VERTICAL HEAT EXCHANGERS
4725.7050 VERTICAL HEAT EXCHANGERS.
A “vertical heat exchanger” may also be referred to as a vertical loop, heat loop, geothermal loop, closed loop,
earth-coupled loop, ground loop, or earth loop. Vertical heat exchangers are a type of boring used to extract heat
from the ground, or dispose of heat in the ground, usually to heat or cool a home, school, business, or other
building. A typical system consists of two 3/4 to 1-1/4-inch plastic pipes with a “U”-shaped connection at the
bottom (a supply and a return line) installed in a hole in the ground, and a pump to force heat exchange fluid
(special antifreeze) through the loop to a compressor where the heat from the ground is removed from the fluid
and used for space heating; or the heat from a building is transferred to the fluid and ultimately to the ground in
order to cool the building.
This rule part regulates the construction of vertical heat exchangers. The rules do not regulate the construction of
horizontal heat exchangers (piping installed horizontally in soil trenches or in coils in the ground or in lakes or
ponds). However, the rules do require an isolation distance between a water-supply well and a horizontal,
“slinky,” or pond heat exchanger piping containing a contaminant.
When installing or constructing a vertical heat exchanger, the materials and methods must comply with the
provisions below and in Minnesota Rules, parts 4725.2010 to 4725.3950. The bore holes must comply with the
electric line, gas pipe, and LP tank isolation distances as well as the 3-foot building and building overhang
separation. The bore holes are prohibited from being in or under a building. The bore holes are not required to be
separated from sources of contamination as specified in Minnesota Rules, parts 4725.4350 and 4725.4450 (the
isolation or “setback” distances do not apply between a vertical heat exchanger and a source of contamination).
Vertical heat exchanger bore holes must be 35 feet from a water-supply well. Horizontal piping from a vertical
heat exchanger (using approved propylene glycol) must be no less than 10 feet from a water-supply well.
Horizontal heat exchangers using other heat transfer fluids such as ethanol or methanol must be 50 feet from a
water-supply well.
The vertical heat exchanger construction permit requirements are located in Minnesota Rules,
part 4725.1833.
Subpart 1. Construction. A vertical heat exchanger must be constructed according to
the general construction standards in parts 4725.2010 to 4725.3875 and the provisions in
this part.
A. Vertical heat exchanger piping must be a minimum160 psi pressure-rated, SDR 11
high density polyethylene, meeting ASTM Standard D3035-03a.
This refers to the piping installed vertically in the ground. This material standard does not apply to the horizontal
piping used to manifold the loops together; however, if this quality of piping is used, the setback to a water-supply
well is 10 feet. If other materials are used, the setback is 50 feet.
ASTM Standard D3035-03a requires that the pipe must be marked in intervals of not less than every
5 feet with the following: the nominal pipe size, type of plastic material, dimension ratio (SDR 11), pressure
rating at 73 degrees F (160 psi), “ASTM D 3035”, and the manufacturer’s name or trademark and code.
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B. Connections to vertical heat exchanger piping must use socket fusion or butt fusion
joining methods.
Socket fusion and butt fusion are joining techniques where the pipe ends are heated in a special fusing machine,
softened, pressed together, and fused forming a joint that is as strong as or stronger than the original pipe. Some
manufacturers provide training. All other joining techniques, such as solvent welding and the use of mechanical
fasteners, are not allowed.
C. Vertical heat exchanger piping must be pressure tested with air or potable water
for 15 minutes at a pressure of 1.5 times the system operating pressure or 75 pounds per
square inch, whichever is greater, after installation in the bore hole.
After each loop is installed vertically in the ground and before the piping is filled with heat-transfer fluid, the loop
must be filled with air or potable water and pressurized to a minimum of 1.5 times the maximum operating
pressure or 75 pounds per square inch, whichever is greater. Loops which fail to hold the pressure must be
removed and repaired by socket-fusion or butt-fusion joining, replaced, or properly sealed.
The Minnesota Mechanical Code, Minnesota Rules, Chapter 1346, adopts the 2006 International Mechanical
Code which requires a pressure test of assembled ground source mechanical systems (geothermal), and a flow and
pressure loss test of the system. Persons contracting to do mechanical work in Minnesota (not including the
vertical loop itself) must have a $25,000 mechanical bond under Minnesota Statutes, section 326B.197. For
further information contact DLI.
D. The annular space between the vertical heat exchanger piping and the bore hole
must be grouted with neat-cement grout or cement-sand grout in bedrock, and neatcement grout, cement-sand grout, thermally enhanced bentonite grout, or bentonite grout
in unconsolidated materials according to the procedures in part 4725.3050, subpart 2.
Thermally enhanced bentonite grout must consist of a fluid mixture of not more than 17.5
gallons of water, not more than 200 pounds of sand with 80 percent or more of the sand
smaller than 0.0117 inch (passing U.S. Sieve #50), and a minimum of 50 pounds of
bentonite.
