Space Cooling Energy Consumption with GSHP

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I am modeling an apartment style dormitory for a project in Atlanta, GA.
Each of the 45 units is served by its own heat pump, along with a few
zones for public areas. For an energy efficiency grant application I
have compared the designed system, a ground source heat pump, with an
air source heat pump prescribed by 90.1. Other than the systems, the
models are identical.
The energy use for space cooling is significantly higher for the GSHP
system (24,000 kWh vs. 17,000 kWh). Because the power required to run
pumps offsets the savings in heating energy, this destroys the rationale
for spending the extra money on the Ground Source Heat Pump, as well as
our chances of receiving the grant.
Has any one encountered this issue?


Thomas Butler, LEED AP

Tom Butler's picture
Joined: 2011-09-30
Reputation: 0


Does the simulated dorm operate 24 hours? What efficiency you specified for GSHP? What is the loop temperature in heating and cooling? Have you checked the pump related input and defaults?

There are lots of thing can affect the results.


Xiaobing Liu2's picture
Joined: 2011-10-01
Reputation: 0

As far as cooling energy goes, what are your design case cooling EIR with the geothermal system, and your baseline case cooling EIR for the air-to-air selections?

Is the heat pump fan power parameter the same in both cases, and modeled separately from the cooling energy?


David S Eldridge's picture
Joined: 2011-09-30
Reputation: 2000


The EER and the COP for the GSHP should be substantially higher than the
air source unit. Your heat pump manufacturer should have separate ARI
ratings for air source and GSHP heat pumps. This is a large source of
added efficiency. If your EERs and COPS are not higher with GSHP, you
are missing out on the advantage entirely. You should contact your heat
pump manufacturer for this information. Both Climate Master and Florida
Heat Pump have separate ratings for their systems and the GSGHP ratings
are even higher than the water source heat pump ratings. Air source
will be much lower, I believe. Please check your EERS!

Other areas to increase efficiency modeling accuracy:
VFD on the main circulating pump: Be sure to use an accurate horsepower
Use the actual HP fan wattages, not the nominal horsepower. This can
make a big difference on some units.

Robert Wichert


RobertWichert's picture
Joined: 2011-10-02
Reputation: 201


You should definitively have more savings than that, even if there is
the pump energy.

One first thing to check: the efficiency of GSHP should be much higher
than that of air-source heat pumps in both heating and cooling modes.


Demba Ndiaye

Demba Ndiaye's picture
Joined: 2011-09-30
Reputation: 200

The efficiency of any water source heat pump is dictated by the temperature
of the water loop. Create an hourly report output showing geothermal loop
temperatures. You can then check this against the heat pump chart to
determine if your heat pumps are operating efficiently. Depending on the
ground temperature, a geothermal system can start to heat up, lowering the
efficiency of attached equipment. However, I would still be surprised to
learn that an air source heat pump still performed better.

Also, in order to make variable volume pumping work properly, you must
specify Isolation valves on the heat pumps. This allows water to bypass any
heat pumps that are not calling for water flow. This lowers the pressure in
the system.


No Username provide's picture
Joined: 2011-09-30
Reputation: 200

Good morning,

I am having similar results with a ground-coupled heat pump and am
following this thread closely. I am only realizing savings of about 5%
to 10% over an air-source HP.

I believe I am incorrectly separating out the fan-energy on the
air-source units. What is the procedure for getting the fan and
compressor energy separate on the air-source units (12 SEER air-source
unit per 90.1-2004, baseline design)? If I understand correctly, the
GCHP efficiency stated under ARI 13256 does not include external fan
energy or external pump energy, and therefore, the stated GCHP
efficiency can be converted into EIR directly. Correct?

Also, for the GCHP the correct number to use is the "GLHP EER" value
from the manufacturer based on ARI 13256, which is higher than the
water-loop HP (WLHP) ARI EER? Is that correct?

Experience tells me a GCHP should realize more savings than what I am
getting, which leads me to believe I am inputting "garbage" into the
model. Thanks for your help!

Adam Young

Adam Young's picture
Joined: 2011-09-30
Reputation: 0