34 LICENSED ARCHITECT • VOL 13 NO. 3 • FALL 2009

Association of

Licensed Architects

Continuing Education

Special to Licensed Architect

Ground Source Heat Pumps - Conditioning a Building Near YouBy Garret W. Graaskamp, P.G., A.I.; American Ground Water Trust

n 2008, approximately one out of every 38 newhomes included a ground source heat pump(GSHP) to condition the interior space. Accordingto Air-conditioning Heating and Refrigeration

Institute (AHRI) statistics, the number of shippedGSHP units more than doubled from 33,000 in 2005to over 71,000 units in 2008. Shipment volume(40,417 units) was 35% ahead of the 2008 pace (29,894 units) through June 2009. The building industryhas taken notice of the environmental efficiency andlow operating and life-cycle costs of GSHPtechnology.

GSHP technology has (unfairly) been the ugly step-sister to solar and wind "renewable energy" fordecades because it does not produce electricitydirectly. The 24/7 reliability of GSHPs and their use ofthe sun’s renewable energy has been overlooked foryears. The American Recovery and Reinvest-ment Actof 2009, signed into law this past February, finallyrecognized the value of GSHPs as a significanttechnology in the nation’s effort to reduce our relianceon foreign fossil fuel supplies and global climatechange impacts. The Act authorizes, among otherincentives, a 30 percent tax credit for residentialinstallations and a 10 percent credit plus accelerateddepreciation for commercial installations through 2016.

The technology is not new. Nearly every home inthe nation uses it 24 hours per day to preserve foodin refrigerators. William Cullen at the University ofGlasgow first demonstrated in 1748 the vaporcompression refrigeration process that is funda-mental to GSHPs. It was not until 1834 whenAmerican Jacob Perkins, while living in London, firstput the vapor compression refrigerator into practicaluse. However, refrigerators did not become acommon consumer product until after the inventionof Freon in 1928. Freon replaced hazardousrefrigerants then in use such as methyl chloride,ammonia and sulfur dioxide. In particular, methylchloride units had been involved with several deaths

in the early 1920’s and people started putting their refrigeratorsout in the backyard. The thought of conditioning the air in abuilding using GSHPs was first mentioned in the commercialpress on October 25th 1948 in Modern Living Section of Lifemagazine in an article entitled "Fireless Furnace." AlthoughGeneral Electric, among other companies, ramped upproduction and marketing of food refrigerators to the nation inthe early 1950s, it took until the late 1970s before GSHPsbegan to establish a foothold in the HVAC marketplace.

EARTH ENERGYGSHPs harvest "low temperature" geothermal heat energy

from the ground beneath our feet. The source of the energy isrenewable everyday from the sun and occurs everywhereacross the world. Forty-seven percent of the energy radiatinginto the earth's atmosphere is absorbed into the ground and,on a sunny day, this is enough energy to replenish all theenergy used in the world on an annual basis (5 x 1020 Joules[U.S. Energy Information Agency]) in about 90 minutes.

This geothermal (little g) energy is different from the morecommonly thought of "hot rocks" Geothermal energy ("Big GGeothermal") the uses steam to create electricity. Big G isassociated with volcanoes and geysers with high temperatureground water above 300 ˚F (150 degrees Celsius). Big Genergy is derived from heat dissipating from hot magma (liquidrock) that has risen from very deep portions of the earth (theearth’s mantle) to within 3 to 5 miles of the earth’s surface. BigG energy is only found in limited areas around the worldassociated with current or former regional volcanism. Thewestern United States has famous examples includingvolcanoes Mount Rainier, Mount St Helens and Yellowstone

Figure 1: Map of the ground water temperatures in degrees Fahrenheit across theUnited States, which approximate the deep earth temperatures at

about 30 feet below ground surface. (Figure courtesy of McQuay International; Application Guide AG 31-008)

Learning Objectives:After taking this course, the reader will have a better understanding of:1. What geothermal energy is and its technology2. The common types of heat pumps used to collect the earth’s energy3. The cost benefits of ground source heat pumps4. How to properly design and install GSHP systems

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National Park. However, in total Big G is limited inthe United States to only portions of the states ofWashington, Oregon, California, Arizona, NewMexico, Nevada, Utah, Idaho, Montana andWyoming. There are no Big G locations east of theMississippi River capable of producing electricity.

