Ground Source Heat Pump 

Sub-Slab Heat Exchange 

Loop Performance in a Cold 

Climate 

Nick Mittereder and Andrew Poerschke 

IBACOS, Inc. 

November 2013 
 

 

 

NOTICE 

This report was prepared as an account of work sponsored by an agency of the United States 

government. Neither the United States government nor any agency thereof, nor any of their 

employees, subcontractors, or affiliated partners makes any warranty, express or implied, or assumes 

any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, 

apparatus, product, or process disclosed, or represents that its use would not infringe privately owned 

rights. Reference herein to any specific commercial product, process, or service by trade name, 

trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, 

recommendation, or favoring by the United States government or any agency thereof. The views and 

opinions of authors expressed herein do not necessarily state or reflect those of the United States 

government or any agency thereof. 

Available electronically at http://www.osti.gov/bridge 

Available for a processing fee to U.S. Department of Energy
 

and its contractors, in paper, from:
 

U.S. Department of Energy
 

Office of Scientific and Technical Information
 

P.O. Box 62
 

Oak Ridge, TN 37831-0062
 

phone: 865.576.8401
 

fax: 865.576.5728
 

email: mailto:reports@adonis.osti.gov
 

Available for sale to the public, in paper, from:
 

U.S. Department of Commerce
 

National Technical Information Service
 

5285 Port Royal Road
 

Springfield, VA 22161
 

phone: 800.553.6847
 

fax: 703.605.6900
 

email: orders@ntis.fedworld.gov
 

online ordering: http://www.ntis.gov/ordering.htm
 

Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste 
 

Ground Source Heat Pump Sub-Slab Heat Exchange Loop
 

Performance in a Cold Climate
 

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:
 

Nick Mittereder and Andrew Poerschke
 

IBACOS, Inc.
 

2214 Liberty Avenue
 

Pittsburgh, PA 15222 

NREL Technical Monitor: Michael Gestwick
 

Prepared under Subcontract No. KNDJ-0-40341-03
 

November 2013 

iii 
[This page left blank]
 

iv
 
v 

Contents

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

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

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

Acknowledgments ................................................................................................................................... viii

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

1 Introduction and Background ............................................................................................................. 1

1.1 Problem Statement ...............................................................................................................1

1.2 Project Objective ..................................................................................................................2

1.3 Ground Loop Description ....................................................................................................2

2 Experimental Approach ....................................................................................................................... 4

2.1 System Operation .................................................................................................................4

2.2 Measured Parameters ...........................................................................................................5

3 Measured Results............................................................................................................................... 10

3.1 Basement Floor Temperatures ...........................................................................................10

3.2 Soil Temperatures ..............................................................................................................10

3.3 Soil Moisture Content ........................................................................................................13

3.4 Soil Thermal Conductivity .................................................................................................13

3.5 Basement Floor Heat Flux .................................................................................................14

3.6 Ground Loop Working Fluid Temperatures ......................................................................16

4 Economic Analysis............................................................................................................................. 20

5 Discussion and Conclusions ............................................................................................................ 21

5.1 Heat Exchanger Parameters ...............................................................................................21

5.2 Design Considerations .......................................................................................................22

5.3 Future Research .................................................................................................................23

References................................................................................................................................................. 24

Appendix A: Excerpt from “Cooperative Agreement DE-FC26-08NT02231. Building America 

Final Technical Report” (Oberg 2010)—Pittsburgh Lab Home Heating, Ventilation, and 

Air Conditioning.................................................................................................................................. 25

Appendix B: Updated TRNSYS Simulation and Model Validation ....................................................... 33

Appendix C: Excerpt from “Cooperative Agreement DE-FC26-08NT02231. Building America 

Final Technical Report” (Oberg and Bolibruck 2009)—Pittsburgh Lab Home Sub-Slab 

TRNSYS Results ................................................................................................................................. 46
 

 

 

 

List of Figures 

Figure 1. The cold-climate Pittsburgh Lab Home .................................................................................... 1
 

Figure 2. Building America climate map, showing the location of the Pittsburgh Lab Home ............ 2
 

Figure 3. GSHP sub-slab ground heat exchange loop composed of 3⁄4-in. high density
 

polyethylene piping .............................................................................................................................. 3
 

Figure 4. Plan view showing the type, location, and name of sub-slab loop instrumentation
 

devices................................................................................................................................................... 6
 

Figure 5. Section view showing the type and location of sub-slab loop instrumentation
 

devices................................................................................................................................................... 7
 

Figure 6. Typical soil TC installation......................................................................................................... 7
 

Figure 7. Typical soil moisture sensor installation ................................................................................. 8
 

Figure 8. Typical TC installation for measuring soil temperature directly below the slab
 

insulation ............................................................................................................................................... 8
 

Figure 9. Typical installation of basement floor TC and heat flux sensor ............................................ 9
 

Figure 10. Basement floor temperatures ................................................................................................ 10
 

Figure 11. Soil temperatures measured directly below the slab insulation........................................ 11
 

Figure 12. Soil temperatures measured at the ground loop depth ...................................................... 11
 

Figure 13. Soil temperatures measured 24 in. below the ground loop depth..................................... 12
 

Figure 14. Soil moisture content ............................................................................................................. 13
 

Figure 15. Measured basement floor heat flux....................................................................................... 15
 

Figure 16. Measured results for basement slab temperatures............................................................. 16
 

Figure 17. Daily average outdoor ambient, ground loop fluid, and soil temperatures ...................... 17
 

Figure 18. TRNSYS results for ground loop temperatures ................................................................... 18
 

Figure 19. Heat pump COP versus EWT ................................................................................................. 18
 

Figure 20. Estimated relationship between soil thermal conductivity and specific peak load ......... 22
 

Figure 21. Typical heating season operation: ground loop fluid temperatures ................................. 35
 

Figure 22. Typical cooling season operation: ground loop fluid temperatures ................................. 36
 

Figure 23. Simulated EWT variation from measured data .................................................................... 37
 

Figure 24. MBE—simulated versus measured EWT .............................................................................. 38
 

Figure 25. Cumulative energy transfer to soil from loop fluid ............................................................. 39
 

Figure 26. Cumulative heat transfer, West slab ..................................................................................... 40
 

Figure 27. Cumulative heat transfer, East slab ...................................................................................... 41
 

Figure 28. Locations of soil temperature slices (not to scale) ............................................................. 41
 

Figure 29. Horizontal temperature profile at loop level during summer operation (°C) .................... 42
 

Figure 30. Horizontal temperature profile at loop level during fall operation (°C) ............................. 43
 

Figure 31. Vertical slice of soil temperatures (°C) ................................................................................. 43
 

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

List of Tables 

Table 1. GSHP Parameters ......................................................................................................................... 4
 

Table 2. Data Logging Equipment ............................................................................................................. 5
 

Table 3. Summary of COP Values ........................................................................................................... 18
 

Table 4. Cost Analysis of Vertical Versus Sub-Slab Heat Exchange Loops ....................................... 20
 

Table 5. TRNSYS Simulation Parameters ............................................................................................... 34
 

Table 6. MBE of Loop and Soil Temperatures........................................................................................ 39
 

Unless otherwise noted, all tables were created by IBACOS. 

vi 
Definitions 

BEopt Building Energy Optimization (software) 

COP Coefficient of performance 

EWT Entering water temperature 

GSHP Ground source heat pump 

MBE Mean bias error 

TC Thermocouple 

TESS Thermal Energy Systems Specialists, LLC 

vii 
 

 

Acknowledgments 

The authors thank Bill Rittelmann for the novel ground loop design and S & A Homes for the 

development and construction of the innovative test house associated with this project. The 

authors also thank the U.S. Department of Energy’s Building America Program for funding this 

research. Additionally, thanks go to Jeff Thornton of Thermal Energy Systems Specialists, LLC 

for his TRNSYS modeling efforts associated with reviewing and updating the latest TRNSYS 

sub-slab analysis module. 

viii 
 

 

 

 

 

Executive Summary 

This report presents a cold-climate project that examines an alternative approach to ground 

source heat pump (GSHP) ground loop design. The innovative ground loop design is an attempt 

to reduce the installed cost of the ground loop heat exchange portion of the system by containing 

the entire ground loop within the excavated location beneath the basement slab. The horizontal 

sub-slab approach presented in this report will reduce installation costs by approximately 

$2,500/ton (Oberg 2010; Appendix A) compared to those of a conventional vertical bore well. 

For the 1.5-ton system installed in the cold-climate, 2,772-ft2

, two-story unoccupied test house in 

Pittsburgh, Pennsylvania (hereinafter referred to as the Pittsburgh Lab Home), a traditional 

vertical well system is expected to cost $17,800, compared to $14,000 for the horizontal sub-slab 

coils. 

