Andrews, Hammock & Powell, Inc. offers a variety of engineering and technical services to Consulting Engineers, Architects, Owners, Contractors and other Clients to help insure the long-term successful operation of Geothermal Heat Pump (GHP) Systems.  Sometimes GHP systems are referred to as Ground Source Heating and Cooling (GSHC) or Geo-Exchange Systems. In the Engineering arena, we provide design services from Ground Loop Heat Exchanger (GHX)-only modeling/engineering, to the integrated design of the entire GHP/GHX system inside and outside the building.  We also provide Research, Development, Engineering and Demonstration capabilities for advanced GHP system “Architectures” like Underground Seasonal Thermal Energy Storage (USTES) systems.  We were recently selected by DoD to demonstrate two forms of USTES: Borehole Thermal Energy Storage (BTES) and Aquifer Thermal Energy Storage (ATES).

On the Technical Services side, we perform in-situ Thermal Conductivity Testing (TCT) of the underground formation for conventional GHP systems and BTES systems, Technical Services for ATES and Open Loop systems as well as other related services. We are also a National Environmental Balancing Bureau (NEBB) certified firm in both Testing/Adjusting/Balancing (TAB) and Commissioning (CxA).  In the CxA arena we provide LEED-compliant Basic and Enhanced CxA, ASTM-compliant GHX Pressure testing, GHX purging/ flushing Monitoring and more.  Beyond having Professional Engineers (PE) on staff registered in multiple states, and the capability of obtaining PE registration in any state, we also have a PE on staff that holds the International Ground Source Heat Pump Association’s (IGSHPA) Certified Geothermal Designer (CGD) credential.

For more information about all of AH&P’s Geothermal services, please contact Chuck Hammock at ext. 6362 or Stephen Sullens at ext. 6382.

Engineering/Computer Modeling/LEED Services:

On the Engineering/Modeling/Computer-Simulation side of GHP/GHX systems, our services typically fall into one of the following categories:

Integrated Building/GHX Modeling/Engineering: Under this approach, we create an hour-by-hour energy model of the building based on data/input from the Architect, Mechanical-Electrical-Plumbing (MEP) Engineer-of-Record (EOR) and Owner and perform a multi-decade integrated simulation model of the building and the GHX for loop sizing purposes. Under this approach, AH&P takes total responsibility for the sizing of the GHX.  We then prepare stamped Plans and Specs of the GHX for inclusion with your Construction Documents.  We often include submittal review of the grout, piping, u-bends, factory or field TC testing of the grout, drilling plan, etc. and usually provide on-site construction reviews of the GHX system and furnish a report of our findings.  Under this scope of services,  if desired, we can add-on the USGBC required “baseline building” to the energy model and perform the EA Credit 1 (Optimize Energy Performance) Energy Modeling required under LEED.

GHX- only Modeling/Engineering: Under this approach, the MEP Engineer-of-Record (EOR) for the MEP systems within the building performs an hour-by-hour simulation of the building and its systems and provides AH&P with the peak monthly heating and cooling loads coil loads (BTU/Hr), as well as cumulative BTU monthly heating and cooling coil loads (heat extraction and heat rejection loads from the Building loop serving the Geothermal Heat Pumps but not including the effect of the compressor operation).  AH&P then performs a multi-decade simulation model of the GHX and the GHPs to determine the proper size of the GHX.  Under this scenario, the building PME Engineer-of-Record has responsibility for the accuracy of the peak and cumulative loads and AH&P’s liability is limited to providing a GHX that will extract or reject the provided/specified loads to the local geology (who’s thermal characteristics were determined by in situ Thermal Conductivity Testing (TCT) performed at the proposed borehole field location.  We also provide the construction documents (plans and specifications) for the construction of the GHX.  Submittal reviews and on-site Construction reviews are often provided.  Under this scenario, the GHX-only modeling efforts are not sufficient to easily expand them for LEED Modeling purposes as those efforts must include a building energy model also, and so the adder for LEED Energy Modeling is significantly more expensive than it would be under paragraph 1.

