"INTRODUCTION Energy utilization in the built environment is of increasing concern@ and geothermal heat pump (GHP) systems (also known as ground-source heat pump or Geoexchange? systems) are now relatively well established as a means of significantly reducing energy consumption in space conditioning of buildings. This improvement in efficiency@ however@ generally comes at a higher first cost@ which must be offset by lower operating and maintenance costs within an acceptable period of time to the building owner. As with most alternative energy systems@ high capital cost is a significant barrier to market penetration. One of the main goals in the design a GHP system is to properly size the total length of the ground-loop heat exchanger so that it provides fluid temperatures to the heat pump within design limits. Unlike with conventional heating and cooling systems@ design of GHP systems requires some type of life-cycle simulation due to the thermal storage effects of the earth. Annual heating loads in a building are rarely balanced with annual cooling loads@ and thus thermal responses of the ground throughout the building's life cycle must be considered. In heating dominated buildings@ annual imbalances in the ground load will lead to progressively lower heat pump entering fluid temperatures@ and in cooling dominated buildings@ progressively higher heat pump entering fluid temperatures will occur. These excursions will result in the heat pump efficiency to progressively deteriorate and the equipment capacity to be ultimately compromised if the ground-loop heat exchanger (GLHE) is not large enough. Borehole fields designed for buildings with relatively large annual ground load imbalances can often be excessively large and costly@ making vertical closed-loop GHP systems noncompetitive with conventional systems. The phenomenon of long-term temperature change in the subsurface due to GHP systems serving buildings with imbalanced annual thermal loads has given rise to the concept of the hybrid GHP system@ where a supplemental component is utilized to effectively balance the annual ground loads. These systems permit the use of smaller@ lower-cost borehole fields@ but their design adds to the complexity of the overall GHP design process because of the addition of another transient component of the system. For example@ acceptable conditions for solar recharging of the ground in a heating-dominated building depends on solar availability and ground loop temperature. Consequently@ hybrid GHP systems should be analyzed on an hourly basis in order to fully understand their behavior. The fundamental task in designing hybrid GHP systems lies in properly balancing the size of the supplemental component and the size of the ground-loop heat exchanger@ while optimizing the control of the supplemental component. Current engineering design manuals such as Caneta Research@ (1995)@ Kavanaugh and Rafferty (1997)@ Kavanaugh (1998)@ and ASHRAE (2003)@ developed from research conducted since the 1980s@ mention the potential use of hybrid cooling tower/fluid cooler GHP systems@ but do not describe a design process for systems that utilize solar collectors for the thermal re-charge of the ground. Proper and reliable design of hybrid GHP systems is quite difficult and cumbersome without the use of a system simulation approach. Further@ without an automated optimization scheme coupled to the system simulation program@ the design activity itself can become tediously impractical and time consuming. The use of system simulation for analyzing complex building systems is ever increasing@ but the necessary computing resources are not at the disposal of every design practitioner@ nor is their use always economically justified. As new technologies and design concepts emerge@ design tools and methodologies must accompany them and be made usable for practitioners. Without reliable design tools@ reluctance of practitioners to implement more complex systems can become a significant barrier. Therefore@ the overall goal of the work presented in this paper is to develop a tool for the design of hybrid GHP systems in heating-dominated buildings that is useful to practitioners. The design tool is based on current ""state-of-the-art"" system simulation methods@ cast in a format that allows straight-forward use."