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Subterranean Energy

Although heating water by using energy found beneath the earth’s surface is far from a new idea, watershape designer/builder Douglas Cook reports that only recently has this geothermal bounty become available for use with pools and spas.   Here, he offers a primer on this emerging technology, discussing ways in which the benefits of a fully renewable energy supply are starting to turn clients’ heads – and transform the way watershapes are heated or cooled.
Although heating water by using energy found beneath the earth’s surface is far from a new idea, watershape designer/builder Douglas Cook reports that only recently has this geothermal bounty become available for use with pools and spas. Here, he offers a primer on this emerging technology, discussing ways in which the benefits of a fully renewable energy supply are starting to turn clients’ heads – and transform the way watershapes are heated or cooled.

By Douglas Cook

Places just below the earth’s surface have been a resource for heating and cooling basically since the planet took shape.  Ever since, all sorts of creatures have taken refuge from excessive heat or extreme cold by burrowing into the soil, and it’s no accident that some of the earliest examples of human self-expression have been found in caves deep below the surface.

With pools and spas, however, up until recently there’s always been a legitimate question about whether this timeless heating/cooling approach was workable on any sort of cost-effective basis.  If recent projects designed by my firm, Aloha Pools Design Studio (Franklin, Tenn.) are any indication, that question has now been answered with a resounding yes.

On the face of it, that answer seems obvious.  After all, the U.S. Department of Energy says that geothermal heating is more cost effective than a 95-percent efficient gas heater – and that the same would be true even if 100-percent efficient gas heaters were available.  Only recently, however, have the suppliers of these systems reached a point where their equipment is properly scaled for residential applications, and we’ve found that they are a perfect fit with clients who want both energy efficiency and a greener world.


In approaching the technological progress that has made these systems available for use with pools and spas, let’s quickly set the scene and see what’s up with this fresh approach to watershape heating and cooling.

[ ]  What is geothermal energy?

Geothermal energy is a perfectly renewable source in the truest sense of the term, because it comes directly from nature in virtually limitless, inexhaustible quantities.  In that sense, it’s comparable to solar and wind energy, except for the fact that it is always there, day and night, no matter the season, the weather or any other factor that might intercede:  It’s always there, just below the earth’s surface.

This is so because the crust of the earth is warmed by the sun above as well as by the planet’s molten core.  So just a few feet beneath the surface, the earth in whatever form it takes – soil, sand or rock – stays at a constant temperature ranging from about 50 to 70 degrees Fahrenheit, depending on location.  And these zones of steady temperature are thick as well, starting about six feet underground and stretching down hundreds of feet in many places.

This is why, even on the hottest days, deep caves stay cool on hot days and relatively warm on cold ones.  Ruby Falls, a massive cave near where I work in Tennessee, is 57 degrees inside every single day of the year, so it should come as no surprise that well water locally is at 56 or 57 degrees straight out of the water tank.

In essence, because of basic principles of heat exchange, this constant temperature is a source of stored and renewable energy.  At the same time, we must recognize that this is quite distinct from the geothermal drama observed in volcanic activity, as with the geysers of Yosemite or Iceland:  Those are isolated regions of a special nature quite distinct from the cool constancy of spaces over which most of us live.

It is this constancy of temperature that is the key to using geothermal energy to heat and cool interior spaces and, at long last, the water in pools and spas.

[ ]  How does pool and spa heating by geothermal energy compare to other energy sources in current use?

The best current means of harnessing geothermal energy for residential use comes in the form of a device known as a water-source heat pump (WSHP), which serve to facilitate the transfer of energy via either open-loop or closed-loop systems.  

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Installation of these systems (including the horizontal closed-loop approach shown here) involves skills that should be familiar to almost all watershapers.  It’s about removing soil and setting up the plumbing runs, manifolds and hydraulic systems needed to move the water from the piping to the heat exchanger and then back through the loop.

