This paper investigates the use of a vertical coaxial borehole heat exchanger (BHE), focusing on those consisting of standard geothermal piping material, as a component in a ground-source heat pump system. The results of a lab-scale experiment are used to verify the trends exhibited by a recent semi analytical model, referred to as the composite coaxial (CCx) model, considering short-term behavior when laminar flow is experienced in the annular space of a coaxial heat exchanger. The discussion on pipe sizes is then expanded upon using the suggested model along with a modified design procedure to compare the performances realized by an example heat pump. A comparison is made here between configurations having various nominal inner pipe diameters while maintaining the same outer pipe. The results of the analysis show that increasing the inner pipe diameter, within the verified limit of the composite coaxial model, will reduce the required length of heat exchanger and increase the overall coefficient of performance realized by the heat pump. Crown Copyright (C) 2017 Published by Elsevier Ltd. All rights reserved.