Prior to the installation and operation of the sub-slab heat exchanger in the Pittsburgh Lab Home, 

energy modeling using TRNSYS software (TRNSYS 2007) (in addition to traditional design 

efforts) was performed to determine the size and orientation of the system. Several design 

considerations were fundamental to the sub-slab GSHP design. Being a low-load home, the 

GSHP capacity is less than that for a typical house of the same size. Standard design of a GSHP 

allows for a margin of error for system undersizing, but with a low-load home, the margin of 

error is reduced. Additionally, a low-load home has longer shoulder seasons with reduced peak 

loads. The impact on the GSHP design is that heat is rejected and withdrawn from the ground in 

smaller, individual loads but over a longer time period than with a traditional GSHP. This design 

characteristic directly relates to the climate region where the house is located. A balanced 

climate region with a relatively equal number of heating and cooling degree days will have the 

effect of balancing the deep earth temperature and will help prevent overheating of the system. 

This report analyzes the performance of the sub-slab heat exchanger for the approximate 11⁄2-year 

monitoring period of October 2011 through February 2013. 

Lower than anticipated soil heat transfer resulted in monitored leaving and entering water 

temperatures that exceeded the initial TRNSYS energy modeling predictions. Initial TRNSYS 

modeling predicted the maximum ground loop inlet temperature to be 29°C (84.2°F), whereas 

the measured maximum sustained inlet temperature was 45°C (113°F). During heating mode, the 

minimum measured inlet temperature was 1.3°C (34.4°F), and the minimum simulated inlet 

temperature was 1.72°C (35.1°F). 

To investigate this discrepancy, an updated model was developed using a new TRNSYS 

subroutine for simulating sub-slab heat exchangers, Type 1267 (TRNSYS 2012). Appendix B 

discusses the updated results and model calibration with measured data. In this new model, the 

fluid piping, basement slab, and soil thermal interactions run as a single subroutine. 

Measurements of fluid temperature, soil temperature, and thermal energy transfer were used to 

validate the updated model. Most significantly, updated soil conductivity values were used in the 

new model, and the results closely corresponded to measured results. 

Soil thermal conductivity plays a major role in the effectiveness of ground coupled heat 

exchangers, such as the sub-slab heat exchanger. The initial estimates for the soil thermal 

conductivity were slightly above the normal range and were based on observed installation 

ix 
conditions. Based on this observation, the selection of soil thermal conductivity values for 

simulating a sub-slab heat exchanger should be closer to the drier end of the soil moisture range 

to allow for more realistic soil properties. 

An engineering economic analysis was completed using the calibrated TRNSYS model to project 

energy use over a 30-year period and actual installed costs of the sub-slab GSHP versus a 

vertical well GSHP. The capital recovery indicates an annual $200 cost savings of a GSHP subslab ground loop heat exchanger. 

Based on the measured data, the research team determined that the ground loop heat exchanger 

met the heating and cooling needs of the Pittsburgh Lab Home. The analysis indicates that a subslab GSHP is an effective strategy for capturing most of the advantages of a GSHP without the 

expense of drilling vertical wells adjacent to a low-load house in a suitable climate region. 

Because of the low margin for error in system design and the greater risk of an undersized 

system causing negative and compounding side effects, the sub-slab heat exchanger design 

requires more verification and stress testing before it will be ready for a production home 

environment. 

x 
 

 

1 Introduction and Background 

1.1 Problem Statement 

The rising cost of energy has become a significant portion of the household budget. Homeowners 

continually find it more and more expensive to heat and air condition their homes to achieve 

year-round comfort. Ground source heat pumps offer a safe and environmentally less impactful 

alternative to conventional heating and air-conditioning systems. Although GSHPs frequently 

offer efficient space conditioning performance, the added labor and material costs for installing 

ground heat exchange loops make these systems less competitive in a price-sensitive market 

(Goetzler et al. 2009). An alternative design, presented in this report, uses the soil under the slab 

as a thermal transfer medium to reduce installation costs while maintaining acceptable system 

performance. 

A GSHP system was installed in the Pittsburgh Lab Home—a cold-climate, 2,772-ft2, two-story 

unoccupied test house located in the Cobblestone community of Pittsburgh, Pennsylvania. The 

Pittsburgh Lab Home is considered a low-load home with a design peak load of 5.84 Btu/h-ft2 

and 3.546 Btu/h-ft2

 for heating and cooling, respectively (Stecher et al. 2013). This research 

investigated the viability of a potential solution that reduces system installation costs by placing 

the ground heat exchange loop in a horizontal plane within the excavated region beneath the 

conditioned basement slab. From March 2011 through February 2013, performance test data 

were collected for a GSHP system having a horizontal ground heat exchange loop. The design of 

this GSHP system was based on TRNSYS Version 16.01 modeling efforts from 2009 (Oberg and 

Bolibruck 2009; Appendix C). Figure 1 shows the Pittsburgh Lab Home, and Figure 2 shows the 

climate zone in which the Pittsburgh Lab Home is located. 

Figure 1. The cold-climate Pittsburgh Lab Home 

1 
 

 

 

Figure 2. Building America climate map, showing the location of the Pittsburgh Lab Home 

1.2 Project Objective

This research focused on the performance aspects related to the ground heat exchange loop 

installed beneath the basement slab of the Pittsburgh Lab Home. Based on cost data taken from 

earlier research projects conducted by IBACOS, the installed cost of the sub-slab loop was 

$2,500/ton less than that of the vertical wells (Oberg 2010; Appendix A). The IBACOS team 

based the design of the ground heat exchange loop on initial TRNSYS modeling. 

The objective of this research project was to answer the following question: Is there a viable, 

lower cost, lower impact alternative to drilling vertical wells? 

This research project helps meet important Building America goals by means of the following: 

•	 The evaluation of the performance of this potentially cost-effective system through actual 

field test data 

•	 Eventual development of a robust system design model, calibrated to test data and 

applicable to other homes in climate zones suitable for GSHP installations. 

This report discusses the results from the summer of 2011 through the fall of 2012, along with 

the correlation of the TRNSYS model to measured results. Analysis of the differences between 

the modeled predictions and the actual measured results also is discussed. 

1.3 Ground Loop Description

Figure 3 shows the installation of the two-pass overlapping horizontal piping loop for the GSHP 

at the Pittsburgh Lab Home. The entire slab area was excavated to the bottom of the wall footers, 

thereby allowing sufficient space for the piping loop, extra limestone fill, and R-10 insulation 

2 
 

installed directly beneath the basement slab. Each pass of the piping loop consisted of 

approximately 200 ft of 3⁄4-in. high density polyethylene pipe, arranged in an overlapping 

serpentine pattern, for a total nominal pipe length of 400 ft. This facilitated a lateral spacing of 

1.0 ft between adjacent sections of pipe. The loop was anchored to heavy-gauge 6 × 6 flat-panel 

wire mesh with zip ties to keep it flat against the soil. Crushed limestone was chosen as the fill 

material because it provides a good balance between cost and material properties. The limestone 

was compacted in two layers to achieve maximum density and contact with pipes. 

Figure 3. GSHP sub-slab ground heat exchange loop composed of 3⁄4-in. 

high density polyethylene piping 

3 
 

 

 

 

2 Experimental Approach 

2.1 System Operation

The subject sub-slab ground heat exchange loop was operational within the GSHP system of the 

Pittsburgh Lab Home from March 2011 to February 2013. Table 1 summarizes the parameters of 

the installed GSHP. Starting in June 2011 and continuing throughout 2012, the research team 

simulated internal sensible and latent heat gains from human occupancy according to the 

Building America Benchmark Definition 2009 schedule used for computer simulation models 

(Hendron 2008). Simulation was implemented with one heater and humidifier on the main floor 

and one heater and two humidifiers in each of three of the four bedrooms. The team simulated 

the different loads by intermittently varying the time of operation for the fixed output heaters and 

humidifiers. 

Table 1. GSHP Parameters 

Model
 

Operational Range
 

Cooling Energy Efficiency Ratio (77°F) 

Heating COP* (32°F)
 

Heating Mode
 

Refrigerant
 

Capacity 

Water Flow Rate 

Cooling Mode 

Carrier 50PTV-026-K-Y-E-30112
 

20°–120°F
 

4.0
 

Three-stage with electric backup 

2 tons 

19.9 

900–950 gpm 

(backup disconnected) 

R-410a 

Two-stage 

* Coefficient of performance 

To accommodate concurrent operation in the home for short-term testing of the GSHP 

desuperheater (Mittereder and Poerschke 2013), a supplemental 2-ton air-conditioning unit was 

used to simulate winter conditions during the beginning of July 2012. Further desuperheater 

short-term testing occurred from the middle of August 2012 to the first week in September 2012. 

These short-term tests had a noticeable impact on the data for this research project and will be 

further discussed later in this report. 

The predicted ground temperatures and heat transfer rates were determined by a horizontal subslab ground loop heat exchanger configuration using a TRNSYS Version 16.01 “Type” model 

developed by Thermal Energy Systems Specialists, LLC (TESS) specifically for this project 

(Oberg and Bolibruck 2009; Appendix C). 