When compared with other water heaters, WSHPs are, in either configuration and according to the U.S. Department of Energy as well as numerous independent research firms, the most efficient.  Indeed, when all pool heaters – gas, air-source heat pumps and water-source heat pumps – are compared on the basis of operating costs or cost per million Btus transferred into water, the clear leader is water-source, with air-source coming in second and gas a distant third.

This is great news for watershapers, because a large percentage of professionals in the pool and spa industry are already familiar with the operating principles and efficiencies of air-source heat pumps, which use the same approach to extracting heat from the air and transferring it into pool or spa water.  The main difference between the two approaches lies in the consistency of the heat source via which this transfer occurs.  Simply put, the earth’s temperature remains the same, but outside air temperature constantly changes.

As a result, the most efficient operating level for an air-source system occurs at 80 degrees of air temperature at 80 percent relative humidity – fairly narrow windows that have made these units best suited to warmer, more humid climates in Florida and the Southeast.  They can nonetheless be excellent choices in all 48 states and Southern Canada – but only for a limited number of months.  In addition, system output is relatively limited, with larger single-phase units topping out at about 135,000 Btus.

When the outside air temperature reaches about 50 degrees, some air-source heat pumps shut down completely – and those that still operate do so only with dramatically reduced capacity.  In other words, although these are great systems where they’re climatically workable, air-source heat pumps are nonetheless an incomplete solution and in many areas must in any event be supplemented by gas heaters to meet the higher heat-up requirements of uncovered spas.

[ ]  How do you decide which geothermal system best fits a particular application?

As we move forward in our general desire to select heating systems that make both economical and environmental sense, the key with geothermal systems is developing an understanding of how the technology works:  While the principles are similar to those familiar to us through use of air-source heat pumps, there are in fact some differences.

To use the earth as either a heat source (or, in the case of cooling systems, as a heat sink), the installer buries a series of pipes commonly called a loop in the ground near the watershape or the building that is being “conditioned.”  Such a loop can be oriented either vertically or horizontally, depending for the most part on site conditions and budget.

This loop circulates a fluid – typically either water or a water/antifreeze mixture – that absorbs heat from or relinquishes heat to the soil that surrounds it, depending on whether the ambient air is colder or warmer than the soil.  As noted previously, these loops take one of two forms:  open or closed.

An open-loop system draws well water for use as the heat source or sink; after use, that water moves to a drainage field or another well.  Closed-loop systems (also known as earth-coupled systems) use the water/antifreeze solution, circulating and recirculating it through buried pipes that extract heat from the earth.

As mentioned above, loops can be installed in either vertical or horizontal configurations, with the former tending to be more expensive but more suited to applications where space is limited.  The length of looping pipe required varies depending on peak Btu demand for the pool or spa as well as upon soil type, loop configuration and system capacity, with a typical range of 250 to 1,000 feet per ton of heating or cooling capacity.

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These drawings, based on illustrations provided by the U.S. Department of Energy, show the four basic approaches to tapping geothermal energy for heating or cooling watershapes as well as homes.  The open-loop system (left) uses well or surface water as the heat-exchange fluid and is only practical where there’s an adequate supply of relatively clean water.  The closed-loop systems (middle left to right) tend to be more practical for residential applications and require only an available piece of land.  If that space is large, a horizontal system can be installed with relative ease (middle left).  If space is limited, a system with a vertical orientation may be used (middle right), although it’s a costlier option.  And properties with large ponds may use those bodies of water to make the system work (right) – an easy, inexpensive option if the water’s at least eight-feet deep.  The good news is, there’s an approach and a system for just about every situation.

So how do you choose the best option among these various possibilities?  Given the novelty of these applications, my basic recommendation is to work with a consultant.  You can then go it alone or call in either a geothermal well driller (to install a vertical closed-loop system) or an HVAC/geothermal contractor (to set up a horizontal closed-loop system), but as I see it, there are so many variables having to do with the heating needs of the pool and/or spa; an open loop’s water-well capacity; a closed loop’s ground temperature; and available space and budget.  As a result, the most sensible approach (for the time being at least) is to rely on the expertise of others.

