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Table 2: Fusion Times for Polyethylene 3408

ASTM Pipe

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 1-1/4” vertical bores 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.

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