A new method for the hourly simulation of fields of long BHEs (borehole heat exchangers) is presented, in dimensionless form. The method considers also the internal structure of the BHE, with reference to double U-tube BHEs. The analytical expressions of new g-functions (time evolutions of the dimensionless temperature, averaged along the BHE length, due to a uniform and constant heat load) are presented, which complement those reported in Zanchini E., Lazzari S., Energy 2013; 59: 570-80. The new g-functions yield the dimensionless temperature at the interface tubes-grout, and allow a more precise determination of the time evolution of the temperature of the operating fluid during hourly peak loads. Moreover, g-functions at the interface tubes-grout and at several radial distances are reported also for BHEs with a ratio 500 between length and diameter, a value not considered in the previous paper. Due to its higher accuracy, the new method is recommended for the hourly simulation and the optimization of ground-coupled heap pump systems with double U-tube BHEs. (C) 2014 Elsevier Ltd. All rights reserved.