Loop Systems
There are two types of loop systems used in geothermal heating, open loop and closed loop.
Closed Loop
A closed loop system is the most common type of geothermal heating/cooling system. In a closed loop system the heat exchanger, a loop of tubing filled with antifreeze is buried underground or under water.
The antifreeze circulates continuously inside the buried pipe, absorbing heat from the earth during the winter for use inside your home or business. In warmer months, the antifreeze takes heat from indoors and transfers it back into the earth.
Horizontal Loop Field
If your home has adequate space and a sand or clay based soil then a horizontal loop field is the most efficient and affordable option. In a horizontal loop field trenches are dug leading from the home outward. I single or multiple tubes are then buried in these trenches. Trenches should be below the frost line, and 8-10 feet deep is recommended in cooler climates.
There are several options for trenches.
- Invidividual trenches with a single pipe run in the trench.
- Narrow trenches with multiple pipes run in the trench.
- Wide trenches with multiple pipes run in the trench.
In areas with rocky ground, a horizontal loop field should have sand layered at the bottom of the trench, and a layer of sand put on top of the loop field to protect the piping.
Vertical Loop Field
A vertical loop field is typicall used when there is limited land space. In a vertical loop field multiple wells are drilled to accomodate the total length of pipe. For example if 2000 feet of pipe are required two 500 foot wells may be drilled, or four 250 foot wells may be drilled. Typically cost is the determining factor of the number and depth of the wells.
A length of pipe, hairpinned, is then dropped into the well and the well is backfilled or grouted to ensure good contact with the earth and the pipe.
Drilling a well for a vertical loop is often cheaper than drilling a well for water. Finding water is not necessary so the driller can simply go for depth. Additional no casing is needed for the well as the tubing is the casing for the fluids and the hole will simply be backfilled.
Coil Loop Field
A coil loop field is becoming more popular, especially in residential applications where land space is limited.
In a coil loop field a trench is dug, similar to the horizontal loop field. Instead of a long straight run of piping, a flexible piping is used and it is coiled on top of itself, somewhat like a slinky. The trench is typically 3 feet wide and should be 8-10 feet deep. The advantages of this is that a much shorter trench can be dug, often 2/3rds shorter than a standard horizontal system. The disadvantage of a coil loop field is that since more fluid is concentrated in a smaller area the heat transfer is less efficient.
As with a horizontal field if the ground is rocky the trench should be layered with sand to protect the piping
Water Loop Field
A water based loop field utilizes water instead of earth as the heat source. A trench is dug from the home to the water source, typically a pond or a lake. Tubing is then coiled at the bottom of the water source. This system can be very economical as very little trenching is needed.
Water loop fields work well as long as the water source is deep enough to prevent complete freezing of the water.
Open Loop
Instead of using an antifreeze solution sealed inside the buried piping, an open loop system uses water from a surface or underground source - such as a pond, lake or well.
This water is piped through your heat pump and the heat extracted from it. The water is then returned to the pond, lake or well. In some cases the water is simply discharged onto your lawn, but this is not recommended.
Well water systems are most often used with an open loop system. The well water provides both your household water, and the input to the geothermal heat pump. With a well driven open loop system you need to ensure your well can support 10-15 gallons per minute water draw. Often this means using a larger pump and water tank.
A major concern of an open loop system is the water quality. Because a continuous flow of freshwater is pumped through your heat pump mineral deposits may build up within the system. Before going with an open loop system you should have your water tested for hardness, iron, acidity and other impurities.
Local regulations should be checked before implementing an open loop system as some areas prohibit such systems. In general open loop systems are not recommended due to the potential environmental issues such as depletion of the aquifer and release of the water if not reinjected into a well. Additionally open loop systems can increase the ownership costs by shortening the life of your heat pump.