NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

DOE Webinar – Residential Geothermal 

Heat Pump Retrofits

NREL

DOE Webinar Series

Erin R. Anderson

December 14, 2010

NREL/PR-6A10-50142
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Applications on Residences 

Costs

Use & Availability – Certified Designers, & Installers

Financing Options

Technology Options –Vertical vs. Horizontal

Key Information for RFPs

Project QA/QC and Monitoring

Tools & More Information on GHP
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

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Technology Options
Innovation for Our Energy Future

NREL Geothermal Technology Team

Geothermal Energy

• Heat energy contained in the 

earth

• Shallow temperature fluctuate 

with the seasonal air 

temperatures

• Temperature become stable at 

depths of 30 ft and approach 

annual average air temps 

• This is all true, absent a 

geothermal anomaly or high 

temperature gradient areas 

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Innovation for Our Energy Future

NREL Geothermal Technology Team

Geothermal Heat Pumps (GHP)

• Highly efficient method 

of providing heating and 

cooling

• Work by using ground 

temperature as a 

renewable resource for 

pumping heat in winter 

and rejecting heat in 

summer

• Cost effective 

• Economic and 

environmental benefits
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NREL Geothermal Technology Team

What is a Heat Pump & How Does it Work?

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The ground loop 

transfers ground 

energy to a 

refrigerant in the 

heat pump 

Increasing the pressure 

increases in the temperature

Heat is transferred to the 

heating fluid or air handler

Heat is transferred to the 

room by radiant floor 

heating or air distribution 

system

Refrigerant expands causing it 

to cool 

Heat Pump

Compressor

Expansion 

Valve

Evaporator Condenser
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NREL Geothermal Technology Team

Geothermal Efficiency 

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• Use the green under your feet...as close to 

the money tree as you can get
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NREL Geothermal Technology Team

Why is GHP useful and efficient?

Ground 

Warmer 

than Air

Ground Cooler 

than Air
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NREL Geothermal Technology Team

Types of Heat Pumps

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• Characterized by medium used for the Heat Source & 

Heat Sink

– Air to Air – Air Source Heat Pump

– Water to Air – Water Source Heat Pump

– Water to Water – Uses hydronic pipes through the building

• Heats, Cools, & Produced Hot Water

– Capacity is measured in tons, like AC units
Innovation for Our Energy Future

NREL Geothermal Technology Team

GHP System & Components

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Air Fan –

circulates 

building air

Air Coils –

heats or 

cools air

Controls

Compressor

Refrigerant 

Loop

Ground Loop & Refrigerant 

Loop Connection
Innovation for Our Energy Future

NREL Geothermal Technology Team

Types of Ground Heat Exchangers

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• Open Loop

– Ground water from a well, river, or pond

– Exchanges Heat & Water with the 

ground

– Returns water to the ground

Closed Loop

– Buried HDPE or copper (direct 

exchange) pipe

– Circulates as secondary fluid (water or 

water/antifreeze mix or refrigerant with 

copper pipe)

– Exchanges Heat with the ground

– Configuration – depends on available 

space and installation costs
Innovation for Our Energy Future

NREL Geothermal Technology Team

Horizontal Trench Loop

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• Cost effective when land is plentiful

• Trench depth is 6 ft or more to go 

below frost line

Rule of thumbs 

• Trench length – typically 300 ft

• Pipe length – out & back = 600 ft 

Source: Klaassen, Iowa Energy Center
Innovation for Our Energy Future

NREL Geothermal Technology Team

Horizontal Layout Options

• Single flow path

• Large land area required

• Larger pipe diameters required 

to reduce friction loss

• Increased antifreeze fluid 

volume

• Single flow path

• Shorter trench required than a 

single pipe system

• Larger pipe diameter than a 

parallel system

• Reduced trench length

• Parallel flow

• Deeper trench is generally 

required Source: IGSHPA

• More pipe area 

per trench length 

• Horizontal or 

vertically installed
Innovation for Our Energy Future

NREL Geothermal Technology Team

Vertical Ground Loop - Borehole

• Used in areas with land 

area restrictions

• Temperatures are typically 

more constant than 

horizontal

• Depths are usually 200-

300 ft

• Pipe length 400-600 ft

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Rule Of Thumb:

