It's always 55F

Sept. 1, 2009

If you have an interest in saving energy and reducing operating costs for your customers, geothermal is something for you to consider. According to Morey McDonnell, owner of Builder's Heating in Denver, geothermal can save up to 60% on the energy costs for heating, cooling and domestic hot water, resulting in a payback in as little as seven years. Additionally, there is a 30% federal tax credit on the entire cost of many systems with no cap.

“We find that new technology differentiates our company among the over 1,200 heating and cooling contractors in Denver,” McDonnell says. “It's a niche market. The competition is tighter on a conventional project than on the innovative ones. And there's no doubt that we're green.”

For many of us in the industry, though, our questions and doubts about geothermal far outweigh what we know about it.

How does geothermal work?

Geothermal is based on the fact that the earth below the frost line is a year-round constant temperature of 45°F-55°F.

Remember the thermodynamic principle that heat moves to cool? In winter when temperatures are cold, the earth is a heat source. Heat is pulled out of the earth and put into the building. In summer when air temperatures are hotter, the same earth is a heat sink. Then heat is pulled out of the building and deposited into the earth.

This is exactly the same idea as heat pumps, but without cold weather limitations. With a conventional air-source heat pump, the outside air, with the disadvantage of considerable temperature variations, is the heat source or heat sink. When the outside temperature drops below 33°F, the heat pump is no longer effective. The great advantage of using the ground instead of air is year-round constant temperature.

The equipment technology used for geothermal is in fact a heat pump — called a ground-source heat pump or a water-source heat pump. The equipment used for geothermal is more like conventional equipment than it is different. Just like with an air-source heat pump, there is a compressor, and a reversing valve. However instead of in a condenser, the heat exchange takes place between an inside refrigerant loop and separate outside loop that circulates into the ground. The outside loop can contain either refrigerant or water.

What are the basic parts?

The HVAC equipment that goes into a geothermal system is familiar to all. There is a condensing unit, air handling equipment and ductwork. Geothermal heat pumps can also supply radiant floor systems. Condensing units can be either water or refrigerant based. There can also be a water heater. For a hydronic system, there is, additionally, either piping for baseboard radiation or radiant tubing.

What about wells, drilling, piping and ground loops?

The heat exchange in the ground is accomplished by circulating either water or refrigerant through piping, called a loop or ground loop. Again, if the fluid is colder than the ground (which is 45°F-55°F), the fluid picks up heat. If the fluid is warmer than the ground, it gives off heat. To create a loop, a well is drilled and a pipe is inserted into it. One loop is needed for approximately every ton of cooling, or 12,000 Btus of heating.

The terms “drilling” and “well” come from the fact that the professionals who insert the pipe into the ground often come from the water well drilling business. The words may give the impression that fresh water is constantly being drawn from the ground, like for drinking water. This is not often the case, though there are a few “open” systems in water-plentiful areas that run well water through the system and then use it to water the lawn. Typically, whether the liquid used is water or refrigerant, it is re-circulated. The wells are simply channels for inserting pipe. Sometimes a number of pipes are placed together in a trench and tied together in a manifold.

The pipe itself is usually 3/8-in. or 5/8-in. plastic or copper. It can be inserted into the ground at an angle slightly off of vertical, or laid horizontally.

For new construction, placement of the pipe is fairly easy. For example, it can go under the front or back yard, or under a driveway. Retrofit is much more of a challenge. McDonnell was fortunate in that when he retrofitted his house there was a large yard space next to his house. It was relatively easy to replace turf once the job was complete. Piping can also be laid in a pond deep enough not to freeze.

The depth and distance between the loops varies according to whether water or refrigerant is used. Water loops need to be 200-ft. deep or deeper, and 20-ft. apart. Each hole has a 3- to 4-in. diameter. Using refrigerant requires less space, and may be more suitable for retrofit. Three-inch holes can be 100-ft. deep and 7-ft. apart.

For example, the five loops needed for a 5-ton system can require a surface area for drilling of only 6-ft. The pipes are trenched together below the frost level, about 7-ft. below the surface and, when the job is completed, there is no evidence of it on the surface. McDonnell found that about three holes a day could be drilled in Denver.

Once the pipe is placed in the holes, mud slurry is forced into the holes for best heat conductivity. If the ground is rock, it is all the better for conductivity.

The hole drilling and placement of pipes is what adds cost to the initial installation of a geothermal system. It is estimated that it costs $2,000-$3,000 for each of the holes.

How is energy saved?

The lot that a home or building sits on is a heat reservoir. It contains 10 times the heat needed to heat a building, and is constantly renewed by the heat of the sun. In fact, it is estimated that half of the sun's energy that reaches the earth is absorbed into the ground. At the same time, because of the mass of the earth, temperatures do not exceed 55° F, so the ground is always available as a place to deposit excess building heat.

There is relatively little energy cost to access this natural heat source and heat sink. It is estimated that a geothermal heat pump consumes just one unit of electricity for every four units of heat it delivers. Using variable speed pumps to circulate the loops can minimize electric consumption. The compressor takes less electricity, too, because with the 45-55°F ground source, it can run at half load.

The issue of whether or not to have a supplemental system is addressed with the concept of designing the complete system, so that a conventional system provides the last 20% of the anticipated maximum load. That way, money is not spent on drilling holes for capacity that is used occasionally. A boiler or water heater is also useful for peak domestic hot water demands. Indoor storage tanks for water can also be used to extend the capacity of a geothermal system.

Water vs. refrigerant

A contractor must choose between water and refrigerant for the ground loops. The efficiency of the two systems is about the same. Builder's Heating has installed both types, ranging from a new construction 22-ton system with water loops to McDonnell's own retrofit 10-ton refrigerant system.

He emphasizes, however, “Geothermal isn't just upscale. Four- to 5-ton systems offer a lot of energy sayings.”

When choosing between water and refrigerant, here are some factors to consider:

  • The federal tax credit applies only to a refrigerant system. That's because of the technicality that in order to qualify, the equipment must have an Air Conditioning, Heating & Refrigeration Institute rating. If only ground water is used for cooling, by definition it does not have an AHRI rating.
  • A refrigerant system may be better suited for retrofit because less surface area is needed to install the loops. The pipes can be closer together and do not need to be as deep.
  • Either plastic or copper pipe may be used for water loops, but refrigerant requires copper because of the pressures.
  • When considering refrigerant, keep in mind that in a geothermal system both the amount of refrigerant and the time for the vacuuming process may be more than you expect.

The educational curve

While equipment for geothermal is similar to conventional heating and cooling equipment, there is a notable educational curve to successfully working in the geothermal business. Most of this has to do with the underground piping. Once the job is complete, the piping is inaccessible, and that means having to get it right the first time.

According to McDonnell, essential to success are accurate load calculations, selection of a driller dedicated to installing tight piping, local support from the equipment manufacturer and realistic scheduling and labor allowance — it may take longer and cost more than a conventional job, especially if you're new at it.

McDonnell is pleased to have the challenges that geothermal presents. His employees enjoy learning new products and techniques. And he himself appreciates the fact that not many of his would-be competitors will take the risk of something different from the ordinary.

Carol Fey is a technical trainer and author of books and a DVD on electricity, troubleshooting and controls. You can e-mail her at [email protected]. Visit her Web site at and blog at:

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