The ground loop is a key part of the geothermal system and it must be carefully designed and installed. Once the loop is in position it is very difficult to modify but, conversely, a well designed and installed loop should never fail.

The loop field is easy to understand but the science behind the design is quite complicated. Careful design is necessary to ensure that there is sufficient capacity available for the geothermal heat pump to operate properly but also that money is not wasted by over sizing the loop. The spacing of the pipes in a loop is important as this affects how much the heating or cooling effect of one pipe influences the heat source or heat sink available to adjacent pipes.

A water and antifreeze mixture is circulated through a toughened polyethylene pipe which is buried in the ground. If the fluid goes into the pipe very cold, as in the heating season, it will warm up whilst circulating underground. In the cooling season the fluid goes into the ground very hot and cools down while underground. The loop is then completed by circulating the warmed or cooled fluid through the heat exchanger in the heat pump.

No matter what loop configuration is used, any joints in the pipe are heat fused to make them stronger than the original pipe. All loops are tested under pressure prior to their being put in the ground.

In this region we usually consider 3 possible configurations for the ground loop. Vertical, horizontal or lake loop. A lake loop is only possible if a good sized pond or lake is available for use. This usually means that the property owner must own the lake.

Vertical Loop

Vertical loops are most common in this region. They require little ground area for installation and cause relatively little disturbance to the ground surface. For domestic systems, holes are drilled to a depth of 150-200 ft and are spaced a minimum of 10 ft apart. As a rule one hole is required for each ton (12,000BThU/h) of heating or cooling although this will depend somewhat on the subsurface material. Usually we can be more confident than we can with horizontal loops that the pipe will be in damp earth or rock with good conductivity.

We usually use a water well rig to drill the holes therefore access to the drilling area for a truck is necessary. When the pipe has been inserted the hole is grouted with a heat conductive grout. This ensures good heat communication between the pipe and the ground and also seals the hole so that there can be no groundwater contamination or flow within the hole.

When the drilling and looping is finished a trench is dug between the holes and to the house to a depth of about 3 ft. The pipes are connected together and into the house using a manifold pipe.

Horizontal Loop

Horizontal loops are usually buried in trenches at a depth of 6-8ft or in a larger pit at a depth of about 10 ft. Trenches can be anywhere up to 150ft long depending on the pipe configuration. The pipe can be laid in a trench in long straight lengths, out and back each side of the trench, and sometimes at 2 levels in each trench. This requires trenches at least 3 ft wide. Alternatively a slinky configuration can be used where the pipe is laid in a trench about 5 ft wide in a semi-coiled manner. Each trench often provides 1 ton of heating or cooling although this depends very much on the pipe configuration and the type of ground. It is therefore necessary to have a sizable area of land available for a complete horizontal loop system.

There can be some cost saving by using a horizontal loop compared to a vertical loop particularly if digging machines are already available at the site. We do have to be sure however that the loops are in good heat conductive ground or they will have to be extremely long to fulfill the requirements of the system.

The critical parameter in designing a horizontal loop is the depth of the water table at your site between December 15th and February 15th. The water table depth at other times of the year is not relevant. The water table depth should be measured via test borings or a pit sometime between December 15th and February 15th. If a horizontal loop is designed and installed based on the assumption that the soils are saturated during the critical winter time period and it turns out the soils are dry, over a period of a few years the ground around the loop will become cooler crippling the ability of the system to heat your home.

When designing a horizontal loop it is therefore critical for us to know what depth the water table might be at and what type of soil the loop will be in. If the water table at your site is at a depth of 4 ft or less then a trench loop will be a suitable choice. If the water table is at 5 to 8 ft then a pit loop may be a better choice. If your water table is at a depth greater than 8 ft then your horizontal loop will be in dry soils and we will need to know if the soils are clayey and heavy or sandy and light. In many cases where the water table is deep, or might become deep, it can be more economical to use vertical holes.

Lake Loop

Lake loops are simple and relatively cheap to install. They are usually placed at depths of at least 8 ft to get well below possible freezing. Coils of pipe are usually connected together on skids and pulled into the water. The water temperature can often be colder, in the winter, or hotter, in the summer, than the ‘constant’ ground temperature. Lake loops still work well because the very high conductivity of water compensates for the cool or warm temperature.