THE BIG DIG
(Page 4 of 5)
Pond loops are typically constructed from 300-foot rolls of
0.75-inch polyethylene pipe that carry SDR11 rating. The
rolls are prepared with spacers between each layer of pipe
- once all spacers are in place and the rolls are banded
tightly, the spacers are attached to a manifold constructed
from one-inch SCH 40 polyethylene pipe. This manifold uses
a connection method called "reverse return", in which the
first circuit on the supply line is connected as the last
circuit on the return side. This requires more manifold
pipe but assures that each parallel circuit of pipe on the
manifold receives equal flow.
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Horizontal Loops Horizontal loops
typically require two or more acres of unrestricted open
area for installation and the cost of a horizontal loop
will vary according to the loop design and the local site
conditions. Typical costs range from $600 to a little more
than $1,000 per ton of loop capacity. Complete systems with
ten-year warranties, installed with loops and attached to
existing duct work range in price from $11,700 for a
2.5-ton system to $18,500 for a six-ton, two-speed system.
The simplest horizontal loop is a single pipe loop
constructed from 1.25-inch polyethylene pipe. Single pipe
loops of up to 1,600 feet may be constructed and placed in
the bottom of a simple out and back trench dug to three to
five feet. Typically, the deeper the pipe is placed, the
more stable the soil temperature will be. The length of
pipe required for horizontal loops ranges from just under
750 feet per ton for light dry soil to as little as 350
feet per ton for heavy, saturated soil.
More involved horizontal loops may assume any number of
configurations depending on lot and soil conditions. In
addition to the single-pipe trench, there are two-, three-,
four- and six-pipe trenches. As the number of pipes in a
trench increases, of course, so does the width, which
allows geoexchange contractors to lay extended-pipe loops
in what is known as "slinky" loops.
A typical slinky-loop field may consist of four or five
parallel trenches separated by ten to 15 feet and laid with
600 feet of coil. with an experienced crew of four -
track-hoe operator, safety-man and two loop installers -
the average work time to install a slinky loop is about an
hour and 20 minutes per ton of coil. After all costs are
accounted for, slinky loop installation averages $800 per
ton.
Vertical Loops
For small lots and locations with little acreage, vertical
loops are an economical alternative. They require the use
of a drilling rig to bore a four-inch diameter hole for the
heat exchanger piping. Depending on soil and rock
conditions, the depth of a vertical loop bore can range 130
to 245 feet per ton of equipment capacity. Drilling costs,
with manifold connections and antifreeze fill, average
$1,500 to $1,800 per ton. Local drilling costs vary, so
vertical loops can range from $12,600 for a 2.5-ton system
to as much as $21,870 for a six-ton, two-speed system with
a ten-year equipment warranty.
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