Outside the Big G areas of the United States andindeed the world, the deep earth temperature atdepth’s between 30 to 500 feet is constant andequal to the average annual air temperature plusabout 2 ˚F (about 1oC) for a particular region. Figure1 is a map of the ground water temperatures foundaround the United States, which approximate theground temperatures at about 30 feet.

Below about 500 feet in non-volcanic regions (thatis, most everywhere), the earth’s temperature beginsto rise again at a rate of about 1.5˚F (about 1˚C) per100 feet of depth due to heat flowing by conduc-tionout from the earth’s interior. There is enough variationin the deep earth temperature that the temperatureshould be confirmed with a Thermal Conductivity (TC)test as part of designing a GSHP system forcommercial structures (> 25 tons [a "ton" is equal to12,000 Btus per hour; the amount of heat needed tomelt a 2,000 pound block of ice at 32˚F in 24 hours.])). TC test data are necessary to optimize theoperating efficiency of a large GSHP system.

COLLECTING THE EARTH’S ENERGYThere are four common types of heat pumps

including air-to-air, air-to-water, water-to-water andwater-to-air. Air-to-air heat pumps have beenpopular for several decades, especially in regionswith moderate air temperatures throughout the year.Air-to-water units are typically used in water heatingactivities and not space conditioning applications.Water-to-water units are an effective technology forradiant heat applications where seasonal cooling isnot necessary or is accomplished with an air-to-airsystem. Water-to-air systems, which comprise mostof the GSHPs being installed today, are the focus ofthe remainder of this article.

Ground source heat pumps have been, andcontinue to be, called by several names includingwater source HPs, ground source HPs,geoexchange HPs, Geothermal HPs and earth-coupled HPs to name a few. Ground source heatpump (GSHP) is used here to emphasize that thesource of the energy is the ground verses an air-sourced system. The name also underscores aGSHP’s capability to gather and reject heat energyto the ground regardless of whether there is

groundwater present. The most common GSHP system consists of three "loops"

including the indoor refrigerant loop, outdoor ground loop andthe indoor air distribution loop. An optional fourth loop fordomestic hot water (DHW) is a common easy addition to thesystem. Design of the air distribution loop including fresh air

exchange, is important to any properly functioning spaceconditioning system, but is beyond the scope of this article.

The refrigerant loop operates like any you will find in arefrigerator, except that the flow path can be reversed toaccommodate a heating cycle in the winter and a cooling cyclein the summer. During the winter, heating season (Figure 2) theliquid refrigerant in the GSHP unit collects heat from the groundloop and evaporates as it accepts the heat energy at aconstant temperature and pressure. The refrigerant gascontinues to the GSHP compressor where the pressure andtemperature of the refrigerant are increased to a superheatedgas as it passes through the compressor. The superheatedrefrigerant travels to a second heat exchanger (e.g., fan coil)where interior return air passes over the refrigerant loop causingthe refrigerant to condense to a liquid. The heat releasedduring the condensation process warms the return air, whichflows back to the living space. The refrigerant then passesthrough an expansion valve that serves to control the flowthrough the refrigerant loop and further cools the refrigerant.The cool refrigerant now starts the process again. In thesummer, the reversing valve is switched and refrigerant flows inthe opposite direction allowing heat from the interior air to beabsorbed by the refrigerant and rejected back into the groundvia the ground loop.

THE GROUND LOOPThere are two types of ground loop designs. One is an

"open system" where ground water is pumped directly as theenergy transfer fluid. The second is a "closed loop" system inwhich a piping circuit, usually made of High DensityPolyethylene (HDPE) pipe, is used to circulate water or an

Figure 2: GSHP system (closed loop) in heating mode. (Figure courtesy of theInternational Ground Source Heat Pump Association)

(Continued on page 36)

36 LICENSED ARCHITECT • VOL 13 NO. 3 • FALL 2009

Association of

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Continuing Education

antifreeze solution between the ground and therefrigerant loop in the GSHP. A closed loop systemis self-contained and there is no direct contact withthe closed loop fluid and groundwater. A lesscommon closed loop system known as directexchange (DX) eliminates the water circuit and runsthe refrigerant loop through copper piping directly

into the ground and then back to the GSHP unit toexchange with the air distribution loop.

