Loop Field Design

Loop fields, like heat pumps, are measured in units called tons. Each loop in a field is designed to provide 1 ton of energy for the system. So if the geoexchange system has a 4 ton heat pump, it will require four loops.

There are three major considerations that go into the design of the loop field:

  1. Space
  2. Thermal Conductivity of the soil
  3. Cost

Space Required to Install a Loop Field

The amount of space available to install the loop field is often the determining factor when designing a loop field.

Vertical Loop Fields

Because vertical loops are drilled straight down they require the least space to install. Vertical loops can be spaced as close a 10 feet apart, allowing them to be installed in a space as small as the driveway on a city lot.

Horizontal Loop Fields

Horizontal loops require the greatest amount of space because the loop is installed horizontal to the surface. Depending on how they are configured,  loops can range from 100 to 800 feet in length.

Pond Loop Fields

A pond loop requires a body of water that is at least 1/3 acre is size and at least 12″ deep to keep the system from freezing up in cold weather.


Thermal Conductivity

Thermal conductivity is the property of a material to conduct heat. Materials with high thermal conductivity conduct heat much better than materials of low thermal conductivity. Since a loop field relies on conductivity to move the heat from the ground into the solution in the loop, it is also a major consideration when designing a loop field.

Different soil types have different levels of thermal conductivity. For example, Dry sand/gravel has some of the lowest thermal conductivity while “saturated clay” has a high level of thermal conductivity. This difference in thermal conductivity means a loop in dry sand/gravel would have to be more than twice as long as a loop in “saturated clay” in order to deliver one ton.

Well reports are an excellent data source of information about soil types in a give area. The reports below shows us the soil types at each depth of the well as well as the static water table. Information that is critical to loop field design.

Capture well Log

Well Log from Minnesota Department of Health County Well Index – on line

In the above well log, the first 120 feet are sand and gravel. The static water level is 25 feet. So the first 25 feet are probably dry and fairly low in thermal conductivity. And in dry conditions that water table can move down even deeper. In this situation a horizontal loop field installed 15 to 20 feet deep would be in the low thermal conductivity dry sand and gravel.

A vertical loop field in this situation would go through the low conductivity soil and into the damp sand and clay that has very high thermal conductivity. By tapping into the higher thermal conductivity of the damp soil the vertical system will be over 50% more effective; allowing the size of the loop field to be cut in half compared to a horizontal loop field. The reduction in size will be accompanied by an almost equal reduction in costs.

In this case the vertical system would be the highest performing, most dependable, and most cost effective choice.


Sizing a Loop Field

Sizing a loop field correctly is affect both performance and costs. An undersized loop field will be cheaper, but will also be less efficient and may go “cold” during an intense cold spell. An over sized loop field typically will not affect performance negatively, but it will drive up the cost of the system.

Consumer should beware of installers who simply install the same size loops on every job they do. They may get away with this if they stay within that one soil type. But soil conditions can vary dramatically with in short distances. And the loop field that worked just fine on one job may well freeze up on a job installed in the next town down the road.

Geothermal Loop System

Geo Link Software Loop Field Analysis

A good installer will use computer software to help calculate the size of the loop field. Once  the loop field type and soil conditions determined, the data can now be loaded into the design software to determine the total length of loop required to deliver the determined amount of Btu’s.

The software analyzes the data and determines the total length of loop needed.  In this example to the right, 945 feet of loop will be required for this system. The analysis also provides additional performance data on the loop field to assess its performance.

Sizing for the Future

The average geothermal equipment today has a COP around 3.2. But the newer equipment is making dramatic increases in performance. The WaterFurnace 7 Series, for example, has a COP of 5.3.  As a result this  new equipment will require less electricity to become even more efficient. But it will also draw more heat out of the loop field that equipment with a lower COP.

For the homeowner this means that when they go to replace their geothermal heat pump twenty or thirty years from now it they will be purchasing higher COP equipment that will require more loop field to maintain its efficiency.  That is why URM routinely designs loop fields to meet the requirements of existing and new equipment. This does not increase the costs of the current system, but assures the replacement system will work just as well.


Cost of Loop Fields

Cost is always an consideration, but when all other factors are equal, costs becomes the final determining factor in choosing a loop field.

Costs can vary dramatically from job to job depending on conditions. The prices below are average ranges for the territory we serve.

Vertical Loop Field: Most Expensive -$1800-$3500 or more per loop*

Horizontal: Less expensive than Vertical – $800-$2800 more per loop*

Pond: Least Expensive – Custom Price

UMR GeothermalUMR Best Practice

 

UMR uses Geo Link software to design geothermal loop fields.