Minnesota Rules, part 4725.3050, subpart 2, relates to grouting methods. Vertical heat exchangers must be
entirely grouted from the bottom of the drilled hole to the land surface. Unlike water-supply wells, the annular
space below 50 feet may not be filled with cuttings; it must be grouted. Grout must be pumped through a tremie
pipe that has been inserted to the bottom of the bore hole and grouted from the bottom up. Drill rods may be used
to pump grout. When grouting in rock, neat-cement or cement-sand grout must be used. When grouting in
unconsolidated materials such as glacial drift, neat-cement grout, cement-sand grout, thermally enhanced
bentonite, or bentonite grout must be used.Grout not only protects the groundwater, but an ungrouted hole has
poor thermal transfer and leads to a system that is very inefficient.
The rules do not allow for greater amounts of sand than a 1:1 mixture of sand and Portland cement for cementsand grout, a 1:1 mixture of sand and bentonite for bentonite grout; or a 4:1 mixture of sand and bentonite for
thermally enhanced bentonite grout.
The sand used for the 1:1 mixture in cement-sand grout or bentonite grout is not required to meet any specific
sand size ratio or analysis, other than the sand must be sand-sized. However, contractors have reported that the
finer-grained sand required in the thermally enhanced bentonite grout pumps easier and does not separate or settle
as much as coarser sand. Sand used in a thermally enhanced bentonite grout (4 parts sand to 1 part bentonite) must
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meet the criteria in the rule above. This finer-grained sand (smaller than a Number 50 sieve) provides lower
permeability and higher thermal transfer compared to coarser-grained sands.
Because of the rapidly evolving geothermal industry, variances may be considered when the grout or other
material can be demonstrated to provide protection equivalent to that required in rule.
E. Only food-grade or USP-grade propylene glycol must be used as heat transfer
fluid. No other materials or additives must be used except for potable water. A permanent
sign must be attached to the heat pump specifying that only approved heat transfer fluids
must be used.
Propylene glycol is commonly sold as recreational vehicle (RV) antifreeze. RV antifreeze is not normally food
grade meeting the standards of the United States Department of Agriculture, nor USP (United States
Pharmacopoeia) grade. The product must not be used unless it is marked to meet one of these standards. A
permanent sign must be attached to the heat pump specifying that only approved heat transfer fluids must be used.
The MDH has approved some propylene glycol products designed for geothermal applications that contain small
quantities of chemicals other than propylene glycol. Check the Geothermal section near the back of this
handbook, the MDH Well Management Section Web site, or the Well Management News newsletter for current
information.
F. Water make-up lines to the vertical heat exchanger must be protected with
backflow prevention in accordance with parts 4715.2010 to 4715.2170.
Part 4715.2110 of the Minnesota Plumbing Code relates to backflow prevention devices required where an air gap
cannot be provided. Backflow prevention devices are explained in Minnesota Rules,
part 4725.3350 and in the appendix.
For vertical heat-exchange systems connected to water make-up lines, a RPZ-type backflow preventer is required
because of the continuous pressure and existence of a high-hazard cross connection. A reduced pressure zone-type
backflow preventer consists of two independent check valves with an intermediate relief valve. It also contains
shut-off valves and ball-type test ports. RPZ’s are subject to the licensing, installation, maintenance and testing
requirements of the Minnesota Plumbing Code, Minnesota Rules, Chapter 4715.
G. A vertical heat exchanger constructed according to this part must be no less than
35 feet from a water-supply well. The horizontal piping must be no less than ten feet from
a water-supply well.
Water-supply wells include public and private drinking water wells, irrigations wells, commercial supply wells,
and others.
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The DLI and the Minnesota Plumbing Board regulate plumbing through the Minnesota Plumbing Code,
Minnesota Rules, Chapter 4715. The Plumbing Code requires that a source of pollution be a minimum of 10 feet
from a potable water service pipe. The DLI considers geothermal piping containing antifreeze, including
propylene glycol, to be a source of pollution that must comply with the 10-foot setback.
Horizontal heat exchangers must also be setback from water-supply wells. A 10-foot distance is required if the
fluid is propylene glycol and the piping is high density polyethylene meeting the standards of these rules. If the
fluid or piping does not meet the standards a 50-foot setback is required.
Subp. 2. Notice of loss or leak. The owner of the vertical heat exchanger must notify the
commissioner of heat loop leakage or loss of pressure within 24 hours after the owner
becomes aware of the loss or leak.
STAT AUTH: MS s 103I.101; 103I.111; 103I.205; 103I.221; 103I.301; 103I.401;
103I.451; 103I.501; 103I.525; 103I.531; 103I.535; 103I.541; 103I.621; 144.05; 144.12;
144.383; 157.04; 157.08; 157.09; 157.13
HIST: 17 SR 2773; 33 SR 211
4725.7100 [Repealed, 17 SR 2773]
4725.7200 [Repealed, 17 SR 2773]
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