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Closed Loop Design/Installation Guidelines

Closed Loop Basics

Closed Loop Earth Coupled Heat Pump systems are commonly

installed in one of three conﬁ gurations: horizontal, vertical and pond

loop. Each conﬁ guration provides the beneﬁ t of using the moderate

temperatures of the earth as a heat source/heat sink. Piping

conﬁ gurations can be either series or parallel.

Series piping conﬁ gurations typically use 1-1/4 inch, 1-1/2 inch or 2

inch pipe. Parallel piping conﬁ gurations typically use 3/4 inch or 1 inch

pipe for loops and 1-1/4 inch, 1-1/2 inch or 2 inch pipe for headers

and service lines. Parallel conﬁ gurations require headers to be either

“closed-coupled” short headers or reverse return design.

Select the installation conﬁ guration which provides you and your

customer the most cost effective method of installation after

considering all application constraints.

Loop design takes into account two basic factors. The ﬁ rst is an

accurately engineered system to function properly with low pumping

requirements (low Watts) and adequate heat transfer to handle

the load of the structure. The second is to design a loop with the

lowest installed cost while still maintaining a high level of quality.

These factors have been taken into account in all of the loop designs

presented in this manual.

In general terms, all loop lengths have been sized by the

GeoDesigner loop sizing software so that every loop has

approximately the same operating costs. In other words, at the

end of the year the homeowner would have paid approximately

the same amount of money for heating, cooling, and hot water no

matter which loop type was installed. This leaves the installed cost

of the loop as the main factor for determining the system payback.

Therefore, the “best” loop is the most economical system possible

given the installation requirements.

Pipe Fusion Methods

Two basic types of pipe joining methods are available for earth

coupled applications. Polyethylene pipe can be socket fused or butt

fused. In both processes the pipe is actually melted together to form

a joint that is even stronger than the original pipe. Although when

either procedure is performed properly the joint will be stronger

than the pipe wall, socket fusion in the joining of 2” pipe or less is

preferred because of the following:

• Allowable tolerance of mating the pipe is much greater in socket

fusion. According to general fusion guidelines, a 3/4” SDR11 butt

fusion joint alignment can be off no more than 10% of the wall

thickness (0.01 in. [2.54mm]). One hundredth of an inch [2-1/2

mm] accuracy while fusing in a difﬁ cult position can be almost

impossible to attain in the ﬁ eld.

• The actual socket fusion joint is 3 to 4 times the cross sectional

area of its butt fusion counterpart in sizes under 2” and therefore

tends to be more forgiving of operator skill.

• Joints are frequently required in difﬁ cult trench connections

and the smaller socket fusion iron is more mobile. Operators

will have less of a tendency to cut corners during the fusion

procedure, which may happen during the facing and alignment

procedure of butt fusion.

In general socket fusion loses these advantages in fusion joints

larger than 2” and of course socket ﬁ ttings become very expensive

and time consuming in these larger sizes. Therefore, butt fusion is

generally used in sizes larger than 2”. In either joining method proper

technique is essential for long lasting joints. All pipe and ﬁ ttings in the

residential price list are IGSHPA (International Ground Source Heat

Pump Association) approved. All fusion joints must be performed by

certiﬁ ed fusion technicians. Table 2 illustrates the proper fusion times

for Geothermal PE 3408 ASTM Pipe.

Parallel vs. Series Conﬁ gurations

Initially, loops were all designed using series style ﬂ ow due to the lack of

fusion ﬁ ttings needed in parallel systems. This resulted in large diameter

pipe (>1-1/4”) being used to reduce pumping requirements due to the

increased pressure drop of the pipe. Since fusion ﬁ ttings have become

available, parallel ﬂ ow using (3/4” IPS) for loops 2 ton [7 kW] and

above has become the standard for a number of reasons.

• Cost of Pipe - The larger diameter (>1-1/4”) pipe is twice the

cost of the smaller (3/4” IPS) pipe. However, the heat transfer

capability due to the reduced surface area of the smaller pipe

is only decreased by approximately 10-20%. In loop designs

using the smaller pipe, the pipe length is simply increased to

compensate for the small heat transfer reduction, although it still

results in around 50% savings in pipe costs over the larger pipe in

series. In some areas vertical bores using 1-1/4” pipe can be more

cost effective, where drilling costs are high.

• Pumping power - Parallel systems generally can have much lower

pressure drop and thus smaller pumps due to the multiple ﬂ ow

paths of smaller pipes in parallel.

•

Installation ease - The smaller pipe is easier to handle during

installation than the larger diameter pipe. The ‘memory’ of the pipe

can be especially cumbersome when installing in cold conditions.

Smaller pipe takes less time to fuse and is easier to cut, bend, etc.

Table 2: Fusion Times for Polyethylene 3408

ASTM Pipe

Pipe Size

Socket

Fusion

Time (Sec)

Butt Fusion

Holding

Time

Curing

Time

(sec.)

Bead,

in [mm]

3/4” IPS 8 - 10 8 1/16 [1.6] 60 Sec 20 min

1” IPS 10 - 14 12 1/16 [1.6] 60 Sec 20 min

1-1/4” IPS 12 - 15 15

1/16 - 1/8

[1.6 - 3.2]

60 Sec 20 min

1-1/2” IPS 15 - 18 15

1/16 - 1/8

[1.6 - 3.2]

60 Sec 20 min

2” IPS 18 - 22 18 1/8 [3.2] 60 Sec 20 min

Always use a timing device

Closed Loop Design/Installation Guidelines