The required length of vertical ground heat exchangers(GHX) used in ground-coupled heat pump (GCHP) systems isdetermined so that the outlet temperature from the GHXremains within certain limits for the worst ground load conditions.These conditions may not necessarily occur after 10 or20 years of operation, as is usually assumed, but often occurduring the first year of operation.The primary objective of this paper is to develop a generalmethodology for the calculation of the total required bore fieldlength on a monthly basis during the first year of operationusing the framework of the ASHRAE bore field sizing method.It is a three phase process. The first phase consists of analyzingandorderinggroundloads according to the first month of operation.Next, a first set of required lengths is determined by usingthe analyzed ground load components and assuming atemperature penalty Tp=0. Then, an iterative process to calculatethe temperature penalty at the end of each month is carriedout to obtain the final required length for the worst conditions.The methodology is exemplified in a particular case witha slight annual cooling thermal imbalance and with a highinfluence of the hourly peak in heating. For this particularcase, it is shown that the required bore field length occursduring the first year and that the starting month of operationhas a strong influence on the results.Finally, it is shown that it is possible to reduce the boreholespacing when the annual ground load is quasi balanced. In thecase studied here, the minimum length occurs for a borehole to-borehole spacing of about 3.2 m (10.50 ft).