Key modeling simulation parameters for the initial sub-slab heat exchange simulation are as 

follows: 

• Simulation length = 17,520 h 

• Time step = 1/10 h 

• Deep earth (average surface) temperature = 10.40°C (50.7°F) 

• Thermal conductivity of soil layer = 9.9 kJ/h∙m∙K (1.589 Btu/h∙ft∙°F) 

4 
 

 

 

• Density of soil layer = 2,080 kg/m3 (129.8 lb/ft3) 

• Specific heat of soil layer = 0.84 kJ/kg∙K (0.2007 Btu/lb∙°F) 

• Fluid through pipes thermal conductivity = 1.65168 kJ/h∙m∙K (0.2651 Btu/h∙ft∙°F) 

• Fluid through pipes density = 1,000 kg/m3 (62.4 lb/ft3) 

• Fluid through pipes specific heat = 4.19 kJ/kg∙K (1.001 Btu/lb∙°F) 

• Fluid through pipes viscosity = 3.2 kg/m∙h (2.150 lb/ft∙h) 

• Pipe inside diameter = 0.0204 m (0.8031 in.) 

• Pipe outside diameter = 0.025 m (0.9843 in.) 

• Pipe thermal conductivity = 1.401895 kJ/h∙m∙K (0.2250 Btu/h∙ft∙°F). 

2.2 Measured Parameters 

Table 2 summarizes the relevant parameters for quantifying ground loop performance, along 

with their associated instrumentation types. The research team used the field data collected for 

these parameters to analyze the degree of relative loop-ground and loop-basement engagements. 

Figure 4 through Figure 9 illustrate instrumentation locations and typical installation examples. 

Table 2. Data Logging Equipment 

Measured Parameter Equipment Quantity Accuracy 

Outdoor Ambient Temperature 

Ground Loop Working Fluid 

Temperature 

Basement Floor Surface 

Temperature 

Soil Temperatures: 

Directly Below the Slab Insulation 

At the Surface of Undisturbed Soil 

24 In. Below Undisturbed Soil 

Basement Floor Heat Flux 

Soil Moisture Content 

Ground Loop Flow Volume 

GSHP Electricity Consumption 

Data Acquisition System 

Type-T TC* 

Type-T TC, 

Pipe-Mount Probe 

Type-T TC 

Type-T TC 

Type-T TC 

Type-T TC 

Hukseflux HFP01 

Water Content 

Reflectometer 

High Resolution Flow 

Meter (75.7 pulse/gal) 

CCS WattNode 

Campbell Scientific 

CR1000 

1 

2 

16 

12 

12 

10 

3 

4 

1 

5 

3 

± 1°C or 0.75% 

± 1°C or 0.75% 

± 1°C or 0.75% 

± 1°C or 0.75% 

± 1°C or 0.75% 

± 1°C or 0.75% 

± 5.0% 

± 2.5% 

± 1.5% 

± 0.5% 

* Thermocouple 

5 
 

North 

NC
 

West 

NW NE 

NM 

EM 

CE 

CW 

SE 

SM LOPPEN 

M = Moisture 

SW SC T = Temperature 

F = Heat Flux 

East 

South 

Figure 4. Plan view showing the type, location, and name of 

sub-slab loop instrumentation devices 

6 
Figure 5. Section view showing the type and location of sub-slab loop instrumentation devices 

Figure 6. Typical soil TC installation 

7 
 

Figure 7. Typical soil moisture sensor installation 

Figure 8. Typical TC installation for measuring 

soil temperature directly below the slab insulation 

8 
Figure 9. Typical installation of basement floor TC and heat flux sensor 

9 
 

 

 

3 Measured Results 

The research team used LoggerNet 4.1 software (LoggerNet 2012) to record field data 

documented in this report. In general, daily average data were used to help characterize the 

performance of the ground heat exchange loop, and all figures in Section 3 display daily average 

quantities. The team also computed averaged quantities at a constant elevation, where applicable. 

3.1 Basement Floor Temperatures

Figure 10 illustrates the daily average basement floor temperature distribution versus time over 

the sampling period from May 1, 2011 through October 23, 2012. Each curve plotted pertains to 

an individual TC whose floor location and name are indicated in Figure 4. 

Figure 10. Basement floor temperatures 

Figure 10 shows that, during the 2011 peak summer cooling season (May 1, 2011 to 

September 30, 2011), measured basement floor temperatures climbed within the range of 81.1°F 

to 86.2°F. With a summer thermostat set point temperature of 76°F, this trend tends to indicate 

that some additional heat load was being imposed on the house by the basement floor from July 

through October 2011. 

3.2 Soil Temperatures

Figure 11 through Figure 13 illustrate the daily average soil temperature distributions at three 

different elevations beneath the basement slab over the monitoring period. 

•	 Figure 11 pertains to temperatures taken at the elevation directly below the R-10 slab 

insulation layer. 

10 
•	 Figure 12 pertains to temperatures taken at the surface of the undisturbed soil, 8 in. below 

the slab insulation, and at the actual ground loop depth. 

•	 Figure 13 pertains to temperatures taken at an elevation 24 in. below ground loop depth. 

Figure 11. Soil temperatures measured directly below the slab insulation 

Figure 12. Soil temperatures measured at the ground loop depth 

11 
 

 

Figure 13. Soil temperatures measured 24 in. below the ground loop depth 

Per Figure 11 through Figure 13, the following trends can be seen: 

•	 As expected, soil temperatures are highest at the ground loop depth, as shown in Figure 

12, particularly during the summer peak cooling seasons. 

•	 Soil temperatures directly below the slab and at the ground loop depth exhibit more 

oscillatory behavior over time than those 24 in. below the ground loop, where thermal 

gradients through the soil have become more attenuated. 

•	 The GSHP fan module failed during the last few days of August 2011, but it was repaired 

and returned to operation on September 9, 2011. The sudden decline in ground 

temperatures can be seen during this period. 

•	 To accommodate concurrent operation in the home for short-term testing of the GSHP 

desuperheater (Mittereder and Poerschke 2013), the research team used a supplemental 

2-ton air-conditioning unit to simulate winter conditions. This is apparent in the drop in 

ground temperatures during the first to second weeks in July 2012. Without this 

additional unmetered cooling, the 2012 summer temperature profile would most likely 

have appeared similar to the 2011 summer temperature profile. 

•	 Further desuperheater short-term testing occurred from mid-August 2012 to the first 

week in September 2012. This testing consisted of three tests (each test lasted 1 week) 

that used the desuperheater and included domestic hot water draws. This diverted a 

portion of the compressor’s rejected heat to the domestic hot water system in lieu of the 

12 
 

 

heat going to the ground loop. This is apparent in the change in soil temperatures that 

occurred in mid-August 2012. 

3.3 Soil Moisture Content 

Figure 14 illustrates the daily average soil moisture content distribution over the sampling 

period. The first three water content reflectometers shown in Figure 14 were located at the 

ground loop depth; the fourth sensor, measuring 23%, was located 2.0 ft below the East–Middle 

device. 

Figure 14. Soil moisture content 

Per these data, the soil moisture content remained relatively constant throughout the sampling 

period, indicating little potential variability in the thermal transport properties of the soil during 

the calendar year. The low and constant moisture levels are the result of a well-installed French 

drain around the structural footing. In a typical horizontal ground loop installed in an open field, 

moisture levels would have been much greater. This has a significant impact on the heat transfer 

rates and associated ground temperatures. 

3.4 Soil Thermal Conductivity 

The 2005 ASHRAE Handbook—Fundamentals describes the effect of moisture on soil thermal 

conductivity as follows (ASHRAE 2005, p. 25.13): 

Effective or apparent soil thermal conductivity (k) is difficult to estimate precisely 

and may change substantially in the same soil at different times due to changed 

moisture conditions and the presence of freezing temperatures in the soil... 

Although thermal conductivity varies greatly over the complete range of possible 

13 
 

 

 

 

 

 

 

moisture contents for a soil, this range can be narrowed if it is assumed that the 

moisture contents of most field soils lie between the “wilting point” of the soil 

(i.e., the moisture content of a soil below which a plant cannot alleviate its wilting 

symptoms) and the “field capacity” of the soil (i.e., the moisture content of a soil 

that has been thoroughly wetted and then drained until the drainage rate has 

become negligibly small)... As heat flows through the soil, the moisture tends to 

move away from the source of heat. The moisture migration provides initial mass 

transport of heat, but it also dries the soil adjacent to the heat source, hence 

lowering the apparent thermal conductivity in that zone of the soil. 

The 2005 ASHRAE Handbook—Fundamentals further states (ASHRAE 2005, pp. 25.13–25.14): 

Trends typical in a soil when other factors are held constant are: 

•	 k increases with moisture content 

•	 k increases with increasing dry density of the soil 

•	 k decreases with increased organic content of the soil 

•	 k tends to decrease for soils with uniform gradations and rounded soil 

grains (because the grain to grain contacts are reduced) 

•	 k of frozen soil may be higher or lower than that of the same unfrozen soil 

(because the conductivity of ice is higher than that of water but lower than 

that of the typical soil grains). Differences in k below moisture contents of 

7 to 8% are quite small. At approximately 15% moisture content, 

differences in the k-factors may vary up to 30% from unfrozen values. 

The thermal conductivity that was estimated during the design phase of the sub-slab heat 

exchanger was 9.9 kJ/h∙m∙K or 2.75 W/(m∙K). This value is slightly above the normal range of 

values indicated in the 2005 ASHRAE Handbook—Fundamentals, Chapter 25, Table 5 

(ASHRAE 2005, p. 25.14), which has a normal range of common soils of 0.6 to 2.5 W/(m∙K). 

