energy metering was performed in the same manner to 

determine the energy output from the DHW system to the fixtures in the house and on the 

circulation loop connecting the desuperheater in the GSHP to the DHW system. 

Figure 4. Diagram showing temperature and flow measurement locations on the GSHP and DHW 

systems

Electricity consumption of the ground loop pump, compressor, air handler unit fan, and 

desuperheater loop pump was measured individually for each device using pulse output Watthour meters (4-Hz full-scale output frequency). These meters were installed with appropriately 

sized current transducers rated at no more than a factor of 2 greater than the anticipated 

maximum load. For circuits with only resistive loads (the electric resistance heating elements in 

the DHW system), voltage output current transducers were installed, and Watt-hours were 

calculated using Equation 4: 

Watt hours- = ampsmeasured ×× hour244 , (4)

where ampsmeasured is the measured amperage from the current transducer and 244 is the typical 

line voltage.

The team took electrical measurements for the DHW system in the main circuit panel. 

Measurements related to the performance of the GSHP were taken at the unit. Type T 

thermocouples were used for the indoor air temperature measurement in each thermostatically 

controlled zone; the reference temperature was taken at the data logger using a thermistor. 

Investigators also measured outdoor temperature and relative humidity. 
7 

All sensors were connected to a central data logger using low-voltage shielded wire, and 60-Hz 

noise-cancelling integration was used to mitigate any noise caused by electrical power lines. The 

team sampled the sensors every 10 s and averaged the data on a minute and hourly basis. A 

cellular data modem was used to allow daily remote collection. 

During the monitoring period from January 2010 to April 2011, the houses were unoccupied, and 

the interior thermostat set points were maintained at constant values of 20.6°C (69°F) heating 

and 23.3°C (74°F) cooling. To determine the efficiency of the desuperheater, DHW needed to be 

used. To facilitate this, the study team installed a solenoid valve controlled by the central data 

logger on the hot water spigot in the washing machine box in each house. This allowed water 

flow on a regular basis according to a slightly modified version of the schedule used in the 

Building America House Simulation Protocols (Hendron and Engebrecht 2010). In this case, 

however, the protocols had to be simplified because only one flow rate was available from the 

fixed orifice opening used in the field test. The flow rate through the solenoid valve was fixed at 

0.095 L/s (1.5 gpm), and the solenoid valve opening duration was varied to simulate different 

volumes. To more accurately simulate the usage patterns of real people, three daily water 

consumption volumes were used during the weekly schedule: 

• Days 1 through 3 used 177 L (46.8 gal) each day. 

• Days 4 and 5 used an additional 99.5 L (26.3 gal) for a total of 277 L (73.1 gal) per day. 

• Days 6 and 7 used an additional 78.0 L (20.6 gal) for a total of 355 L (93.7 gal) per day. 

Typical flow events simulated the water used for showering, hand washing at a sink, and 

operating the dishwasher and the clothes washer. Individual flow events ranged from 1.9 to 57 L 

(0.5 to 15 gal). Hot water was consumed according to this schedule for the duration of the 

monitoring period. 
8 

4 Analysis

4.1 Space Conditioning Coefficient of Performance

The study team calculated the COP values every 10 s and averaged the data (weighted with 

respect to GSHP runtime) on a minute and hourly basis. Investigators assumed that during a 

given hour, the system operated in only one mode: heating or cooling. Based on hourly data 

outputs, monthly average COPs were created, weighted by runtime in each mode. The 

manufacturer’s rated performance might have been met at various times throughout the 

monitoring period; however, over the course of the year, the values were lower. Table 2 shows 

seasonal average COPs and system runtime for heating or cooling. The results plotted in Figure 5 

are the COPs for the mode in which most of the runtime occurred. Measured COPs while the 

system was in cooling mode were closer to the range of the manufacturer’s values shown in 

Table 3 (Waterfurnace 2007) than measured COPs while the system was in heating mode. Note 

that the wide range in the manufacturer’s values results from two stages of system operating 

capacity and two fan speeds for each stage. 