1 ton for every 200-300 ft borehole
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NREL Geothermal Technology Team

Vertical Configuration Options

• Small diameter pipe than series 

system

• Larger capacity heat exchanger 

can be designed

• 3⁄4 and 1 inch pipe loops are 

common

• Bore hole depths can be 

increased in limited land areas

Parallel

• Larger pipe required for series 

system

• Large pipe difficult to handle

• Heat exchanger limited to about 

3-tons because of pipe friction

Series

Source: IGSHPA
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Applications on Residences
Innovation for Our Energy Future

NREL Geothermal Technology Team

Process for Residential

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• Determine the heating/cooling loads (Btuh)

• Select heat pump size

• Select indoor air/water distribution system

• Estimate the building’s energy requirement

• Estimate the ground heat exchanger loads

– Annual load

– Design month’s load
Innovation for Our Energy Future

NREL Geothermal Technology Team

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Innovation for Our Energy Future

NREL Geothermal Technology Team

Building Heating and Cooling Load Profile

Outdoor Temperature F 13 96

50,000 Btuh 50,000 Btuh 36,000 Btuh 36,000 Btuh

62 6767

Design Heating Load Design Cooling Load
Innovation for Our Energy Future

NREL Geothermal Technology Team

Free Hot Water

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• Domestic Water Heating Applications

– Desuperheater to heat domestic water 

• Cooling Season = Free Water Heating

• Heating Season = High COP water heating

– Water to water heat pumps preheat Domestic Water at COP of 

3.0 to 5.0
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

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Costs
Innovation for Our Energy Future

NREL Geothermal Technology Team

First Costs

22

• Generally, the geothermal system cost inside the building 

is less than or equal to conventional systems

• The incremental cost over a conventional system is in the 

Geothermal Heat Exchanger (aka. ground loop) 

• Manage the installed costs

– Reduce heating/cooling loads

• Efficient building envelopes

– TC (thermal conductivity) test as applicable for project size

– Organize & minimize geothermal system piping
Innovation for Our Energy Future

NREL Geothermal Technology Team

Cost of Installation

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• Installed cost – ground loops

– Ground coupled – horizontal: $1,100/ton

– Ground coupled – slinky: $1,300/ton

– Ground coupled – vertical: $1,500/ton

– Ground water (3 ton, w/o well): $950/ton 

• Water-to-air heat pump units

– 8.8 kW (2.5 tons): $2,750 ($1,100/ton)

– 10.5 kW (3.0 tons): $3,000 ($1,000/ton)

– 14.1 kW (4.0 tons): $3,600 ($900/ton)

• Substantial variations possible

Ref: Kavanaugh & Gilbreath (1995), and Rafferty (2008)
Innovation for Our Energy Future

NREL Geothermal Technology Team

Estimations on Installed Costs

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2008 Mature 

Market Est. from 

Graph

2008 Immature 

Market

$ per Ton

ASHP $2,000 $4,000

Gas w/AC $1,967 $3,933

Ground Water $3,333 $6,667

Horizontal $3,933 $7,867

Slinky $4,033 $8,067

Vertical $4,300 $8,600

Source: Kevin Rafferty, 2008 - "An Information Survival Kit For The Prospective Geothermal Heat Pump Owner"

Estimates includes both inside (ductwork, heat pump unit, controls 

etc) and outside (ground loop, piping, excavation and/or drilling)
Innovation for Our Energy Future

