ABSTRACT Geothermal heat pumps exchange heat with the ground through the use of ground heat exchangers where the heat transfer rate depends on the thermal conductivity of the surrounding soil. An in-situ test is often performed on a vertical borehole to estimate soil thermal conductivity@ but the test must have sufficient duration in order to obtain an accurate estimate. Conventional analysis methods usually do not check to see if the test duration is sufficient. This paper validates a procedure to perform this check as a supplement to current methods. The procedure uses an analytical composite model of borehole heat transfer to estimate the minimum test duration necessary to determine soil thermal conductivity within 5% of the estimated value from a very long test. Data sets from 16 field tests are used in the validation process. The minimum test duration ranges from approximately 10 to 53 hours among the tests when a simplified line-source model is used for evaluating soil thermal conductivity. The results indicate no simple rule for minimum duration applies to all cases. Instead@ the proposed procedure based on the analytical composite model can determine if test duration is sufficient.