Abstract The duct storage (DST) model is one of the most widely used borehole heat exchanger (BHE) models owing to its fast calculation scheme and usability. However, the DST model can simulate only regularly placed BHEs, particularly in cylindrical borefield configurations. As a possibility to describe irregular borefield configurations by adjusting BHE spacing has been reported, we propose optimal values of BHE spacing for typical irregular BHE cases using the GenOpt optimization tool. A regression model based on a set of results obtained from the optimization is proposed to calculate the modified spacing for the DST model directly without optimization simulations. The proposed regression model fits to the datasets of the optimization with an accuracy of 96.5% in predicting the modified BHE spacing. The differences in time-averaged EWTs between the DST model with the proposed modified spacing and a reference model are close to zero, and they are less than 0.5 °C. However, other borefield configurations that have not been used for the model development can result in higher errors.