This Recommendation sets out the principles of good practice to limit the internal air leakage in bidirectional air handling units, notably those fitted with a rotary heat exchanger. It outlines the most important measures to be considered in the design and setting of a unit to minimise the leakage. The recommendation also presents guidelines on how to correct the declared performance due to leakages. Eurovent holds that these guidelines should become a common industry standard. The potential magnitude of internal leakage rates resulting from incorrect unit design and the corresponding consequences are also discussed.