ATES and Open-Loop GHX Modeling/Engineering: Under this approach, we provide the engineering necessary for successful implementation of an ATES or Open-Loop GHX System. We create an hour-by-hour energy model of the building based on data/input from the Architect, Mechanical-Electrical-Plumbing (MEP) Engineer-of-Record (EOR) and Owner. We also input the parameters calculated/observed from the hydro-geological study to perform a multi-decade simulation model of the ATES or Open-Loop GHX system.  Under this approach, AH&P takes total responsibility for the sizing of the ATES or Open-Loop GHX system.  We then prepare stamped Plans and Specs for inclusion with your Construction Documents.  We often include submittal review of the submersible pumps, piping, HVAC equipment, drilling plan, etc. and usually provide on-site construction reviews and furnish a report of our findings.  Under this scope of services, if desired, we can add-on the USGBC required “baseline building” to the energy model and perform the EA Credit 1 (Optimize Energy Performance) Energy Modeling required under LEED.

NEBB Commissioning (CxA) Services:

AH&P is fairly unique amongst Consulting Engineering firms in that we are certified by the National Environmental Balancing Bureau (NEBB) in both Testing, Adjusting and Balancing (TAB) and seven different disciplines of Commissioning (CxA), for the whole building and its systems.  We therefore offer LEED-compliant “Basic” CxA Services and on projects where we are not the EOR on the MEP systems, we also offer LEED-compliant “Enhanced” CxA Services.  On projects where there is already a Prime CxA, we can assist them with just the nuances of proper GHX commissioning including ultra-sonic based velocity-monitoring during purge/flush, ASTM-compliant HDPE piping pressure testing, grout TC testing, etc.

Thermal Conductivity Testing Services:

Introduction: AH&P performs In-Situ Thermal Conductivity Testing (TCT) to determine underground thermal properties that are critical to the proper design and sizing of Ground Loop Heat Exchangers (GHX).  TCTs are accomplished by injection of heat at a constant rate into an underground formation for 36-48 hours via a grouted u-bend installed in a deep borehole and the subsequent analysis of the data collected from that procedure.  Highly efficient Geothermal Heat Pump (GHP) systems utilize GHXs to transfer heat in and out of the underground formation to heat and cool buildings.  GHP/GHX systems cannot be properly engineered for a commercial building without a preceding high-quality TCT.

Thermal properties determined by the TCT include Thermal Conductivity (k), Thermal Diffusivity (α) and the “undisturbed deep earth” temperature.   “Drill Logs” from the TCT report provide insight into geological and hydro-geological characteristics of the underground formation at the site (“the Borefield”) and drilling issues encountered during the test bore .  The TCT Drill Log therefore provides potential production drillers insight which can reduce the uncertainty/cost/potential change-orders associated with their proposals.

Procedures/Protocols: The procedures and protocols AH&P follows in the execution of a TCT are listed below.  These tests provide at least three critical properties of the underground geology at a project site that are critical to the proper engineering, sizing and modeling of the Ground-loop Heat Exchanger (GHX).  GHX’s are normally utilized by Geothermal Heat Pumps (GHP) systems (sometimes referred to as Geo-Exchange or Ground Source Heat Pump systems) for heating/cooling/process loads at a nearby proposed facility to the site where the TCT is conducted.

