Geothermal - Energy Policy Analysis: A Conceptual Framework - page 130

Most loops for residential systems are installed either horizontally or vertically in the ground or
submerged in water in a pond or lake. In most cases, fluid runs through the loop in a closed system,
but open-loop systems may be used where local codes permit. Each type of loop configuration
has advantages and disadvantages.
Horizontal Closed Loops
For a horizontal closed loop, trenchers or backhoes are used to dig trenches three to six feet
below ground level for a series of parallel plastic pipes. The trench is backfilled, taking care not
to allow sharp rocks or debris to damage pipes. Fluid runs through the pipes in a closed system.
A typical horizontal loop will be 400 to 600 feet long per ton of heating and cooling capacity.
This configuration is usually the most cost-effective when adequate yard space is available and
trenches are easy to dig. The pipe may be curled into a Slinky shape in order to fit more of it into
shorter trenches, but while this reduces the amount of land space needed it may require more
pipe. Horizontal ground loops are easiest to install while a home is under construction. However,
new types of digging equipment that allow horizontal boring are making it possible to retrofit
geothermal heat pump systems into existing homes with minimal disturbance to lawns, at some
expense. Horizontal boring machines can even allow loops to be installed under existing buildings
or driveways (GeoExchange 2011).
Vertical Closed Loops
For a vertical closed loop, contractors bore vertical holes in the ground 150 to 450 feet deep. Each
hole contains a single length of pipe bent double with a U-bend at the bottom. After the pipe is
inserted, the hole is backfilled or grouted. Each vertical pipe is then connected to a horizontal pipe,
which is also underground. The horizontal pipe then carries fluid in a closed system to and from
a geothermal heat pump. This configuration is ideal for homes where yard space is insufficient to
permit horizontal loops, for buildings with large heating and cooling loads, when the ground is
rocky near the surface, or for retrofit applications where minimum disruption of the landscaping
is desired. Vertical loops are generally more expensive to install due to drilling costs, but require
less piping than horizontal loops because the earth deeper down is cooler in summer and warmer
in winter than the surface (GeoExchange 2011).
Pond Closed Loops
In a pond closed loop, fluid circulates through polyethylene piping in a closed system, just as it
does in ground loops. Typically, pipe is run to the water and coils of piping are submerged under
water. If a home is near a body of surface water, such as a pond or lake, this loop design may be
the most economical. Pond loops are typically used only if the water level above the piping never
drops below six to eight feet at its lowest level, assuring sufficient heat-transfer capability. Properly
designed pond loops result in no adverse impacts on the aquatic system (GeoExchange 2011).
Open Loop Systems
Open loop systems are the simplest to install and have been used successfully for decades in areas
where local codes permit. In this system, groundwater from an aquifer is piped directly from a
well to a building, where it transfers its heat to a heat pump. After it leaves the building, the water