During the monitoring period, the soil temperature did not approach the freeze point. Thus, the 

soil conductivity was not influenced by freezing conditions. During the construction period after 

the installation of the sub-slab heat exchanger and French drain system but before the monitoring 

period, it is possible that the soil dried out, causing the soil thermal conductivity to decrease. In 

effect, a moisture shadow exists under the house foundation and French drain. 

3.5 Basement Floor Heat Flux 

Figure 15 illustrates the daily average basement floor heat flux distribution over the sampling 

period. For these curves, positive values indicate heat flow from the house into the slab; negative 

values indicate heat flow from the slab into the house. All three heat flux transducers were 

affixed to the surface of the basement floor using thermally conductive grease to minimize the 

thermal contact resistance at the interface. 

14 
15 

Figure 15. Measured basement floor heat flux

Per Figure 15, the following trends can be seen: 

• For most of the summer sampling periods (i.e., for any day having a non-zero daily 

average heat flux), some net thermal exchange occurred between the slab and the house, 

thereby contributing additional load on the GSHP system. 

• During the heating seasons, a net heat flow occurred from the house into the slab. 

• During the swing season months, the basement floor heat flux was very low, indicating 

negligible thermal exchange between the slab and the house.

Initial TRNSYS modeling results for the heat transfer rates of the basement predicted that the 

heat transfer direction would always be from the basement into the ground and that the basement 

slab temperature would never be greater than the zone air temperature. These results differ from 

the measured data and the final TRNSYS model used for validation due to the change in thermal 

conductivity for the soil. 

TRNSYS modeling predicted a summertime slab peak temperature of approximately 22.7°C 

(73°F), indicating that heat transfer upward through the slab was not anticipated because the 

peak temperature was lower than the cooling indoor set point of 24.4°C (76°F). Figure 16 shows 

the measured results for the basement slab temperatures. The peak slab temperature was higher 

than 25°C (77°F) for a 1-week period during the summer; however, this was only for a portion of 

the slab. Once the outdoor temperature extremes relaxed, the excess heat in the slab quickly 

dissipated. The higher-than-expected slab temperatures during the end of 2012 are an artifact of 

the position of the unfinished basement supply air duct. This duct was blowing directly onto the 

slab surface, giving an apparent rise to the measured temperature.
 

Figure 16. Measured results for basement slab temperatures 

3.6 Ground Loop Working Fluid Temperatures

Figure 17 illustrates the daily average temperatures for the outdoor ambient, ground loop fluid, 

and elevation-averaged soil over the sampling period. Entering water temperature (EWT) and 

leaving water temperature were averaged only for values occurring during the system run. 

Because of an undetected malfunctioning pipe-mounted TC probe, successful recording of 

ground loop fluid temperatures did not begin until August 2011. 

16 
 

 

 

 

Figure 17. Daily average outdoor ambient, ground loop fluid, and soil temperatures 

Figure 17 shows the following trends: 

•	 The largest loop-soil temperature difference occurred during the peak summer cooling 

seasons. 

•	 A rapid drop in loop fluid temperature occurred in late August 2011 during a short GSHP 

shutdown period and then again in mid-July 2012 as a result of desuperheater testing. 

This rapid decay of soil temperatures indicates that short breaks in outdoor ambient 

temperature extremes can effectively prevent overheating below the slab. 

•	 Throughout nearly all of September through December 2011, the loop inlet temperatures 

continued to remain higher than the loop outlet temperatures, indicating the GSHP was 

operating in the cooling mode. This further indicated continued heat flow into the ground 

because the outdoor ambient temperatures fell only modestly during the 2011 autumn 

season. 

•	 Because of the unusually mild winter season, there were several occasions where space 

cooling was taking place in the home as a result of high thermal insulation and consistent 

simulated internal loads. 

Figure 18 is a composite of measured and simulated fluid temperatures. The significant 

temperature increase in the horizontal loop during July is attributed to the performance of the 

heat exchanger and not an abnormally warm summer as indicated by the average July outdoor air 

temperature relative to Typical Meteorological Year 3 data. 

17 
 

Temperature [F] 

120 

100 

80 

60 

40 

20 

0 

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

Vertical Loop Simulated EWT Horizontal Loop Mesured EWT 

2012 Measured Outdoor Air TMY3 Outdoor Air 

Figure 18. TRNSYS results for ground loop temperatures 

Decreased heat pump COP due to less advantageous EWT is a concern with the sub-slab heat 

exchanger. Table 3 summarizes the COP for a TRNSYS simulated vertical well system and 

measured data from the sub-slab heat exchanger. COP values are determined from the average 

EWT during system operation and expected heat pump performance trends from manufacturersupplied catalog data. Figure 19 indicates the average COP for average monthly EWTs. 

Table 3. Summary of COP Values 

Heating Cooling 

Vertical Loop COP (Simulated) 

Horizontal (Sub-Slab) COP (Measured) 

Percent Change 

4.58 

4.11 

–11% 

5.16 

4.41 

–17% 

COP

7 Vertical Loop Cooling 

COP 

6 

Horizontal Loop 

5 Cooling COP 

Vertical Loop Heating 

COP 

4 

3 

Horizontal Loop 

2 Heating COP 

1 

0 

0 10 20 30 40 50 

EWT 

Figure 19. Heat pump COP versus EWT 

18 
 

Continuous operation of the desuperheater, with regular hot water draws, could reduce the 

necessary heat rejection to the ground, lowering the horizontal loop summer temperatures. 

Although this could possibly improve the cooling COP, occupant behavior can vary widely, 

making this an unreliable design strategy. 

19 
 

 

 

 

 

4 Economic Analysis 

To confirm the net economic gain of using a sub-slab heat exchanger, the research team 

conducted a simple 30-year economic analysis. The team used BEopt+ 1.4 (BEopt) to predict the 

cost of a vertical well heat exchanger for a 1.5-ton system in the Pittsburgh Lab Home. The team 

then subtracted the assumption of $2,500/ton savings from this value to determine the equivalent 

cost of a sub-slab heat exchanger installation (Oberg 2010; Appendix A). The capital recovery 

factor predicts the annual payment on a 30-year mortgage (Masters 2004).1 

Finally, the research team calculated the annual electricity cost by using measured monthly 

electricity consumption and then reducing this amount by the ratio of COPs for the sub-slab heat 

exchanger to the vertical well heat exchanger as predicted by TRNSYS. Based on simulated and 

measured monthly average EWTs, the team used catalog performance data to estimate the COP, 

calculated an annual cost of the loan plus energy, and then compared that cost to the cost of the 

standard vertical well system. 

Table 4 summarizes the results of the economic analysis, which indicate that, compared to the 

standard vertical well system, the sub-slab heat exchanger is $200/year less expensive to operate. 

Additionally, the team determined that the annual fuel escalation rate would have to be greater 

than 10% for the increased costs of electricity consumption to negate the reduced installation 

cost. 

Table 4. Cost Analysis of Vertical Versus Sub-Slab Heat Exchange Loops 

Vertical 

Heat Exchanger 

Sub-Slab 

Heat Exchanger 

Annual Interest Rate, i (%) 

Duration of Mortgage, n (years) 

Capital Recovery Factor 

Average COP 

Electricity Use (kWh) 

Additional Loan (System Cost) 

Annual Payment 

Monthly Payment 

Annual Utilities 

Annual Loan Plus Energy 

0.05 

30 

0.065 

4.867 

2,768 

$17,763 

$1,155 

$96.29 

$312 

$1,467 

0.05 

30 

0.065 

4.262 

3,167 

$14,000 

$911 

$76 

$357 

$1,267 

Because of the experimental nature of the sub-slab heat exchanger, the team did not conduct a 

more in-depth analysis involving maintenance cost and tax incentives. Likewise, the team did not 

conduct a comparison to other heat pump systems because this was beyond the scope of the 

research. 

n i(1+ i) 1 CRF = , where i is the annual interest rate and n is the duration of mortgage in years (Masters 2004). n (1+ i) −1 

20 
 

 

 

 

 

 

5 Discussion and Conclusions 

5.1 Heat Exchanger Parameters

Based on the measured data, the ground loop heat exchanger met the heating and cooling needs 

of the Pittsburgh Lab Home. Furthermore, the measured data and analysis indicate that a sub-slab 

GSHP is an effective strategy for capturing the advantages of a GSHP without the expense of 

drilling vertical wells adjacent to a low-load house in a suitable climate region. Although the 

installed cost is lower, the overall COP of the system is lower due to the reduced effectiveness of 

the sub-slab heat exchanger at rejecting thermal energy to the soil. With strategies to raise the 

thermal conductivity and improve the pipe layout, the negative thermal consequences could be 

mitigated. Despite the slightly increased annual energy usage, over a 30-year economic analysis, 

the sub-slab system is less expensive than a vertical well system. 

