not easy but possible..if it is a combined washer dryer its probably REALLY not worth it but if its just a tumble dryer you need to find out how the air circulates. The fan usually draws air through a heating element which then goes into the tub and through the clothes, at the other end it comes out and flows into a chamber where a fine cold water jet condenses the moisture the air has picked up from the clothes, and returns the dried air back to the heater. You need to leave the fan and heater operating, but stop the water cooling jet by disconnecting the water valve that feeds it, and then remove the ducting to provide an air outlet that can be ducted outside the house . this will also provide a fresh air inlet. It all depends if this can be done on your particular model, and that depends on where the fan is situated in relation to the heater, and how it all works. this job is not to be tackled lightly, and the more I think about it the more I would advise trading it in. condenser dryers are crap! the main problem seems to be that the water jet gets blocked and the thing goes on for hours wasting electricity, and then gives you damp clothes.
Phil, December 2008