TCT (Testing) Procedures: Andrews, Hammock & Powell Inc. (AH&P) utilizes the procedures outlined in Chapter 32 of the American Society of Heating, Refrigeration and Air Conditioning (ASHRAE) HVAC Applications Handbook, 2007 Edition when conducting this Formation Thermal Conductivity Testing (TCT).  Some of those procedures, and the enhancements AH&P adds to those recommendations are as follows:

  • ASHRAE recommends the test duration be 36 to 48 hours.  The total duration of AH&P test will fall within or exceed those guidelines.
  • ASHRAE recommends that heat be applied to the borehole U-tube at a rate of 15-25 watts per ft of bore (141 to 234 ft of bore/U-tube per ton). For AH&P TCTs, energy will be delivered to the underground formation within those guidelines, with a preference toward the “middle” of the specified 15-25 watt/ft range.
  • ASHRAE recommends a maximum standard deviation of +/-1.5% of the input power from the average power level and peak variations be kept to less that 10% of the average power level OR the resulting temperature variation of the actual average loop temperature, from that predicted by a linear regression analysis of temperature vs. the natural logarithm of time, be held to less than +/- 0.5°F. Our TCT will better those specifications and possible comply with both parameters, not just one parameter as the ASHRAE standard allows.
  • ASHRAE recommended temperature measurement/recording accuracy is 0.5°F. Our temperature probes and data loggers will have a combined accuracy of 0.50 °F or better.
  • ASHRAE recommended power transducer/recorder accuracy is +/-2%. Our equipment will meet or exceed this level.
  • ASHRAE recommends pumping sufficient flow to obtain a temperature differential of the “actual heat pump system” and mentions the range of 6-12 °F. AH&P’s experience is that actual heat pump systems can have annualized temperature differentials that may fall below this range. Also, as “line source” is the heat energy “architecture” modeled in this analysis, it is felt that higher flow (lower delta T) values may be more appropriate when the grouted, dual piped u-bends are desired to serve as the “line source” of heat energy for the TCT. Accordingly, our test can often be run at higher flow rates with resultant lower temperature rise, allowing the u-bend to more accurately represent a “line source” if desired by the client, or we will operate at the ASHRAE 6-12 °F if preferred by the Client.
  • ASHRAE recommends a 5 day waiting period for k<1.0 formations and 3 days for k>1.0 formations. Some TCT practitioners adjust this waiting period to 5 days when high energy/friction/heat drilling is involved (down-hole-hammer /air drilling in rock) vs. as little as 2 days waiting when lower energy/friction “mud-rotary” drilling is utilized. Unless directed by the Client due to time constraints, this test will be conducted after at least five days have passed between drilling operations and the TCT commencement to insure ASHRAE compliance although thermodynamically, this may be unnecessarily conservative.
  • ASHRAE allows either direct temperature recording of the underground undisturbed formation temperature via probe insertion in the liquid filled u-bend at three locations or loop exit temperature during startup. AH&P is capable of performing both of these test procedures. We often utilize the more accurate direct measurements by recording the temperature at multiple depths (50’ below grade down to the u-bend’s deepest depth, on 50’ intervals) by means of a miniature rapid-response submersible temperature data-logger. This device is carefully inserted into the u-bend and lowered to the bottom of the U-bend then raised to the surface in 50’ increments, with the last recording taken at 50’ below grade and the complete temperature profile and overall average provided to the Client.
  • ASHRAE recommends data collection on maximum 10 minute intervals. AH&P increases the frequency of recording at least two-fold by recording data at least every five minutes.
  • ASHRAE recommends ½” of above ground piping insulation and a minimum of 1” fiberglass equivalent test rig insulation. AH&P provides at least the recommended ½” pipe insulation and insulates the test rig with closed cell polyisocyanurate insulation that is equivalent to more than 3” of fiberglass batt insulation.
  • ASHRAE publishes “re-testing” protocols, which will be followed if retesting is needed.

