This paper investigates the sensitivity on choice of heat pump concept to uncertainties and variations in boundary conditions for large scale heat pumps with a danish district heating system as case example. Large scale heat pumps have recently received increased interest due to tax-reductions, green initiatives and potential for ancillary services. A load-dependent performance map based heat pump model is used to evaluate performance of different heat pump concepts. Since large scale heat pumps have great potential for providing ancillary services, this as well as performance under normal operation should be considered in the choice of heat pump concept. A performance map based heat pump model is implemented in the multi-domain modelling language, Modelica and a micro genetic algorithm is proposed as a nonlinear optimizer for finding the optimised concept design. A sensitivity analysis is additionally carried out to identify whether and which variations in boundary conditions and cost function weighting affects the optimisation and how this consequently affects the choice of heat pump concept.