Because of the low margin for error in system design and the greater risk of an undersized 

system causing negative and compounding side effects, the sub-slab heat exchanger design 

requires more verification and stress testing before it is ready for a production home 

environment. Heat waves or prolonged cold snaps will more significantly affect the system 

performance due to the small immediate soil volume for heat transfer. The shallow depth of the 

basement slab also results in greater influence from the soil surface temperature relative to a 

deep vertical well system. The R-410A refrigerant used by the heat pump loses one-fourth of its 

ability to transfer energy per cycle at the peak sub-slab EWT of 50°C (122°F) compared to the 

design EWT of 30°C (86°F). Beyond 50°C, the ability of the heat pump to operate efficiently is 

significantly compromised (ASHRAE 2005, Figure 14, p. 20.30). Additional study may indicate 

that certain climate zones and soil types will result in refrigerant temperature extremes beyond 

the operating limits of a standard heat pump compressor. Regardless, this system is effective for 

a low-load-density building such as a building meeting the Passivhaus standard. Temperate 

regions with a balanced number of heating and cooling degree days and a neutral deep earth 

temperature around 10°C (50°F) are best suited for this system. Because basement surface area is 

the limiting factor in system size, a single-story ranch house would be most effective for sub-slab 

installations. 

Several factors could improve the system performance and increase the range of acceptable 

houses. One possibility includes increasing the excavation depth for the loop to a level below the 

base of the footing or installing the loop in a different geometric layout that extends deeper into 

the earth. Another solution could be to control the moisture content by adding small amounts of 

water beneath the slab to increase the moisture content, thus improving the thermal heat transfer 

of the ground. Increasing the R-value of the insulation below the slab would reduce the heat flux 

through the basement floor. 

The research team was able to show a high degree of accuracy using the latest TRNSYS model 

for predicting the working fluid temperatures of a sub-slab GSHP as described in Appendix B. 

When possible, the team used known physical properties of materials as inputs. Of all the inputs, 

the team adjusted only the soil thermal conductivity within reasonable ranges to improve the 

accuracy of the simulation. The TRNSYS model could be used to accurately design future 

systems if the soil properties are well known and understood. Additionally, the TRNSYS module 

could accurately function as part of a larger, whole-house model to predict overall performance 

of building systems that use sub-slab heat exchangers. 

21 
 

 

 

 

5.2 Design Considerations

Several important factors must be considered when designing a sub-slab heat exchanger: 

• House specific peak heating and cooling loads 

• Annual heating and cooling loads 

• Soil thermal conductivity 

• Soil thermal diffusivity 

• Fill aggregate conductivity 

• Excavation depth 

• Heat exchanger geometry 

• Slab insulation 

• Balance between number of heating degree days and cooling degree days. 

When considering the feasibility of a sub-slab heat exchanger, the two most important 

characteristics to consider are the house-specific peak load relative to the basement floor area 

and the soil thermal conductivity. Initial analysis points toward a potentially strong relationship 

between these two values, indicating the approximate range of load densities possible for the soil 

thermal conductivity of a given location. Figure 20 illustrates a possible relationship useful in the 

early stages of system design. This relationship would be derived from many simulations across 

climate zones. Typically, only the specific peak load is a controllable parameter in system 

design. Soil thermal conductivity is a property of the house site; however, in some situations, 

conductivity may be modified by controlling moisture content. The relative location of the 

Pittsburgh Lab Home is indicated on Figure 20. Specific Peak Load [Btu/h*ft2] 

Feasible System 

Pittsburgh Lab Home 

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 

Soil Thermal Conductivity [Btu/h*ft*F] 

Figure 20. Estimated relationship between soil thermal conductivity and specific peak load 

22 
 

Once it has been determined that a sub-slab heat exchanger is feasible, the other factors 

contribute to an optimal system design. 

Between geographical regions, soil thermal conductivity can vary widely. As such, each system 

design will need to consider the local soil conditions directly under the house (ASHRAE 2005, 

p. 25.13). Results from the updated modeling effort suggest that knowledge of soil properties to a 

depth of 30 ft is sufficient for system design. 

Soil thermal conductivity plays a major role in the effectiveness of ground coupled heat 

exchangers, such as the sub-slab heat exchanger. The initial estimates for the soil thermal 

conductivity were slightly higher than normal ranges and were based on initial site conditions in 

an open field. The soil thermal conductivity may have decreased during the period after 

installation of the French drain and sub-slab heat exchanger and prior to beginning soil moisture 

content measurement. Based on this observation, the chosen soil thermal conductivity values for 

a sub-slab heat exchanger design should be closer to the “wilting point” or drier end of the 

thermal conductivity range of the soil. 

5.3 Future Research 

Initial research and analysis indicate valuable topics for future work. Much of this future work 

would be similar to the research and modeling that have been undertaken for vertical well 

systems, with the intent to understand the additional sensitivities of the sub-slab system. The 

following are topics for future research: 

•	 Analyze the effect of a heat wave or long cold snap on soil temperatures and system 

operation to determine if the heat pump will fail to meet the building load. 

•	 Study the effects of the depth of the ground loop on system performance. Consider the 

extremes of a slab-on-grade house or a deep basement with additional excavation. 

•	 Develop the relationships between soil conductivity and diffusivity, balance between the 

number of heating degree days and cooling degree days, and load density. Guidance 

could be developed for feasible system parameters. 

•	 Develop a regional map outlining soil conductivity and diffusivity, balance between the 

peak heating and cooling loads, and building load density. 

•	 Study the effect of continuous desuperheater usage on the peak summer loop 

temperatures and COP. 

•	 Empirically validate a model across multiple locations and climate zones. 

23 
 

 

 

References 

ASHRAE. (2005). 2005 ASHRAE Handbook—Fundamentals. Atlanta, GA: ASHRAE. 

BEopt. BEopt E+ 1.4 Building Energy Optimization Simulation, Version 1.4. Golden, CO: 

National Renewable Energy Laboratory. 

Goetzler, W.; Zogg, R.; Lisle, H.; Burgos, J. (2009). Ground‐Source Heat Pumps: Overview of 

Market Status, Barriers to Adoption, and Options for Overcoming Barriers. U.S. Department of 

Energy, Energy Efficiency and Renewable Energy Geothermal Technologies Program. 

http://www1.eere.energy.gov/geothermal/pdfs/gshp_overview.pdf. 

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

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

LoggerNet. (2012). LoggerNet 4.1 Datalogger Support Software. Logan, UT: Campbell 

Scientific. www.campbellsci.com/loggernet. 

Masters, G. (2004). Renewable and Efficient Electric Power Systems. Hoboken, NJ: John Wiley 

& Sons. 

Mittereder, N.; Poerschke, A. (2013). Desuperheater Performance and Modeling in a Cold 

Climate. Golden, CO: National Renewable Energy Laboratory (under review). 

Oberg, B. (2010). Cooperative Agreement DE-FC26-08NT02231. Building America Final 

Technical Report. Pittsburgh, PA: IBACOS. 

Oberg, B.; Bolibruck, S. (2009). Cooperative Agreement DE-FC26-08NT02231. Building 

America Final Technical Report. Pittsburgh, PA: IBACOS. 

Stecher, D.; Prahl, D.; Burdick, A. (2013). Final Expert Meeting Report: Simplified Space 

Conditioning Strategies for Energy Efficient Houses. Golden, CO: National Renewable Energy 

Laboratory. NREL/SR-5500-52160. www.nrel.gov/docs/fy11osti/52160.pdf. Accessed 

November 2013. 

TRNSYS. (2007). Transient System Simulation Tool, Version 16.01. Madison, WI: Thermal 

Energy System Specialists, LLC (TESS). www.trnsys.com/. 

TRNSYS. (2012). Transient System Simulation Tool, Version 17.1. Madison, WI: Thermal 

Energy System Specialists, LLC (TESS). www.trnsys.com/. 

24 
 

Appendix A: Excerpt from “Cooperative Agreement DE-FC26­

08NT02231. Building America Final Technical Report” (Oberg 

2010)—Pittsburgh Lab Home Heating, Ventilation, and Air 

Conditioning 

25 
50 

Building America Final Technical Report 

Cooperative Agreement DE-FC26-08NT02231 

IBACOS, B. Oberg – Project Director / Principal Investigator 

•	 When insulating sheathing, housewrap, and drywall ceiling on the second floor installed, and only 

penetrations through sidewalls and top and bottom plates were sealed, blower door tested 

enclosure airtightness was 2.86 air changes per hour at 50 Pa. 

•	 After strategic airsealing with closed cell spray foam insulation at the attic floor, blower door 

tested enclosure airtightness was 0.91 air changes per hour at 50 Pa. 

•	 After closed cell spray foam insulation was applied to the band joists, blower door tested 

airtightness was 0.87 air changes per hour at 50 Pa. 

•	 After the wall cavity was insulated with blown fiberglass and interior drywall applied, blower 

door tested enclosure airtightness was 0.65 air changes per hour at 50 Pa. 

