espace condensation leak

Hi everybody have a renaulte espace 2.2 dt diesel 1998 model and all is working fine except for the air con which has a condensation leak. If i place it on for 3 or 4 hours it starts to leak under the dashboard and wets all the carpets in front. brought it to the mechanic and he cleaned the condensation pipe but still the same brought it back to him and to another one but there seems to be no solution according to them can anyone help... thanks a lot