Installing a Geothermal Heatingand Cooling SystemGround-source heat pumps rely on buried water loops to heat and coolefficiently and economically

BY WILLIAM J. EVANGELIST

B

ack in the early 1980s, my me-chanical-contracting business in-stalled air-to-air heat pumps and

gas furnaces. Period. Then I started design-ing a new house for myself. I oriented thehouse for passive-solar heat, chose premiumwindows and doors, and after some intensiveresearch, settled on a geothermal heat pumpto heat and cool the house. It was the clearfirst choice.

Soon after I moved in, my brother built ahouse. He also wanted a geothermal system.Two neighbors were next. Before I knew it,geothermal heating and cooling had becomemy full-time business, and business is good.

In eastern North Carolina, where I live,air-source heat pumps are common. Duringthe winter, however, these heat pumps haveto work harder to extract heat from the air,and they become less efficient. When out-

side temperatures fall to about 30°F, electri-cal-resistance heaters kick in, making heatpumps relatively expensive to run. Althoughgeothermal heat pumps are similar, they aremore efficient. Both types work on the prin-ciple of vapor compression—just like airconditioners and refrigerators (sidebar p. 106).Electrically operated pumps and compres-sors move heat from one medium to another.But because no fuel is burned, the heat pump

D E S I G N B A S E D ON THE S ITEGround-source heat pumps can rely on any one of four undergroundwater systems, depending on conditions at the site. They includepond loops (top), horizontal loops (center) and vertical loops(bottom). Another option is an open loop (not shown)in which well water is used and then discarded.

does not have to be vented. The same unitcan heat a house during the winter and coolit in summer. Although many ground-sourceheat pumps run forced-air heating systems,some models can power hydronic heat or ra-diant-floor heating systems as well.

Geothermal systems pick up heat from wa-ter circulating through plastic pipe buried inthe ground. They work well no matter whereyou live because underground temperatures

stay relatively constant all year, even whereoutdoor temperatures vary widely. Once youget down about 6 ft., the temperature of theearth in this region is a steady 55°F.

Typically, the installed cost of one of ourheat pumps is at least 20% and as much as80% more than an air-to-air heat pump or agas furnace plus central air-conditioning.However, the energy use can be 40% to 60%less. There's little maintenance required, and

geothermal units are quiet. Mechanicalequipment is inside the house, which is notthe case with air-to-air heat pumps or cen-tral air-conditioning units, so the landscapeis much more appealing.

System is sized according toheating and cooling demandHeat-loss and heat-gain calculations are thekey to choosing the right equipment. For anexisting house, I measure wall lengths andceiling heights, note the size and type ofwindows, check the orientation of the houseto the sun and determine what kind of insu-lation the house has. For a new house, all theinformation I need to determine the loadsshould be on the house plans.

Using a room-by-room calculation helpsme to size the ductwork properly. WaterFur-nace, the brand of heat pump we use (FortWayne, IN; 800-436-7283; www.waterfur-nace.com), allows us to offer zoned heatingand cooling with a single piece of equip-ment. These units use multiple thermostats,motorized dampers, and variable-speedblowers and compressors. They can be in-stalled in basements, crawlspaces, attics,closets or garages. Slightly larger and heavierthan the air-to-air heat pumps, ground-source heat pumps are still light enough tobe placed on standard framing in a second-floor installation.

I also ask whether anyone in the family hasallergies that may require special air filtra-tion. Set-point temperatures for both heat-ing and cooling—where the equipment willturn on and off—also are important in sizingthe equipment correctly. We typically use70°F for the heating set point and 75°F forthe cooling set point, but even relativelysmall changes in these numbers can affectthe size and cost of the equipment we use.

Our system also helps to heat domestic hotwater with a built-in device called a desuper-heater. These systems generally supply be-tween 50% and 70% of hot-water needs.

The ground loop can be one of fourdifferent designsGeothermal heat pumps can use any one offour types of ground loops (drawing left). Thedesign is based on the conditions at thebuilding site. At my own house, I was able touse the 30-gal. per minute (gpm) well thathad been drilled for our domestic-water sup-ply in what is called an open loop. This ar-rangement is possible only if the well producesenough water volume to meet the demand ofthe heat pump— gpm per ton of heatingand cooling (a ton is the equivalent of 12,000Btus). That amounts to at least 5 gpm for a

How the heck does a geothermal heat pump work?Any fool would want toknow how cold water canheat a house in the winter. Intwo words, the answer is"vapor compression," a prin-ciple of refrigeration thathas been in practical usesince the mid-19th century.