In summary, the testing demonstrated that the exterior housewrap and insulating sheathing system, with 

strategic airsealing of penetrations acts as a very effective air barrier. The fact that the closed cell spray 

foam insulation application at the band joists had a minimal airtightness impact reinforces that the wall 

system was already airtight even though wall cavities were un-insulated and no interior drywall was 

installed. Strategic airsealing of the second floor ceiling with closed cell spray foam insulation was shown 

to have the largest airtightness impact during the testing, highlighting the importance of this practice. 

2.1.4 HVAC

During 2010, IBACOS continued its focus on researching very high performance HVAC technologies. In 

particular, IBACOS completed constructability research necessary to establish cold climate options for 

the lab house and broadened energy use and system performance modeling studies to include other 

climate zones. The investigations focused on the applicability of a ground-source heat pump system and 

sub-slab heat exchanger; the same system that is in the IBACOS Lab House. 

Earlier simulation work as documented in the “2009 IBACOS Building America Annual Report for 

Budget Period 2” (IBACOS 2010) identified a ground-source heat pump system as the lowest energy 

solution when it was also integrated with the domestic hot water system (Figure 2.31). The system was 

further optimized for energy and cost savings by modeling the system with a variety of water storage tank 

configurations and ground-loop configurations. A dual-tank DHW system achieved the lowest energy use 

and a horizontal sub-slab ground-source loop is now being tested as a viable cost-saving alternative to 

conventional vertical wells or horizontal loops outside the footprint of the house. Detailed research 

questions can be found in Appendix 2.H of this report. 

26
 

 

51 

Building America Final Technical Report 

Cooperative Agreement DE-FC26-08NT02231 

IBACOS, B. Oberg – Project Director / Principal Investigator 

Lab House HVAC System Options 

4 Bedrooms 

8000 

Energy Use - (kWh/yr) 

7000 

6000 

5000 

4000 

3000 

2000 

1000 

0 

1943 

989 

2231 2129 

13 57 1541 

69 0 

796 

68 2 705 

688 64 1 

4721 

4721 

3151 

2609 

1505 1386 

DHW (direct) 

Cooling 

Heating 

Soil Type: shale 

System nom. cap.: 2 tons 

Grout conductivity: 1.15 Btu/hr•ft•°F (med-high) 

3549 

6506 

7354 

3568 

6064 

5443 

ASHP GSHP No ASHP ASHP & HPWH GSHP 50 m GSHP Sub-slab 

Desuperheater w/HPWH w/subslab HX wells -1 m 

Figure 2.31: Lab House System Options 

Figure is a schematic depiction of the HVAC/DHW system installed in the Lab House. Much of the 

apparent complexity is due to thermocouples, meters, extra valves, and bypasses that were installed for 

research purposes. A production system would be much less complicated and therefore less expensive. 

Missing from this schematic are any representation of the sub-slab and vertical-well ground-coupled 

piping loops on the right side of the diagram. The two different ground-loop configurations are redundant 

and each contains two piping circuits. Eight shut-off valves enable each circuit to be individually isolated. 

27
 

 

52 

Building America Final Technical Report 

Cooperative Agreement DE-FC26-08NT02231 

IBACOS, B. Oberg – Project Director / Principal Investigator 

To washer in Laundry 

Room and to tub & 

sinks in Master Bath 

T 

T 

T 

T 

T 

M 

W 240 VAC 

T 

200 liter 

Water 

Heater 

400 liter 

Pr eheat 

Tank 

T 

T 

HW Manifold 

Expansion 

Tank 

Heat 

Pump Drain 

water 

heat 

recovery 

and Powder Room 

From 

Cold 

M 

M 

M 

T 

W 240 VAC 

M 

T 

T 

Flow 

Center 

Figure 2.32: Lab House HVAC/DHW System Schematic 

Sub-Slab Ground-Source Heat Pump Loop 

Progress in this area of HVAC research within the Lab Home in 2010 focused on acquisition of 

equipment, installation of equipment and monitoring sensors, determination of specific research questions 

and the creation of testing and monitoring schedules to be implemented in 2011. 

While it is not expected to be as efficient as a vertical well configuration, the horizontal sub-slab approach 

will reduce installation costs by approximately $2500/ton. With simulated peak hourly heating and 

cooling loads reduced to not much over one ton and peak average daily loads well under that, the load on 

the heat exchanger is reduced to the point where the area of the slab is sufficient to handle the load. 

Figure illustrates the distribution of the annual system heating load on a daily basis. The total number of 

heating days are 222 with 30% requiring less than 1,000 Btu/hr and less than 9% requiring more than 

8,000 Btu/hr. At an operational COP of 4.0, the load on the sub-slab heat exchanger is 25% less than the 

loads shown in Figure 2.33. This means the maximum daily average load on the heat exchanger is 7,150 

Btu/hr and 83% of the heating season imparts a load of less than 4,500 Btu/hr. These loads include 

operation of the DHW desuperheater, which could be disabled during the heating season to reduce the 

heat exchanger load if necessary. 

28
53 

Building America Final Technical Report 

Cooperative Agreement DE-FC26-08NT02231 

IBACOS, B. Oberg – Project Director / Principal Investigator 

0 

10 

20 

30 

40 

50 

60 

70 

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 

Days 

Average Daily Heating Load - (Btu/hr) 

Distribution of Annual Heating Load 

Figure 2.33: Lab Home - Distribution of Space Heating Load 

Figure 2.34 shows the installation of two horizontal sub-slab piping loops for the Lab House ground 

source heat pump. To prepare for the installation, the entire slab area was excavated to the bottom of the 

wall footer to allow more room for the installation of the piping, extra limestone fill, and insulation 

beneath the slab. Each piping circuit consists of approximately 400 feet of 3⁄4” HDPE pipe installed 

beneath the basement floor slab. A “folded” serpentine configuration was chosen to serve two purposes; 

it enables the circuit inlet and outlet to be in the same location, and it allows for a larger bending radius 

while maintaining a maximum spacing of one foot. A horizontal slinky configuration was also 

considered, but was ruled out due to the numerous pipe crossings and less-even distribution of the pipe 

within the limited slab area. The piping is anchored to heavy-gauge 6 x 6 flat-panel wire mesh with zip 

ties to keep it flat. Wire mesh from roll stock should NOT be used. Long steel pins such as those used for 

securing pipes to the side of a trench would be an acceptable alternative method. The one-foot radius 

bends are about the limitation for cold bending. 

29
54 

Building America Final Technical Report 

Cooperative Agreement DE-FC26-08NT02231 

IBACOS, B. Oberg – Project Director / Principal Investigator 


 

Figure 2.34: HDPE 3⁄4” tubing installed beneath the basement slab will serve as the ground loop for the ground-source 

heat pump. 

The sub-slab system will serve the ground-source heat pump system for the entire calendar year of 2011. 

A redundant vertical-well system was also installed at the time of construction. This system will be used 

in 2012 to provide a benchmark comparison to the experimental sub-slab system. 

30
55 

Building America Final Technical Report 

Cooperative Agreement DE-FC26-08NT02231 

IBACOS, B. Oberg – Project Director / Principal Investigator 

Figure 2.35: GSHP Sub-Slab Heat Exchanger - Minimum monthly average fluid temperatures 

As expected, the annual space conditioning and DHW energy use in Figure is inversely proportional to 

the minimum sub-slab temperatures shown in Figure 2.35. Figure 2.36 also shows the relatively small 

contribution the desuperheater is providing to reducing the domestic hot water load in mixed and cold 

climates. This indicates the potential for a dedicated domestic hot water function on the heat pump, 

provided it is controlled to eliminate the risk of freezing the ground beneath the slab. 

31
56 

Building America Final Technical Report 

Cooperative Agreement DE-FC26-08NT02231 

IBACOS, B. Oberg – Project Director / Principal Investigator 

Figure 2.36: GSHP with Sub-Slab Heat Exchanger: Annual space conditioning and DHW energy use 

Air Distribution System 

Progress in the area of air distribution systems research within the Lab Home in 2010 focused on 

acquisition of equipment, installation of equipment and monitoring sensors, determination of specific 

research questions and the creation of testing and monitoring schedules to be implemented in 2011. 

Research questions can be found in Appendix D.1 of this report. 

Table 2.6 illustrates the three different test modes of the air distribution system. Mode “A” is the most 

conventional system, designed using ACCA guidelines for duct velocity and ASHRAE guidelines for 

supply air outlet velocity. Mode “B” utilizes one large supply air outlet will serve each floor and the only 

air circulation will be via the ventilation system. Mode “C” will utilize the PVC distribution of the 

ventilation system, which will act as a high-velocity system. Main duct velocities will be approximately 

1,750 fpm compared to a typical design maximum of 900 fpm. Diffuser outlet velocities will remain the 

same in Mode “A” and “C”. 

Table 2.6: Air Distribution System Operation Schedule 

Mode System Zones Outlets Cycle Time 

A Ductboard 2 Multiple 2 weeks 

B Ductboard 1 2 2 weeks 

C PVC 1 Multiple 2 weeks 

32
 

Appendix B: Updated TRNSYS Simulation and Model Validation 

Simulation Results 

After the Pittsburgh Lab Home was in an operational state for more than one year, the IBACOS 

team began a renewed modeling effort to take advantage of the latest subroutines available. The 

initial design stage TRNSYS simulation used a combination of the Type 701b basement module 

as well as Type 706 sub-slab heat exchanger module. The results from this simulation indicated 

favorable heat transfer into the soil and that the slab surface temperature would not rise to a level 

that would transfer energy into the basement zone. In 2012, TESS released a new sub-slab 

module that combines the capabilities of Type 701 and Type 706. This new module is called 

Type 1267 (TRNSYS 2012). The research team compared the results of this model to simulated 

data to determine the accuracy of the model. 

