ASHRAE Journal - December 2023 - 28

FEATURE
FIGURE 1 Predicted 20-year ground exchanger (GHX) average water
temperatures.1
reduce use of excessive safety factors.
The 1817-TRP proposal " State of the Art (Background) "
Time in Years
90
80
70
60
50
40
48
96
144
Time in Months
suitable for GSHP operation. However, these increases
are unacceptable at sites with much warmer ground
temperature, (Tg), especially beyond year 20, because
many high-density polyethylene (HDPE) GHXs are
approaching 40 years of service.
An annual temperature decline can also occur when
the amount of heat removed from the ground exceeds
the amount rejected. The decline is somewhat mitigated
because the heat of compressors, fans and pumps must
be rejected into the formation in the cooling mode. In
heating, this contribution is delivered into the building,
which lowers the relative amount of heat required from
the formation. Annual heat balance would be achieved
if heat pumps operate 160 to 180 hours in heating for
every 100 hours in cooling. However, the heat transfer
mechanisms that can mitigate long-term
temperature rise are different in the heating
mode. Long-term formation temperature
decline is likely to be more pronounced in
heating than the rise in cooling for an equal
amount of annual heat imbalance.
In 2020 ASHRAE initiated Research Project
1817-TRP, Long-Term Temperature Change
of Ground Heat Exchangers (GHXs).2
Goals of the project were to improve GHX
design methods via expanded insight into
long-term performance. Data was to be
collected and compared to current GHX
models and design methods. A stated
goal was to improve current methods to
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ASHRAE JOURNAL ashrae.org D ECEMBER 2 0 2 3
95
90
85
80
75
70
65
60
2012
192
240
4
8
12
16
20
stated: " ...there are other important variables
controlling the temperature change that are poorly
understood; the impact of heat induced moisture
migration, groundwater flow and phase change have
not been adequately addressed. The positive cooling
effect of evaporation and the potential negative
impact of reduced conductivity due to lower moisture
concentration are complex and not incorporated
in current design methods. Likewise, the impact of
moisture freezing in cold-climate applications has not
been widely addressed... "
This statement proved prophetic in that the best
available data, shown in Figure 2, did not correlate longterm
imbalance with expected ground temperature
change as suggested in Figure 1.3 The project was
terminated due to a project requirement that sufficient
data with acceptable uncertainty must be located and
vetted for it to proceed. A large number of sites were
located with measured temperatures, but vexing issues
resulted in high levels of uncertainty in the heat transfer
rates and annual heat imbalance. These included
the absence of liquid flowmeters, limited long-term
building automation system (BAS) management and
GHX differential temperatures that were often less than
a ±1.0°F (±0.6°C) accuracy of the measurement devices.
Results of a previous project to assess the long-term
performance of GSHPs were provided in a series of seven
articles in ASHRAE Journal. The third article focused on
ground loop temperature primarily in warm climates.4
Figure 3 demonstrates the maximum rise in average GHX
FIGURE 2 Average GHX multiyear temperatures from the ASHRAE 1817-TRP project progress report.3
Furman University
University of Alabama
MW Office Campus BF1
MW Office Campus BF2
MW Office Campus BF3
MW Office Campus BF4
Furman University
University of Alabama
Load on loop different
than in 2013 - 2014.
ASHRAE 1817-TRP Conclusions:
No obvious relationships observed between change of loop temperature vs. operating years.
Some sites have a positive relationship between heat imbalance and leaving temperature.
Annual heat imbalance uncertainties 20% to 30% in cooling and >30% in heating with ±1 (°F).
2013
2014
2015
2016
Year
2017
2018
2019
2020
Temperature (°F)
Temperature (°F)

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ASHRAE Journal - December 2023

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