ASHRAE Journal - February 2013 - 32

Characteristics of Successful GSHPs
• The ENERGY STAR rating of the building exceeds 90.
• Maximum loop temperatures returning from the ground
tend to be below 90°F (32°C) for systems in which the
cooling mode determines loop length.
• The systems surveyed during this project were primarily
10-month schools and 8 a.m. to 5 p.m. offices located in
areas where the measured ground thermal conductivity
was between 1.0 and 1.5 Btu/h·ft·°F (1.7 and 2.6 W/m·°C).
Under these circumstances, the successful vertical ground
loops tend to be in the range of 200 to 240 ft of vertical
bore per installed ton (17 to 21 m/kW) of cooling capacity for a ground temperature of 63°F (17°C). This corresponds to a range of 155 to 185 ft per ton (13 to 16 m/
kW) for 55°F (13°C) ground temperature and 270 to 320
ft per ton (23 to 28 m/kW) for 70°F (21°C) ground.
• The ground loop lengths of systems in this survey were
all dictated by the cooling mode requirements. This resulted in advantageous heating mode ground loop temperatures even at the coldest sites in Central Illinois.
At the one site that was monitored continuously, the
ground loop return temperature remained above 46°F
when the outdoor temperature was –6°F (–21°C).
• The primary equipment type tends to be water-to-air
heat pumps.

• Pump control tends to be on-off for these smaller loop
circuits rather than variable speed.
• Ground loop pump power tends to not exceed 10
hp/100 tons (kWPump/kWHeatPump). This value is
deemed to be average (Grade = C) using recommended guidelines.7
• Due to the selection of piping materials and pH level of
the fill water, piping systems tend to not require chemical treatment. However, caution is advised against using PVC pipe. It is not recommended for service when
contact with polyolester oil is possible8 if leaks occur in
the water coils of HFC-refrigerant systems.
• Control is provided by individual thermostats or a building automation system that is simple with a clear and
concise sequence of operation so program adjustments
(or retrocommissioning) can be performed by the
maintenance staff.
• When surveyed, occupants rate indoor comfort, indoor
air quality, acoustics, lighting, maintenance responsiveness, and system controllability as satisfactory.
• When surveyed, the maintenance staff rates system serviceability, quality of design, and quality of installation as
satisfactory.

• Installed outdoor ventilation air equipment capacity
tends to be 20 cfm/person (9.4 L/s per person) or less.

• Owners and designers are satisfied with utility cost and
they openly share results (and permit ENERGY STAR
rating).

• Systems with heat pumps circuited to individual ground
loops, small central, or multiple common loop circuits
out-performed systems with large central loop circuits
by a significant margin.

• Owners and designers are satisfied with the installation
costs, they will openly share itemized results, and they
are confident the project provides positive economic
value.

ergy use cannot be recommended with GSHPs. It is recommended that the Standard 90.1 mechanical section be revised
to allow the best performing GSHP systems to be compliant
and to disallow systems with excessive auxiliary power requirements (i.e., 2.3 hp/1,000 cfm).

Heat Pump Efficiencies
Manufacturers of water-to-air heat pumps are currently advertising products with cooling EERs in excess of 40 Btu/W·h
(11.7 W/W). These values are attained at part-load rating
points that do not reflect actual operating conditions.4
• External static pressure: 0 in. w.g. (0 Pa);
• External water pressure: 0 ft of water (0 kPa);
• Full load airflow used with part-load capacity (no cooling dehumidification in cooling and cool air delivery in
heating);
• Water temperature entering the condenser: 68°F (20°C); and
• Return air conditions: 80.6°F (27°C) db/66.2°F (19°C) wb.
32

ASHRAE Journal

(Car mileage per gallon is not rated going downhill with a
tailwind, so why is heat pump cooling efficiency rated with a
condenser fluid colder than the evaporator fluid?)
Figure 2 indicates cooling and heating efficiencies of constant speed heat pumps are higher than multi-capacity and
variable speed units at ground loop temperatures experienced
in commercial applications (cooling: >80°F [27°C], heating:
<50°F [10°C]). Trends indicate ratings at higher entering water temperatures are needed since almost all the systems in
this survey exceed 86°F (30°C). Furthermore, the efficiency
improvements of multi-capacity and variable speed machines
are minimal or non-existent when corrected to typical partload field conditions.5
This field study demonstrates it is possible to achieve ENERGY STAR ratings of 95 to 100 with simple equipment.
Higher cost heat pumps exacerbate the discrepancy in the
cost increases of HVAC components compared to the increases in ground loops costs. A standard that rates heat
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ASHRAE Journal - February 2013

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