Quality Assurance: In addition to the ASHRAE procedures outlined above, AH&P strives to provide the client with the highest level of formation thermal conductivity testing and data analysis by adhering to the following practices: AH&P owns and engineered our TCT testing apparatus. We maintain, calibrate, repair and constantly improve our TCT rig to provide the highest quality TCT analysis we can deliver. We have an in-house test lab where we can startup and operate the test rig under simulated conditions prior to utilizing it in the field so that we can insure it is functioning properly before it is deployed on your project. Many practitioners utilize rented equipment and are trained in drilling methodologies, but are not formally trained in the Thermal Sciences that are the focus of an accurate/comprehensive TCT. Unless specified otherwise, we utilize thermally conductive bentonite grout for each grout-able borehole, installed via tremie tube from the bottom to the top the borehole. A “grout-able borehole” is one installed in a formation capable of being grouted via (up to) 133% of the theoretical annulus grout volume needed to fill the borehole/u-bends annulus. In the event of highly porous formations with “un-grout-able” regions (regions requiring grout volume exceeding 133% of theoretical), the annulus will be filled with pea-gravel or similar material until a “grout-able” region has been encountered, at which time grouting will resume to the surface (or until another “un-grout-able” region has been encountered). In no circumstances will less than the last 10’ of borehole (up to the surface) be grouted with a bentonite grout meeting NSF standards. ¾”, 1” or 1-1/4” SDR-11 HDPE piping with u-bend will be installed in the borehole with a minimum of 3’ of excess pipe extended above the adjacent grade left for testing and future connections. Temporary use of and fuel for an on-site diesel generator to power the TCT test apparatus for up to 24 hours/day during the test procedure shall also be provided unless specified otherwise. A Thermal Conductivity Test (TCT) Report shall include, but not be limited to:

  • Thermal Conductivity of formation
  • Deep Earth Temperature
  • Estimated Diffusivity
  • General log of geologic formation encountered during drilling procedures

We utilize graduate engineers, NEBB certified Testing/Adjusting/Balancing (TAB) Supervisors or our in-house trained technicians in the field to conduct the actual TCT test. We do not normally subcontract this work. If time/cost/Contract constraints mandate the use of field sub-contract labor, we will only utilize technicians that have been directly trained by AH&P. After our in-house field personnel return to the office, an IGSHPA Certified Geothermal Designer (CGD) and more importantly, a Registered Professional Engineer (PE) will supervise the data analysis and preparation of your TCT report at our main office located in Macon, GA. We utilize linear regression to analyze the appropriate portion of the data set to insure the above ASHRAE standards are met in regards to power consistency and temperature vs. ln(time) “slope” quality. Specifically, if we determine the “correlation coefficient” (R2) of our slope line does not exceed 0.95 we will analyze the anomaly and will re-run the TCT at no cost to the Client if it is determined that the lower than normal R2 is due to any errors in our execution or equipment.

Hydro-geological Services:

Either through in-house personnel and/or consulting hydro-geologist and drillers, we can provide the following hydro-geological services.

  • Aquifer Testing
    • Install Monitoring Wells
    • Install Production Wells
    • Well Development
    • Perform Pump Tests (Time-Drawdown and/or Distance Drawdown)
    • Determine Aquifer Specific Yield and Recovery Rates
    • Perform Slug Tests
    • Determine Hydraulic Properties (hydraulic gradient, transmissivity, groundwater velocity, estimated porosity)
  • Determine position and thickness of Aquofers and confining layers
  • Collect Groundwater samples for laboratory testing (Iron, Oxygen, etc.)
  • Geophysical Logging (Gamma, Self-Potential, Single-point Resistance, Normal Resistivity, Caliper, etc.)
  • Provide detailed cutting descriptions at various depths from drilling activities
  • Provide sieve analysis of drill cuttings

The information gathered from these technical services are compiled to prepare a hydro-geological report detailing the testing process, observations and calculations, which is also utilized to properly model and design ATES and Open-Loop GHX systems.


Mr. Jackson Kane, AIA
Lord Aeck Sargent, Architects
1201 Peachtree, St. 300
Atlanta, GA 30361-3500
(w) 404-253-1428

Mr. Clint Hardy, PE
Clark Nexsen (Consulting Engineers +)
440 Martin Luther King, Jr., Blvd.
Macon, GA
(w) 478-743-8415