The impact of unintentional air flows on the performance of ventilation units with heat recovery is discussed on the basis of single room ventilation units. Assuming an external short circuit (outdoor) and internal (inside the ventilation unit) air leakages, which lead to internal short circuits, a model is developed and characteristic numbers for ventilation efficiency, efficiency of heating load reduction and effectiveness of electrical energy use are derived. Differences between supply and extract air flow rates, resulting in increased air flows through cracks in the building envelope, are taken into consideration too. The use of tracer gas techniques to measure air leakage rates from ventilation units is described briefly. It is shown by numerical examples that unintentional air flows can considerably reduce the performance of ventilation units in terms of ventilation efficiency and, in combination with unintentional heat flows through the casing, energy savings. Therefore, these flows should be avoided or at least reduced to an acceptable level by an appropriate construction, manufacturing process and installation of the units. (C) 2001 Elsevier Science B.V. All rights reserved.