Type 1267 utilizes a fully implicit finite difference solver to calculate energy exchange through a 

number of predefined nodes. Nodes are manually defined by a process that considers the 

geometry of the pipe network, building, and soil. An optimized node layout is generated to 

provide smaller grid spacing in areas of interest, while balancing system run time. This node 

specification file is then used by the Type 1267 module. Currently, because of the manual nature 

of the process, a large portion of the set-up time for a simulation is configuring the node file. For 

this analysis, the file contained approximately 600,000 nodes. Due to the level of detail and the 

number of soil nodes, an annual simulation of 8,760 hours takes approximately four days to 

complete on a quad core laptop. 

The IBACOS research team provided TESS with a data set with all measurements pertinent to 

the sub-slab heat exchanger on a five-minute-period basis to use as a calibration aid. In addition 

to the measured data, the team gave TESS pertinent information regarding the system operation, 

including information on the duct switch-over schedule and other abnormal behavior in the 

house. TESS ultimately provided the IBACOS research team with a working model and soil 

node file, which the team was able to run for the final simulation. Table 5 compares the initial 

and final simulation values. 

33 
 

 

Table 5. TRNSYS Simulation Parameters 

Parameter Initial Value Final Value Units 

Simulation Length 17,520 8,760 h 

Time Step 1/10 1/12 h 

Deep Earth (Average Surface) Temperature 10.40 (50.72) 10.40 (50.72) °C (°F) 

Thermal Conductivity of Soil Layer 9.9 (1.589) 6.24 (1.002) kJ/h∙m∙K 

(Btu/h∙ft∙°F) 

Density of Soil Layer 2,080 (129.854) 2,100 (131.103) kg/m3 (lb/ft3

) 

Specific Heat of Soil Layer 0.84 (0.2007) 0.96 (0.2293) kJ/kg∙K (Btu/ lb∙°F) 

Fluid Through Pipes Thermal Conductivity 1.65168 (0.2651) 1.775 (0.2849) kJ/h∙m∙K 

(Btu/h∙ft∙°F) 

Fluid Through Pipes Density 1,000 (62.43) 963.2 (60.133) kg/m3 (lb/ft3

) 

Fluid Through Pipes Specific Heat 4.19 (1.001) 4.097 (0.9788) kJ/kg∙K (Btu/ lb∙°F) 

Fluid Through Pipes Viscosity 3.2 (2.150) 4.387 (2.948) kg/m∙h (lb/ft∙h) 

Pipe Inside Diameter 0.0204 (0.803148) 0.021539 

(0.84799) m (in.) 

Pipe Outside Diameter 0.0250 (0.98425) 0.02667 

(0.08988) m (in.) 

Pipe Thermal Conductivity 1.401895 (0.2250) 1.728 (0.2774) kJ/h∙m∙K 

(Btu/h∙ft∙°F) 

34 
During verification, the model was driven using measured fluid temperature and flow rate 

leaving the heat pump. As an additional level of detail, the fluid temperature was set to decay 

toward the measured air temperature; however, all calculations regarding the validation of the 

model occurred only when the system was operating at full flow. Outputs were collected from 

the temperature of fluid returning from the sub-slab heat exchanger, as well as the temperature 

and heat transfer to all materials. These initial tests appear to show good correlation between 

measured and simulated temperatures. 

Figure 21 represents typical winter operation of the GSHP. The simulated and measured EWTs 

closely aligned during continuous system run. The results of this initial simulation indicate that 

the module developed by TESS is accurately predicting the heat rejected to the ground. As 

indicated in Figure 22, the simulation slightly underpredicted the summer EWT. This aligns with 

the simulation underpredicting the soil temperature, likely the result of assumed input parameters 

such as deep earth soil temperature. 

Figure 21. Typical heating season operation: ground loop fluid temperatures 

35 
 

Figure 22. Typical cooling season operation: ground loop fluid temperatures 

Error Analysis

Figure 23 provides a seasonal breakdown of the error associated with the simulated fluid 

temperatures. Seasonally, the summer was the least accurate because the model overpredicted 

heat transfer to the deep earth. There was significantly less system operation during the swing 

seasons, and the total variation relative to the mean was slightly greater as well, indicating the 

simulation is not as accurate at predicting the fluid temperature when the system mode is 

changing. Winter includes December through February, and spring includes March through May. 

Summer includes June and July, and fall includes August through November. 

36 
 

Figure 23. Simulated EWT variation from measured data 

To estimate the error in the model, the research team performed a mean bias error (MBE) 

calculation on the measured entering fluid temperature, as compared to the simulated entering 

fluid temperature during system operation. Figure 24 shows a slight negative offset to the 

simulated EWT. 

37 
 

 

Figure 24. MBE—simulated versus measured EWT 

The MBE was calculated using Equation 1, 

Σ(Simulated − Measured ) MBE = , (1) 

n 

where n is the sample size. 

Table 6 summarizes the results of the error analysis. Each percentage value was calculated by 

dividing the MBE by the average of the value. For this analysis, the most important value is the 

percentage error in the ground loop fluid temperatures. The simulated EWT shows accurate 

correlation, and the overall percentage of the mean shows minimal error. 

Another metric to consider is the average error of the absolute value of the change in fluid 

temperature. This percentage is slightly higher and gives an indication of the percentage of error 

associated with the energy transfer to the ground. On an annual basis, the model is predicting 

slightly lower fluid temperatures than reality. 

Overall, the soil temperature also exhibits low error and tracks with the fluid temperatures. The 

western portion of the slab under the finished basement exhibits slightly greater error, indicating 

that, in reality, heat is building up to a greater extent than predicted by the model. 

38 
 

 

Table 6. MBE of Loop and Soil Temperatures 

Variable MBE Percentage 

Fluid Temperature Returning from the Ground Loop (°C) 

Fluid Temperature Delta T [°C] 

North–West Soil Temperature 24 In. Below the Loop (°C) 

South–West Soil Temperature 24 In. Below the Loop (°C) 

East–Middle Soil Temperature 24 In. Below the Loop (°C) 

LOPPEN* Soil Temperature 24 In. Below the Loop (°C) 

Energy Rejected to the Loop (kWh) 

–0.34 

–0.34 

–1.58 

–0.99 

–0.46 

–1.26 

–0.01 

–1.7 

–12 

–9.0 

–5.5 

–2.5 

–7.1 

–10.7 

*The portion of soil directly below the loop piping entrance 

Deep earth soil temperature influences the overall rate of heat transfer. For this simulation, the 

deep earth temperature was assumed to be the annual average surface temperature. Some of the 

model’s inaccuracy during summer months can be explained if the deep earth soil temperature is 

higher than assumed, based on regional climate data. The Pittsburgh Lab Home is located on 

southern sloped terrain with no surrounding trees, resulting in greater energy flux into the soil 

from solar irradiance. 

Another important measure of error during system design is the overall heat transfer into the soil. 

The calculated MBE for the energy rejected to the loop is based on the absolute value of the 

calculated energy transfer. Figure 25 illustrates the cumulative annual energy transferred into the 

soil through the ground loop. These values are calculated from measured temperatures and flow 

rates, as well as the simulated water temperature leaving the ground loop. Although there is a 

slight skew in the absolute value of the simulated results, the slopes of the two values match 

throughout the simulation period. 

Figure 25. Cumulative energy transfer to soil from loop fluid 

39 
 

 

Basement Heat Flux 

Another area of interest is the accuracy of the simulation to predict the heat transfer from the 

sub-slab heat exchanger into the basement zone. Energy transferred through the slab will 

contribute to the cooling load in the summer and the heating load in the winter, thereby 

increasing the overall energy use of the system. Initial TRNSYS predictions indicated that soil 

and slab temperatures would not be elevated to a degree that would contribute to this exchange. 

However, measured results indicate higher than expected soil temperatures and consequently 

greater transfer of energy into the building zone. Figure 26 and Figure 27 represent the 

cumulative heat transfer as measured and simulated by the model for the West and East sections 

of the slab, respectively. These results indicate that the model is accurate at predicting the net 

heat transfer for each season. Discrepancies could arise from the assumption of a constant 

coefficient of convection from the slab into the zone air. The East section of the slab was subject 

to greater variation in airflows due to changing of air distribution layouts for additional testing in 

the rest of the house. Throughout the summer months, the direction of heat transfer was into the 

basement. This is apparent during the period of July through October, when the curve undergoes 

a minimum and has a positive slope. 

Figure 26. Cumulative heat transfer, West slab 

40 
Figure 27. Cumulative heat transfer, East slab 

Heat Exchanger Geometry

Results from the modeling efforts suggest the large impact of heat exchanger geometry on the 

ability of the system to reject heat to the ground. Figure 28 indicates the location of the 

temperature slices relative to the house basement. 