Table 2. Seasonal Average COP

House 1 COP House 2 COP

Winter early 2010 (heating) 4.86 2.65

Summer 2010 (cooling) 5.24 4.26

Winter 2010 to 2011 (heating) 3.44 2.36

10

20

30

40

50

60

70

80

90

-8

-6

-4

-2

0

2

4

6

8

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr

Entering Water Temperature (TGroundLoopIn)

Coefficient of Performance - (COP)

Month

House 1 COP

House 2 COP

House 1 G-loop Temperature

House 2 G-Loop Temperature

Outdoor temperature

25

30

-10

-5

0

5

10

15

20

°F °C

2010 2011

Figure 5. EWT and monthly average COP for both houses; heating COPs are shown as negative 

values, and cooling COPs are shown as positive values. (Data for House 2 in April 2010 are 

unavailable because a data logger malfunctioned.)
9 

Table 3. Equipment Rated COPs at Given EWTs

EWT 

Range 

(°F)

Rated 

Heating 

COP Range

Measured 

Heating 

COP 

Range

Rated 

Cooling 

COP Range

Measured 

Cooling 

COP Range

House 1

60–70 4.86 6.25 2.4 5.3 6.21 9.23 3.4 6.1

70–80 5.15 6.68 5.42 8.09 4.9 5.5

80–90 5.28 7.15 4.66 6.71

House 2

60–70 4.85 6.15 1.8 3.8 5.95 9.17 5.0 5.0

70–80 5.05 6.58 5.25 8.00 5.4 6.8

80–90 5.17 7.01 4.57 6.74 4.9 5.0

The measured COPs for the GSHP systems were lower than the manufacturer’s ratings in 

cooling, with House 1 performing better than House 2. The measured 2010 cooling season COP 

was 5.24 for House 1, 3% to 35% below the manufacturer’s range of 5.42 to 8.09 for the summer 

EWT. The measured 2010 cooling season COP was 4.26 for House 2, 16% to 47% below the 

manufacturer’s range of 5.09 to 8.06 for the summer EWT. 

The heating season COP was 4.86 for House 1 during the winter of early 2010, 4% to 22% lower 

than the manufacturer’s range of 5.05 to 6.25 for the winter EWT. During the 2010–2011 winter 

season, the observed COP of 3.44 was substantially worse. Although this performance is worse 

than anticipated, after accounting for differences in source energy, this is comparable to the rated 

performance of a high-efficiency (90+% annual fuel utilization efficiency) furnace. 

The heating season COP for House 2 was 2.65 in early 2010. This COP also declined during the 

2010–2011 winter, averaging 2.36 for the season. This is at least 50% lower than the 

manufacturer’s listed COP values for the winter EWT.

It is unclear why the heating season COPs varied substantially between the two houses; however, 

the GSHP installed in the two-story house was slightly oversized. Figure 5 shows the monthly 

average COPs and the corresponding EWTs for both houses. The inside temperature of House 1 

was 17.2°C (63°F) in January, February, and March 2010, and in January and February 2011. 

The low temperature in the latter months was caused by a compressor malfunction that prevented 

the system from heating the house. Based on observations from adjoining data in 2011, when the 

inside temperature was 20.6°C (69°F), this appeared to have no impact on the measured COP. 

In House 2, during January and February 2011, the air handler unit fan operated continuously at 

high speed, although the team excluded the extraneous runtime from COP calculations. Although 

the stage or fan speed at which the system was operating was not measured, knowing this 

information would help determine why differences occur. The substantial difference in 

performance between two houses with fairly similar characteristics is perplexing and highlights 

the need for more critical analysis of the possible differences between the houses, such as the 

quality of the ground loop installation. 
10 

4.2 Domestic Water Heating Efficiency

The “as-measured” EF of a DHW system can be approximated based on the measured thermal 

energy output of the water heater and the measured electrical input to the water heater (see 

Equation 5 and Equation 7). In this study, however, calculating the EF is somewhat more 

complicated because the thermal energy provided by the GSHP in heating mode is not free; it is 

created from electricity proportionately to the COP of the GSHP. The EF calculation is modified 

to account for this in Equation 6. 

ER

output

E

TE

EF = , (5)

















+

=

GSHP

de erheater

ER

output

COP

TE

E

TE

EF

sup

, (6)

where

TEdesuperheater is the measured thermal energy provided to the DHW system from the 

GSHP.

TEoutput is the measured thermal energy provided from the DHW system to the house. 

EER is the measured electricity supplied to the heating elements in the DHW system. 

COPGSHP is the seasonally averaged measured COP of the GSHP in heating mode.

( ) output Out In Tank Water CP TE QTT ρ ×××−= , (7)

where

TEoutput is the thermal energy delivered by the DHW system.

TOut is the temperature of water leaving the DHW system.

TIn is the temperature of water entering the DHW system.

QTank is the volume of water flowing through the tank. 