NREL Geothermal Technology Team

Savings

• Average Energy savings: 31% to 71% 

• Average Cost savings ($): 18% to 54%

• Heating efficiencies 50 to 70% higher than other heating 

systems 

• Cooling efficiencies 20 to 40% higher than available air 

conditioners

• 4x for electric resistance

• 3x for propane

• 2x for ASHP

• 2x for fuel oil

• 2x for natural gas

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Sources: Geo-Heat Center, http://geoheat.oit.edu/pdf/hp1.pdf, IGSHPA

Savings in Energy Costs in USA Compared to GHP

($ per Btu or kWh)
Innovation for Our Energy Future

NREL Geothermal Technology Team

Operational & Maintenance Costs

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• Make sure circulating pumps are sized correctly

– Oversized pumps are energy hogs

Source: Kavanaugh, K & McInerny, S, “Energy Use of Pumping Options for GSHP”
Innovation for Our Energy Future

NREL Geothermal Technology Team

Habitat for Humanity Project – Hope Crossing, OK

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Standard Gas House Low Energy GHP & 

PV House

Energy Use 95 MMBtu 19 MMBtu

Energy Cost $1,739 $522

CO2 Emissions 25,460 lbs 9,825 lbs

Standard Gas House Low Energy GHP + PV House

Source: Ellis, D. (presented by Paul Bony 

2010, April 19), ClimateMaster
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

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Use & Availability – Certified Designers & Installers
Innovation for Our Energy Future

NREL Geothermal Technology Team

Feel Confident with the Team

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• Find certified contractors, designers, & installers:

• Call around to University's in the area who might do 

research on GHP and find out who are the best contractors

• Get firms that understand the critical importance of 

designing a GHP system that fits the building use and loads 

IGSHPA GEO
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

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Financing Options
Innovation for Our Energy Future

NREL Geothermal Technology Team

Help with the Costs

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• ESCOs - Energy Services Company 

– Successful with Fort Polk 

– Energy Saving Performance Contract (ESPC)

• Utility Negotiations 

– Reduce rates due to reduction of summertime peak loads with GHP

• Utility & Tax Incentives

– DSIRE - http://www.dsireusa.org/
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

32

Key Information for RFPs
Innovation for Our Energy Future

NREL Geothermal Technology Team

Things you should have in an RFP

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1. All contractors, installers, designers, & drillers must be 

IGSHPA certified

2. Require a minimum number of previous successful GHP

projects in your area and references 

3. Contact the bidders previous clients

4. Costs 

1. What are the actual performance savings of previous projects 

2. Clearly state how you want the GHP components broken down –

NO LUMP SUM Contracts

Project Specifications

• GHP General Specifications – by Oak Ridge Lab

– http://www.ornl.gov/~webworks/cpr/v823/rpt/107119.pdf
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

34

Project QA/QC and Monitoring
Innovation for Our Energy Future

NREL Geothermal Technology Team

QA/QC

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• Get GHP Smart...Fast

• Know the processes and what to look for

• If you get qualified owners rep, make sure that they are 

IGSHPA certified or have worked on several successful 

GHP projects. 
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

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Tools & More Information on GHP
Innovation for Our Energy Future

NREL Geothermal Technology Team

Software & Certified Installer Directories

• RETScreen

– http://www.retscreen.net/ang/

g_ground.php

• GeoKISS

– http://www.geokiss.com/gsoft

ware.htm

• Gaia Geothermal

– http://www.gaiageo.com/

• IGSHPA

– http://www.igshpa.okstate.edu/

directory/directory.asp

• GEO

– http://www.geoexchange.org/fi

nd-a-pro/geothermal-heatpump-directory.html

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Software Industry Directory

More Information

• Geo-Heat Center 

– http://geoheat.oit.edu/techpap.htm#heatpump

• GROUND-MED

– http://www.groundmed.eu/

• Geothermal Surviver Kit - Rafferty 

– http://www.heatspring.com/downloads/intro/GeothermalSurvivalKit.pdf