Answer from Radleys FAQ : If a flask is over filled the condensation load on the Findenser will exceed its ability to cool and condense effectively. This is called ‘overloading’. In such circumstances Findenser may not contain the solvent. Working solvent volumes should be a maximum of half the flask volume, e.g. 250ml in a 500ml flask, 1 litre in a 2 litre flask. This is standard ‘good practise’ when heating solvents.