|Home Heating from the Good Earth||| Print ||
copyright by REX A. EWING
(originally published in Countryside in 2006)
Imagine for a moment that you hear someone talking about a magic battery capable of collecting the oppressive warmth of the summer sun and storing it until winter, when you really need it to heat your house. By using the magic battery, he tells you, you will never have to burn another gallon of propane, natural gas or fuel oil to keep your house warm, and—as an added bonus—the magic battery also stores the waste heat from your home's air conditioning until the mercury plummets and the snow flies.
Too good to be true?
Not at all. In fact, the magic battery is nothing more otherworldly than the ground beneath your feet, and the means used to move the heat back and forth is the same clever science that makes refrigerators and air conditioners possible: the common heat exchanger.
The Principle of the Heat Exchanger
Most of us know the earth stays a fairly constant temperature several feet below the surface. That's because it takes so long for the ground to gain or lose heat, by the time it begins to react to one season, the next season is already underway. So once you dig down 5 to 8 feet, the soil will be a fairly constant 45 – 50 degrees Fahrenheit in the northern latitudes, and 50 – 70 degrees in the south.
This is all easy enough to understand. But now it's about to get counterintuitive, because I'm going to tell you that you can heat your home cheaply and conveniently by extracting heat from soil that is only 15 or 20 degrees above freezing. How? By circulating, through buried tubing, water that is even colder than the earth itself. Since heat always flows toward where it's colder, the water—mixed with an environmentally-safe antifreeze solution—picks up heat from the ground as it circulates. Thus it will be warmer when it returns to your house than when it left it.
The water is still cold, of course; no warmer than the earth it came from. So it has to give up its heat to something even colder—a refrigerant, such as liquid Freon. Now we're getting somewhere, though it may not seem like it, yet. Inside a heat pump unit, the very cold Freon circulates in a double coil with the earth-warmed water, absorbing its heat and making the water cold again, relatively speaking. But even though the Freon has absorbed most of the heat the water gained from the soil (becoming a low-pressure gas in the process), it's still no warmer than the ground outside. And we need to make it hot; hotter even than the air inside our house. How? By compressing it to a very high pressure. This concentrates the gas and raises its temperature to approximately 165 degrees. Then, by running the hot high-pressure gas through a second heat exchanger (either an air duct coil or a hot water tank), the heat is given up into your house. In the meantime, the gas cools to a liquid, runs through an expansion valve, and returns to a cold, low-pressure state, ready for the next go-round.
In summer the process is reversed: the excess heat inside your home is returned to the soil where, as I mentioned earlier, it will be available in a few months for winter heating.
With three separate phases of heat exchange (ground to water; water to Freon; Freon to air or water), the use of a ground-source (geothermal) heat pump to heat and cool your home may not, at first blush, appear to be a very efficient process. But in fact it is extremely efficient—right in the neighborhood of 400 percent. This means that the ground surrenders 4 units of heat energy for every 1 unit of electrical energy you use to extract it. It's almost like stealing.
Types of Systems
There are two basic types of underground pipe systems —open loop and closed loop—with several variations. The one you use will depend on where you live, how much land you have, and the characteristics of the soil and ground water.
Closed Loop - In a horizontal closed-loop system, loops of special heat-conducting polyethylene pipe are buried in a series of 3-foot-wide trenches 6 to 8 feet below the surface. The length of each trench depends on the amount of moisture in the soil. David Petroy, owner of Blue Sky Energy Solutions in Boulder, Colorado (www.bluesky-energy.com), has found that three trenches, each 100 feet long, will be sufficient for an average-sized house in saturated soil, though in Colorado's dry climate he most often has to go 200 feet per trench for optimal heat absorption.
Since many of us simply don't have enough land for such a sprawling system, David frequently uses a vertical closed-loop system, in which loops of ¾-inch high-density polyethylene pipe are set in concrete in a series of 4½-inch holes. The holes are bored to a depth of 175 to 220 feet, and placed 10 to 15 feet apart. All the separate pipes converge at a manifold, where they are joined into two pipes—one in, one out.
A third incarnation of the closed-loop system is the pond loop. As the name implies, the tubing is floated over a body of water, then sunk to the bottom. If you're lucky enough to have a lake nearby, it could spare you the cost of trenching or drilling.
Open Loop - In open-loop systems, ground water from a series of wells is used. Water is pumped out of one set of wells, run through a heat exchanger, and is then pumped back into a different set of wells. Since water conducts heat better than dirt, open-loop systems are very efficient and can be less expensive to install than vertical closed loops.
System Size, Cost and Applicability
In a temperate climate, a geothermal system will heat and cool roughly 750 square feet of space per each ton of capacity (12,000 Btu/hour). Installed systems range from $3,000 per ton for horizontal closed-loop systems in ideal soil, to $5,000 per ton for vertical closed-loop systems. If, on the other hand, you have adequate groundwater flow, vertical open-loop systems can save you money.
Are you building a new house? If so, the increase in your mortgage payment from choosing a ground-source heat-pump system over a conventional system will be more than offset by the savings on your utility bill, since a geothermal system will be 2½ to 3 times more efficient. If you instead plan to retrofit an existing house, the payback will take 10 to 15 years, depending on numerous factors. The good news is, the equipment should last for 20 to 30 years with little maintenance, while the underground tubing will perform trouble-free for at least 100 years.
Practically speaking, ground-source heat pumps work best with forced-air heating systems, but can also work well with radiant-floor heating systems. When adapted to an existing forced-air setup, the system efficiency can be greatly augmented by sealing gaps and holes in the ducts, adding return-air ducts, and setting up multiple zones. With the addition of a desuper water heater, you can enjoy virtually-free hot water in summer and more efficient water heating in winter.
When installed as a hot-water heating system for radiant in-floor heating, a geothermal system can only heat water to 110 - 120 degrees, so for most homes it will require some amount of boiler-heated water on really cold days, though overall it should handle 80 to 90 percent of the heating chores over the course of a winter. Summer cooling with ground-source hot-water systems is accomplished by reversing the process. The heat pump unit produces cold water which is circulated to fan coil units which blow air past the cool coils. The cool air is then distributed through a separate duct system.
Want to do something good for both the planet and your pocketbook? According to the EPA, geothermal systems are the most energy-efficient, environmentally clean, and cost-effective space conditioning systems available. To learn more about geothermal heating and cooling, or to find a certified installer near you, visit the GeoExchange web site at: www.geoexchange.org
Rex Ewing is the author of several books, including Got Sun? Go Solar, Power With Nature, and Crafting Log Homes Solar Style.