Figure 28. Locations of soil temperature slices (not to scale) 

41 
 

Figure 29 indicates that, for the Pittsburgh Lab Home, the flow arrangement associated with the 

piping below the eastern unfinished basement offers a more efficient design. The hottest fluid is 

running through the perimeter of the system; thus, the greatest amount of energy can be 

conducted away from the exchanger. Alternatively, the flow arrangement associated with the 

western portion under the finished basement routes the hottest fluid directly into the center of the 

system, causing a buildup of temperature in the center and reducing performance. Curiously, the 

structural engineering recommendations for the system design were to group the loops as close to 

the center of the slab as possible to avoid excessive excavation near the perimeter. The ideal 

design would route the hottest fluid near the perimeter of the slab while maintaining a margin of 

safety around the footings. 

Figure 29. Horizontal temperature profile at loop level during summer operation (°C) 

The model developed by TESS is capable of simulating a wide variety of heat exchanger 

geometries, including placing the loops on the outside perimeter of the foundation as proposed 

by Oak Ridge National Laboratory (Hughes and Im 2013) and investigated in several 

publications. 

The seasonal soil temperature changes are an interesting output of the model and can be 

analyzed. The soil temperature at various depths exhibits extensive seasonal variation, as shown 

in Figure 29 through Figure 31. A pocket of warm soil exists under the slab during autumn, 

which improves the efficiency of the system during the early heating season. The horizontal slice 

is located at the pipe layer. 

42 
 

 

Figure 30. Horizontal temperature profile at loop level during fall operation (°C) 

Figure 31. Vertical slice of soil temperatures (°C) 

Modeling Conclusions

The final test is to determine whether accuracy is maintained when the sub-slab model is driven 

with simulated heat pump operation and water temperatures. The assumption is that if individual 

components of a model are accurate, the model as a whole will be accurate. Although not 

presented in this report, simulation results indicate that when using catalog performance data to 

43 
 

 

 

simulate leaving water temperatures, the accuracy of the model is maintained. The simulation 

runtime of 4–5 days offers a significant hurdle for using this model in a parametric manner to run 

a large number of simulations. Provided there was a strong interest in using this model for 

system design, an effort to reduce system run time through multicore code or code utilizing the 

latest graphics processing unit parallel computing technology could be undertaken. Additionally, 

the manual process of creating the soil node layout could be automated by updating an existing 

tool provided by TESS. 

As with any computer-based simulation, the results are only as accurate as the assumed input 

values. Given ideal knowledge of the soil and material properties, the TESS model can be used 

as an accurate design tool in a wide variety of system geometries, not limited to underslab loops. 

However, the sub-slab heat exchanger model has a number of inputs with values that are difficult 

to measure and that change over time. Because the sub-slab system is more sensitive to 

variations in simulation inputs than a vertical well, worst-case scenario values should be 

considered to give a range of operational conditions. 

44 
References 

Hughes, P.; Im, P. (2013). Foundation Heat Exchanger Final Report: Demonstration, Measured 

Performance, and Validated Model and Design Tool, January 2012, Revised January 2013. Oak 

Ridge, TN: Oak Ridge National Laboratory. http://web.ornl.gov/sci/ees/etsd/btric/publications/ 

ORNL-FHX%20Final%20Report_Jan%202012%20for%20Distribution_rev_01152013.pdf. 

TRNSYS (2012). Transient System Simulation Tool, Version 17.1. Madison, WI: Thermal 

Energy System Specialists, LLC (TESS). www.trnsys.com/. 

45 
 

Appendix C: Excerpt from “Cooperative Agreement DE-FC26­

08NT02231. Building America Final Technical Report” (Oberg and

Bolibruck 2009)—Pittsburgh Lab Home Sub-Slab TRNSYS 

Results 

46 
 2214 Liberty Avenue, Pittsburgh, PA 15222 1-800-611-7052 www.ibacos.com

Advanced Systems Research Annual Report 22 

Cooperative Agreement DE-FC26-08NT02231 

IBACOS, B. Oberg and S. Bolibruck – Project Directors/Principal Investigators 

Sub-slab heat exchanger 

IBACOS constructed a TRNSYS model of the ground source heat pump (GSHP) system with a 

horizontal sub-slab heat exchanger configuration using a new TRNSYS “Type” developed by 

TESS, Inc. specifically for this project. The component combines a ground-coupled basement 

model with a sub-slab heat exchanger into one sub-routine. This eliminates the need for certain 

approximations and assumptions that were being used in the previous IBACOS model as a 

means of calculating realistic ground temperatures around the heat exchanger. Simulation results 

indicated that the annual energy usage is only 4% more than that of a comparable system using 

two 50-meter vertical wells. Combined annual energy use is 3,802 kWh/yr. Monthly average 

temperatures ranged from 42°F to 73°F for bottom-of-slab temperatures (see Figure 10) and 

from 39°F to 76°F for outlet fluid temperatures (see Figure 11) from the sub-slab heat 

exchanger. Inlet fluid temperatures were slightly lower, which may still make antifreeze a 

requirement, but a freeze stat could be used to avoid frost heave damage to the foundation. Temperature ‐ (°F)

75

70

65

60

55

50

45

40

35

30

Fro

Ba

nt_W

ck_Wall_T

all_T

Lef

Rig

t_Wal

ht_Wa

l_T

ll_T

Slab_T

Jan Jul Jan Jul Jan Jul Jan Jul

Time ‐ (months)

Figure 10: Foundation temperatures - horizontal ground heat exchanger (GHX) 

47
 

   

   

   

   

23 

2214 Liberty Avenue, Pittsburgh, PA 15222 1-800-611-7052 www.ibacos.com

Advanced Systems Research Annual Report 

Cooperative Agreement DE-FC26-08NT02231 

IBACOS, B. Oberg and S. Bolibruck – Project Directors/Principal Investigators 


 

35

40

45

50

55

60

65

70

75

80

85

Jan Jul Jan Jul Jan Jul Jan Jul

Temperature ‐ (°F)

Time ‐ (months)

Vertical Loop Inlet

Vertical Loop Outlet

Horizontal Loop Inlet

Horizontal LoopOutlet

Figure 11: Heat pump ground loop temperatures 

IBACOS completed further simulations to evaluate the energy impact due to changes in the 

proximity of the heat exchanger to the slab. Results illustrated in Figure 12 indicate that the 

horizontal heat exchanger piping actual performs better at shallower depths beneath the slab, 

which will help limit the amount of over-excavation and reduce costs. 

48
 

   

   

2214 Liberty Avenue, Pittsburgh, PA 15222 1-800-611-7052 www.ibacos.com

Advanced Systems Research Annual Report 24 

Cooperative Agreement DE-FC26-08NT02231 

IBACOS, B. Oberg and S. Bolibruck – Project Directors/Principal Investigators 


 

1258 1462

596

595

1947 1705

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

Sub‐slab

Horiz. GHX

‐1.0/‐0.5 M

Sub‐slab

Horiz. GHX

‐0.5/‐0.25 M

Energy Use ‐ (kWh/yr)

DHW (direct)

Cooling

Heating

Soil Type: shale

Systemnom. cap.: 2 tons

Grout conductivity: 1.15 Btu/hr•ft•°F (med‐high)

3802 3762

Figure 12: Energy impact of GHX proximity to slab (3-bedroom occupancy) 

Dehumidifier performance 

IBACOS assembled a TRNSYS model to simulate the performance of a stand-alone 

dehumidifier over a wide range of inlet air conditions. The model relies on an empirically-based 

performance map provided by the manufacturer and can be used to evaluate the necessity, energy 

consumption, and annual efficiency of a high performance dehumidifier used with typical 

heating and cooling systems in the context of a high performance home. IBACOS evaluated the 

performance as a function of infiltration, latent loads, and maximum humidity set points. 

IBACOS evaluated the impacts of infiltration and humidity capacitance using basic constantvolume and linear models and then using more enhanced models. The enhanced infiltration 

model was adapted to TRNSYS from chapter 27 of the ASHRAE Handbook of Fundamentals and 

the enhanced humidity capacitance model was already an integral part of TRNSYS. The models 

were used to evaluate a home built to the 30% energy savings level. In almost all cases, annual 

heating energy for the enhanced models (see Figure 13) was slightly higher than that of the same 

cases using the basic models (see Figure 14), while cooling energy needs were much lower. 

With dehumidification energy, the results were split, showing energy increases for the cases with 

high infiltration rates and no dehumidification energy use at all for the enhanced model cases 

with low infiltration rates. Overall energy use, including dehumidification, was lower using the 

enhanced models for every case; the largest differences occurred in the cases with lower annual 

infiltration rates, lower internal latent gains, and higher humidity set points. In the cases that 

most closely represented the lab home, the enhanced models resulted in a total annual space 

conditioning energy usage approximately 56% less than previously calculated using the basic 

models. 

49
buildingamerica.gov 

DOE/GO-102013-4295 ▪ November 2013 

Printed with a renewable-source ink on paper containing at 

least 50% wastepaper, including 10% post-consumer waste.