ρWater is the density of water. 

CP is the specific heat of water.

Based on monitored data (see Figure 6), the annual site EF of the DHW system in House 1 

ranged from 2.8 to 3.3, depending on the volume of water consumed. By comparison, electric 

resistance water heaters range in EF from 0.85 to 0.95. If compared to an instantaneous gas water 
11 

heater, the difference in source energy between natural gas and electricity must be accounted for 

by converting the EF of each water heater to source energy weighted EF using Equation 8 and 

Equation 9: 

.3653

EF EF ElectricDHW

SourceEnergyWeighted = , (8)

.0921

EF EF GasDHW

SourceEnergyWeighted = , (9)

where

EFElectricDHW is the EF for an electric DHW system.

EFGasDHW is the EF for a gas DHW system.

3.365 is the site-to-source conversion factor for electricity.

1.092 is the site-to-source conversion factor for natural gas. 

Figure 6 shows that, after accounting for the difference in source energy, the DHW system in 

House 1 is on par with or better than the rated source energy efficiency of tankless gas water 

heaters. The DHW system is also more efficient than other options, including tank type gas, tank 

type electric, and heat pump DHW systems. Rated performance is plotted as a line in Figure 6 to 

more clearly provide context with respect to measured results at specific flow rates; the plotted 

lines are not indicative of rated performance at those flow rates. In House 2, the desuperheater 

provided, on average, 20% of the thermal energy input to the tank, which is comparable to 

results obtained by Fanney and Dougherty (1992); however, there was no apparent improvement 

in EF over the rated performance of a conventional tank type electric water heater. Average 

delivered temperature of hot water from each tank was similar, with the 151.4-L (40-gal) tank in 

House 1 delivering at 51.1°C (124°F) and the 322-L (85-gal) tank in House 2 delivering at 

46.7°C (116°F). More detailed measurements are necessary to determine if other factors, such as 

tank standby losses or disruption of tank stratification by the desuperheater, influenced the 

performance.
13 

Figure 6. As-measured EF for three different daily water consumption volumes with rated 

efficiencies of four other water heating systems shown; all efficiencies are weighted with respect 

to source energy type
13 

5 Conclusions

The study results indicate that GSHPs can be an effective means of providing space heating and 

cooling in warm climates. The variation in heating performance between the units in the two 

houses in this study, however, is a cause for concern. In general, a desuperheater is a more 

efficient means of providing DHW, but its efficiency varies, depending on the components to 

which it is connected. Based on these results, the study team recommends further research on the 

performance of desuperheaters in other residential applications, such as those connected to air 

source heat pumps. 

As the desire to increase energy efficiency continues, integration of space conditioning and hot 

water generation systems, either directly (desuperheaters) or indirectly (heat pump water 

heaters), will create new challenges for calculating the in situ operational efficiency of both 

devices.
14 

6 References

Fanney, A.; Dougherty, B. (1992). “Performance of a Residential Desuperheater.” Proceedings 

of the ASHRAE Winter Meeting, Anaheim, CA, January 25–29, 1992. Atlanta, GA: ASHRAE; 

pp. 489–499. 

Hendron, R. (2008). Building America Research Benchmark Definition, Updated December 20, 

2007. NREL/TP-550-42662. Golden, CO: National Renewable Energy Laboratory. 

Hendron, R.; Engebrecht, C. (2010). Building America House Simulation Protocols. NREL/TP550-47246.Golden, CO: National Renewable Energy Laboratory. 

IBACOS (2008). Building America Program Annual Report BP 1. DE-FC26-08NT02231. 

Pittsburgh, PA: IBACOS. 

IBACOS (2010). Building America Program Annual Report BP 2. DE-FC26-08NT02231. 

Pittsburgh, PA: IBACOS. 

Lund, J.; Sanner, B.; Rybach, L.; Curtis, R.; Hellström, G. (2004). “Geothermal (Ground-Source) 

Heat Pumps—A World Overview.” GHC Bulletin (September).

Merrigan, T.; Parker, D. (1990). “Electrical Use, Efficiency, and Peak Demand of Electric 

Resistance, Heat Pump, Desuperheater, and Solar Hot Water Systems.” FSEC-PF-215-90. 

Florida Solar Energy Center, Cape Canaveral, FL.

Waterfurnace International (2007). Specifications Catalog Number SP1585. Fort Wayne, IN: 

Waterfurnace International, Inc.
DOE/GO-102012-3516 ▪ November 2012

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