Applications Manual Bosch Geothermal Heat Pumps
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Bosch Thermotechnology Corp.
Data subject to change
Another type of horizontal con guration is called the
“Slinky™” loop (Fig. 73). While this horizontal loop will
require less land area, it will generally require more pipe
than a parallel type horizontal loop con guration. As with a
horizontal loop the earth is excavated to a depth of about
5-8 feet and the pipe is coiled like a attened “Slinky™”
and then buried (either horizontally or vertically in the
excavation).
Fig. 73 "Slinky" loop
Courtesy of International Ground Source Heat Pump Association/Oklahoma
State University
Pond or lake loops are one of the most cost-effective
closed-loop installations available today (Fig. 74). These
closed-loop heat exchangers typically require limited
excavation from the structure to the pond or lake. Pond
or lake loops require a minimum of about ½ acre of land
per ton of cooling capacity and a minimum depth of 8 to
10 feet for proper and ef cient heat transfer to occur. Like
other closed loop installations, pond or lake loops utilize
polyethylene pipe, but are typically laid out in a coil or
“Slinky™” arrangement (discussed later).
Surface water (lake or pond loops) are less common
for residential applications since fewer homes are
located on or near a suitable body of water. Additionally,
environmental concerns often are concerns when using
natural ponds and lakes. Ponds and lakes are good heat
rejecters, but often less ef cient heat producers. Even so,
pond and lake loops can be 30% to 60% of the cost of a
vertical or horizontal loop.
Fig. 74 Pond or lake loop
11.1 Horizontal - Loop Layout
The deeper a horizontal ground loop the more stable the
ground temperatures. A typical horizontal loop system
consists of buried polyethylene pipe containing water and
antifreeze solution, which is circulated by means of a small
pump. Fluid runs through the pipe in a closed system in a
certain order. Many loop layouts exist (Fig. 75).
Fig. 75
Courtesy of International Ground Source Heat Pump Association/Oklahoma
State University
The typical length of a trench must be based on the
amount of pipe in the trench, burial depth, and average
ground temperature and conductivity.
The typical extremes of normal ground temperature are
44ºF for the northern continental US and 70ºF in the
southern US. The lengths should provide a maximum
loop temperature of approximately 90ºF entering the
geothermal heat pump in normal applications. In homes
with excessive run times this temperature will typically
be 3ºF to 5ºF higher. The typical recommendations for
vertical ground loops is ft. of bore per ton for ¾” and
1-1/4” HDPE, which will operate about 5ºF cooler than the
horizontal loops.
The greater the distance between buried horizontal
closed-loops, the higher the ef ciency. Industry guidelines
suggest that there should be 10 feet between sections
of buried loop, in order to allow the pipe to collect heat
from the surrounding earth without thermal interference
from the neighboring loop (Fig. 76). This spacing can be
reduced under certain conditions. It is common to bury
one set of loops above another set with a deeper trench.
A “rule of thumb” is “more ground mass is always better
than less”. Another “rule of thumb” is “more piping, more
uid and deeper is good practice”.
