A heating system that combines a ground source heat pump and solar collectors with a combistore was measured in detail in the laboratory by means of “whole system testing”. The test rig thereby emulated a building with floor heating, hot water tappings, flat plate solar thermal collectors and a ground source heat exchanger. The tested heating system covered the heat demand for space heating and domestic hot water preparation in a reliable way with a good annual system performance factor of 4.5 that was determined after the physical test by means of test-calibrated simulation models. Nevertheless the measurements revealed that there is still room for improvements. It was found that the heat pump operates in space heating mode in an oscillating on/off behavior with short runtimes and respectively with a high number of starts and in addition with a supply temperature that is in average 8K above the temperature that is needed in the heat distribution. Furthermore a significant transport of energy, from the upper part of the store that is reserved for domestic hot water preparation to the lower part, reserved for space heating, was detected.