Heat pumps are refrigera-tion devices running back-ward. They do not generateheat like a conventional fur-nace or boiler; they justmove heat from one place toanother. At the heart of aground-source heat pump isa sealed loop containing arefrigerant (drawing right)that interacts with both wa-ter circulated through un-derground pipes and with airfrom the house. Workingpressures and temperaturesof water and refrigerant sys-tems inside a heat pump canvary widely, depending onthe size of the system andentering air and water tem-peratures. The numbersused in the following sce-nario do not apply to allground-source heat pumpsor all installations.

FROM A LIQUID TO A GASAND THEN BACK AGAINAt the beginning of the heat-ing cycle, water circulatedthrough underground pipesthen enters a heat exchanger.At the beginning of the heat-ing season, the water temper-ature might be 50°F or 55°F. Bythe end of the season, it mayhave cooled to 30°F. Even atthat temperature, the wateris warmer than the refriger-ant. When the two meet inthe heat pump's evaporator,the refrigerant warms up andchanges from a liquid to agas. The refrigerant is sucked

Boosting earth's natural heat. A ground-source heat pump relies on the principle of vaporcompression to extract heat from a closed, underground loop of water-filled polyethylene tubing.Inside the heat pump, a sealed refrigerant system is the heat-exchange medium. During summer,the direction of the refrigerant is reversed, allowing the pump to extract heat from the air.

Heat pump in heating mode

into an electrically drivencompressor at 55 to 65pounds per square inch (psi),where it is heated to 94°F andreleased at 180 psi. This gasnow enters a condenser,where it gives off heat to astream of air moved by fans.This 95°F air is what getsducted around the house andkeeps you warm. The refriger-ant goes through an expan-sion valve, where it cools fur-ther and once again becomesa low-pressure liquid.

That takes care of coldweather. In summer, when theproblem is too much heat,the flow of the refrigerant isreversed, and the heat pumpbecomes an air conditioner.Air from the house circulatesthrough the heat pump,where it is cooled and dehu-midified. The refrigeranttransfers the heat to the cir-culating water in the groundloop, so the earth becomes aheat sink. Excess heat also isrouted through another heat

exchanger called a desuper-heater (not shown in draw-ing), where it warms domesticwater. In the cooling season,hot water is essentially free.

The refrigerant in WaterFur-nace heat pumps is R-22, a hy-drochlorofluorocarbon(HCFC). HCFCs have a lowerozone-depletion potentialthan chlorofluorocarbons(CFCs), whose production wasbanned in the 1990s.—Scott Gibson, a free-lancewriter in Middlebury, CT.

Heat pump in cooling mode

By Scott Gibson

2,000-sq. ft. house. Well water is pumpedinto the unit, where its heat is extracted andthen discharged, in my case into a creek.

If there's enough well capacity and gettingrid of the water is not a problem, this systemis usually my first choice because it has thelowest installed cost. In this area, an open-loop system for a 2,000-sq. ft. house wouldcost between $6,000 and $10,000. Althoughan open-loop design can include a returnwell for water that has run through the heatpump, this system gets expensive.

When a well doesn't produce enough waterto support the heat pump, a closed-loop sys-tem is used. There are three kinds: a pondloop, a vertical loop and a horizontal loop.The simplest is a pond loop, which consistsof coils of -in. polyethylene pipe that aresubmerged in a pond. Of course, you've gotto have a pond (8-ft. minimum depth). Butif you do, the cost would run between $9,000and $13,000 for the same 2,000-sq. ft. house.

A vertical loop is the way to go when thereisn't a lot of land to work with. Typically, two

-in. pipes with a U-bend at the bottomare dropped into a 6-in. dia. hole, which isthen backfilled with high-solids bentonitegrout. Depths range from 100 ft. to 400 ft.,depending on the design of the system. Anaverage 2,000-sq. ft. house would need be-tween 350 ft. and 400 ft. of depth, either inone deep shaft or two shallower ones. Thissystem is the most expensive approach be-cause of the drilling costs, and a system suchas this one would probably cost between$14,000 and $19,000.