There are several types of open loop systems.Groundwater pumped through an open system maybe returned to an aquifer (i.e., zone of undergroundwater storage) after exchanging its heat with theGSHP unit refrigerant or it may be discharged to asurface water body such as a pond, lake or river. Inthe first case, most state regulatory agencies requirethat the groundwater be returned to the sameaquifer from which it was obtained to maintain thewater balance (out volume = returned volume) andto protect water quality (i.e., not mixing water from

one aquifer with another). "Pump and Dump" is the common term for water discharged

to a surface location (Figure 3). This arrangement frequentlyrequires state or federal discharge permits related to wetlandconcerns and water quality and quantity control issues. It isalso extremely important to conduct an aquifer pump test at thedesign phase to ensure that there is enough ground water inthe aquifer to satisfy the water demand from the GSHP systemplus all other water needs for the structure. This is true whetherone well or multiple supply wells are used on the property. TheGSHP system will need between 1.5 to 3 gallons per minuteper ton of conditioning load. An average home with a four tonconditioning load would require 1.6 million to 3.1 million gallonsper year if the GSHP unit were operating only half-timethroughout the year.

Knowing the ground water quality is important for opensystems. Mineralized water (Iron is a common problem) canprecipitate and foul the well screens, reducing the flow andefficiency of the GSHP system. Damage or failure of therefrigerant-to-water heat exchanger inside the GSHP is also a

Figure 3: "Pump and Dump" open loop systems pump groundwaterfrom a water well and release it to a surface water body or second

(discharge) well. (Courtesy of ClimateMaster Inc.)

Figure 4: Horizontal ground loop systems are installed in shallowtrenches or excavations. They are a type of closed loop system.

(Courtesy of ClimateMaster Inc.)

Figure 5: A "pond loop" collects heat energy from thesurface water bodies and is an example of a closed loop

system. (Courtesy of ClimateMaster Inc.)

Figure 6: A vertical loop system can be installed on very small lots and does notrequire groundwater to be effective although the system must be designed to

accommodate "dry" conditions if ground water is deep and not in contact with theloop pipe. (Courtesy of ClimateMaster Inc.)

(Continued from page 35)

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Continuing Education

consideration. The system design must addressthese concerns and include mitigating operatingand/or treatment procedures.

Closed loop systems can be designed as shallow(typically 4 to 8 feet deep) horizontal pipingconfigurations (figure 4), submerged loops in pondsand lakes (figure 5), or in deep vertical boreholes(Figure 6). The choice of design depends on geology(local soil type and thickness) and the availability ofopen land space or a large surface water body.Vertical ground loops can be installed on "near zero"footprint lots but horizontal and pond loops requiresignificantly larger footprint space.

Loop spacing (separation) and piping length arecritical components of any ground loop design.Less is never more. In 1954, professors at theUniversity of Wisconsin-Madison demonstrated that acubic foot of "soil" contained 40 Btus of energy in theform of heat. We now know that this quantity willvary widely depending on the geology and moisturecontent of the ground. In any case, a certain volumeof ground will hold a certain quantity of heat energyjust as a certain size gas tank holds a fixed numberof gallons of gasoline (fixed amount of energy). If theground loop spacing is too close and/or the totalpipe loop length is too short for the actual energyloads to be conditioned in the structure, then theGSHP will not operate efficiently and will not be ableto meet adequately the space conditioning demandand may ultimately fail. The inefficiency and demandresponse will become progressively worse with time.However, if the GSHP system is designed to accessenough ground energy and the energy gathered fromthe ground is in balance with the energy returned tothe ground, a GSHP system can satisfy the spaceconditioning needs of a building indefinitely.

GSHP EFFICIENCYIn 1993, the Environmental Protection Agency

called GSHPs "the most energy-efficient,environmentally clean, and cost-effective space-conditioning system" available. This fact has notchanged. The basis for this statement lies with aGSHP’s ability to move the energy stored asmoderate year-round deep earth temperatures into(or out of) a structure as opposed to (1) air-to-air heatpump systems that exchange energy withseasonally-extreme outdoor air temperatures or (2)burning fossil fuels in on-site in relatively inefficientboilers and furnaces to create "new" heat energy.Traditional fossil fuel heating, ventilation and air-conditioning (HVAC) systems are typically only 80 to95 percent efficient when new or just after amaintenance treatment. Air-to-air systems may reach

efficiencies of 300 percent, but not when you need them most inthe depths of winter’s cold or during the hottest days of summer.Current GSHP technology routinely operates at efficiencies of400 to 500 percent. For each dollar spent on electricity to run aGSHP system, four to five dollars of heating or cooling isproduced. Figure 7 presents a comparison of Btus per dollarspent on energy based on 2009 April/May fuel pricinginformation for Illinois compiled by the U.S. Energy InformationAgency (EIA). Maintenance of a GSHP system is minimalcompared to other traditional HVAC systems providing a clearoperating cost advantage for GSHPs.

