Manned space laboratories like the US Space Station Freedom or the european COLUMBUS APM are equipped with so-called racks for subsystem and payload accommodation. An important resource is air for cooling the unit internal heat sources, the avionics air. Each unit inside the rack must be supplied with sufficient amount of air to cool down the unit to the allowable maximum temperature.In the course of the COLUMBUS ECLSS project, a thermohydraulic mathematical model (THMM) of a representative COLUMBUS rack was developed to analyse and optimise the distribution of avionics air inside this rack. A sensitivity and accuracy study was performed to determine the accuracy range of the calculated avionics air flow rate distribution to the units. These calculations were then compared to measurement results gained in a rack airflow distribution test, which was performed with an equipped COLUMBUS subsystem rack to show the pressure distribution inside the rack.In addition to that cold flow study, the influence of the avionics air heating due to the unit dissipations on the air flow distribution and the cooling temperature was investigated in a detailed warm flow analysis.