I put in more horizontal systems than anyother kind. You don't need a pond, and it'sless expensive than a vertical bore. The onlycatch is that you need a little bit of land(photo p. 104). We dig a 6-ft. deep trenchabout 2 ft. wide and lay in -in. polyethylenepipe at the 6-ft. level and at the 4-ft. level.We need about 150 ft. of trench for each tonof heating and cooling load, so the house I'vebeen using as an example would need about400 ft. of trench. A system such as this onewould cost between $11,000 and $15,000.

Check for underground utilitiesbefore you start diggingBefore any excavation begins, we measureand mark off the area for trenching, and wealways contact local utilities for informationon buried gas, water and electric lines. In thisinstallation, we were lucky. There was plentyof room behind the house for the trenches. Idecided to go with two separate trenchesthat are parallel for about half their length.As I neared the tree line, I turned the trench-es back toward the house, so from the air,

Heat pumps can live virtually anywhere. Even a crawlspace with crouch-only head-room is large enough for this style of heat pump. Other models stand upright. Supplyand return ducts are insulated.

Sealing foundationagainst leaks. Wheresupply and returnlines penetrate thefoundation, the authoruses plastic sleevesand boots to seal outwater. Corrugatedplastic drain line pro-tects the water loopfrom damage.

Color-coding keeps loops straight. Manifolds divide the single supply and return linesinto four separate circuits. Colored vinyl tape identifies each circuit, two stripes forsupply and a single stripe for return.

Heart of the system is underground. After coils of tubing have been pressure-tested, they are rolled into a waiting trench. Note thatthe shoulders of the trench have been stepped back to minimize the threat of a collapse.

Two loops in the same trench. To make the most of the heat potential in soil that sur-rounds buried tubing, loops that share a common trench must be separated. One isplaced at the full 6-ft. depth; the other is stapled to the wall 2 ft. above it.

each of the trenches would have appeared tobe a big J.

In the crawlspace near the heat pump, weinstalled a flow center, which is the pump as-sembly for the loop water (top photo, p. 107).We ran one supply line and one return linefrom the flow center through the foundationwall (center photo, p. 107) to what we call aheader pit outside. That's the area, about 6 ft.below grade, where these lines are connectedto manifolds that separate supply and returnlines into a number of separate circuits.

In the installation shown in these photos,there are four loops of polyethylene pipe, twoloops in each trench. Each manifold has fourports (bottom photo, p. 107). On the supplyside, each port channels water into a loop; on

the return side, the manifold gathers waterfrom each loop and sends it back to the heatpump via the return line. Before the polyeth-ylene is actually laid in the trenches, we pres-sure-test and color-code each loop. The cod-ing, strips of colored vinyl tape, keeps usfrom confusing the big rolls of tubing whenthey are hooked up to the manifolds. Alltubing connections are heat-fused, making ajoint that is as strong as the tubing itself.

Putting the loops in the ground is simple.A roll of tubing starts at the header pit. It isunrolled down the trench (photo facing page),turned around in an enlarged pit at the endof the run, then returned to the header pit.Tubing at the 6-ft. level at the bottom of thetrench is held in place temporarily with soil

while we run the second loop. Tubing at the4-ft. level is attached to the walls of thetrench with landscaping staples (photo aboveleft). When all is in place, the trench can bebackfilled. I tamp down the soil with the back-hoe's bucket as I go to minimize settling.

To charge the system, air must beremoved from the linesBefore the header pit can be filled in, severalsteps remain. We must fill the lines with wa-ter, add ethanol to prevent freezing and thenpressure-test the circuits again. To add waterand remove air from the loops, we use some-thing called a flush cart from WaterFurnace.When the system is full of water, we use thesame device to add ethanol—about 20% ofthe total volume of the loop (photo aboveright). The ethanol prevents freezing andmakes the system more efficient so that wecan use shorter trenches. As ethanol is added,an equal volume of water is taken out of theloop, keeping the system free of air pockets.

Finally, we install interior grates and ther-mostats and start the system. We brief cus-tomers on how to operate the system andprovide maps of buried water loops.

William J. Evangelist's company, Evangelist ServiceCompany (www.geoevangelist.com), is in Apex,North Carolina. Photos by Scott Gibson.

Air out, antifreeze in. A flush cart is usedto add water and antifreeze to the sys-tem while removing trapped air. Theethanol antifreeze boosts the system'sefficiency and allows for shorter runsof tubing.