One advantageous detail about these systems that I’ve already learned is that the operating unit and control system can be installed just about anywhere – outdoors or indoors, in a vault below the pool or in the home’s basement or attic.  This flexibility is made possible by the fact that, unlike gas heaters, these devices do not require ventilation – nor do they need outside air or an upward exhaust, as do air-source heat pumps.

[ ]  What are the cost factors involved in using geothermal heating and cooling?

Geothermal technology does not come cheap, with systems ranging in cost from about $20,000 to levels well up in the six-digit range.  This means that any discussion of the subject with clients will involve discussions of return on investment, financing and, helpfully, the possibility of tax breaks that may be available to them.

If that seems outlandish, consider this:  In the “good ol’ days” just four or five years back, few upscale clients expressed much care about pool- or spa-heating costs and thought even less about systems relative to the “green movement.”  Now, however, most homeowners are appreciably aware of these issues and seek out both energy-efficient and earth-friendly systems for their projects.   

The way I approach these discussions is simple:  All I do is point out that two very good things happen when a geothermal heating/cooling system is installed in conjunction with a watershape and other buildings on the property.

A Warm Complement

In discussing geothermal heating, there’s a natural tendency to think of it as a replacement for fossil-fuel heating systems, basically because the cost per Btu is so much lower.  

I look at things differently:  The ability of geothermal systems to maintain the water at even temperatures in pools and spas at a low cost makes the use of gas heaters for fast heat-ups or temperature maintenance far more cost effective because the water starts out at a higher level and the temperature doesn’t need to be raised nearly as much.  (It also bears mentioning that, in the right climate conditions, air-source heat pumps combined with fossil fuel systems offer similar benefits.)

I’m fine with it when my more environmentally conscious clients want to eliminate fossil-fuel heating altogether, but I’m also encountering homeowners who appreciate the convenience of combining two approaches:  Either way, my clients stand to save in the long run.

-- D.C.  

First, assuming that the client pays taxes and is interested in minimizing them, purchasing a geothermal system will, through 2016 at least, entitle them to a 30-percent tax credit on the installation.  (It’s important to note, as of this writing, that this tax credit applies to applications in heating and cooling buildings rather than pools and spas.  There is apparently some progress toward including a provision for watershapes, but in the meantime, the credit is granted for projects that heat and cool buildings and, incidentally, pool and spa water.)  In addition, some local power companies have been financing system installations on loans with little or no interest.

Second, there’s a synergy between a pool/spa heating system and an air-conditioning system used to cool interior spaces in that the watershape becomes a place to dump the heat generated by the interior-cooling process.  In fact, many of the water-source heating and cooling systems can be specifically programmed to work in this way.

Working through all of this with a sharp pencil and a bit of paper, I’m able to show clients that the return on investment comes faster and more easily than they might think.   


The best thing about geothermal systems in heating/cooling applications is that the technology is time-tested and reliable.  Many commercial buildings, schools and residences worldwide have reaped the benefits of this clean, low-cost, renewable source of energy, and extending those applications to encompass pools and spas is simply a matter of using a familiar approach in a slightly different way.

As I see it, we watershapers stand to benefit from reaching out to professionals in other trades who can help us make our clients happy in the long run.  In addition, in situations in which a watershape is being installed during construction of a new home, the synergistic benefits of tying their heating/cooling systems together makes simple, elegant sense.

As a builder, it was tough for me to get past the sense that geothermal heating was another fad being palmed off on us by tree huggers and wild-eyed environmentalists.  In this case, however, the numbers told a story so compelling that I’m all for saving my clients money while also helping the planet!


Douglas Cook is owner of Aloha Pools Design Studio, a Franklin, Tenn.-based design/construction firm that focuses on custom swimming pools for residential clients.  He began his career in water-related industries in 1980, working as a sales representative for Sta-Rite Industries and selling not only swimming pool equipment but also products for commercial irrigation and wastewater management.  Among the many accomplishments in his long tenure in the watershaping industry, Cook was salesman of the year for Sta-Rite in 1982 and qualified for membership in the Society of Watershape Designers in 2006.  He also sits on the board of directors for the Dyer County Literacy Program.

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