A FINAL CONSIDERATION"Rules of Thumb" are currently very common chatter among

many new entrants into the GSHP design and installationindustry. One must be extremely careful with these "rules" inorder not to become thumbless as the result of an improperlydesigned or installed GSHP system. GSHPs are the mostefficient space conditioning HVAC method available today.However, in order to earn this honor, the GSHP systems mustbe designed and installed properly by trained and experiencedprofessionals. A team approach is usually the best strategy.When qualified in-house expertise is not available (especially fora large home or commercial project), it is recommended that theArchitect should seek out an experienced International GroundSource Heat Pump Association (IGSHPA) Certified GeothermalDesigner who is also a professional engineer, an IGSHPA ormanufacturer accredited [ground loop] Installer and anappropriately licensed and experienced HVAC contractor toinstall the indoor equipment, piping and air distribution system.

The buildings you design today will be in service for the next 50to 100 years. Your HVAC choice will have an immediate andlong-lasting impact on energy consumption and the carbonfootprint created by operating your building creations. Pleasechoose wisely.■

Figure 7: Properly designed and installed GSHP systems in Illinois have the lowest operatingcosts per 100,000 Btu produced. They have a low carbon footprint impact compared to

burning fuel oil or natural gas in an on-site furnace/boiler because 48 percent of theelectricity consumed in Illinois is from non-carbon emitting renewable energy sources or

nuclear power plants. (Data from the Energy Information Agency, August 2009)

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Ground Source Heat Pumps - Conditioning a Building Near You

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QUIZ QUESTIONS

1. Which type of GSHP is the mostcommonly installed version today?a) steam-to-air heat pumpb) water-to-air heat pumpc) air-to-air heat pumpd) water-to-steam heat pump

2. The undisturbed deep earth temperature ofthe ground at a depth of 50 feet beneath aproperty is approximately equal to:

a) the average annual air temperatureplus 2˚F

b) the average annual water temperaturein the nearest surface water body tothe property

c) the average annual summertime airtemperature divided by four.

d) one degree Fahrenheit per foot timesthe depth in feet of the verticalground loop borehole

3. The most common GSHP system,without a domestic hot water option,consists of how many heat exchangeloops?a) one b) twoc) three d) four

4. During evaporation of the refrigerant inthe refrigerant loop, heat is released bythe refrigerant.a) Trueb) False

5. The most common installation depth forthe pipe in a horizontal ground loopconfiguration is.a) 4 to 8 feetb) 8 to 12 feetc) 12 to 16 feetd) 16 to 20 feet

6. Ground source heat pumps can movethe renewable energy from the sun that isstored beneath the ground to heatinterior spaces of a building.a) True b) False

7. What percentage of the sun’s energy isabsorbed by the ground on an annualbasis?a) 37 percent b) 41 percentc) 47 percent d) 53 percent

8. Standard ground source heat pumpscannot be used to produce electricity.a) True b) False

9. The vapor compression refrigerationprocess is fundamental to the operationof GSHPs. It was first demonstrated inwhat year?a) 1748 b) 1834c) 1928 d) 1948

10. Which statement below is true?a) A closed loop system exchanges ground

water with the refrigerant loop to transferenergy and an open loop system dumpsground water to either a surface waterbody or second well.

b) A shallow horizontal closed loopconfriguration uses groundwater toexchange heat energy with the GSHPrefrigerant loop.

c) An open ground loop system can pumpgroundwater directly to the refrigerantloop to exchange energy but a closedloop system pumps water or anantifreeze solution to the refrigerant loop.

d) A "Pump and Dump" loop is a commondescription for an outdoor vertical closedloop configuration.

Learning Objectives:

After taking this course, the reader will have a better understanding of:

1. What geothermal energy is and its technology2. The common types of heat pumps used to collect the earth’s energy

3. The cost benefits of ground source heat pumps4. How to properly design and install GSHP systems