Les Ward
Hi Robin, if the ducting from the fan to the outside is a significant length the new fan may not have sufficient power to create an effective flow of air. If the fan vents straight through an outside wall this is unlikely to be the problem but if back-draught shutters are fitted these may be stuck in the closed position. If possible ask the installer to return to diagnose the problem. If you hold a smoking joss stick near the front of the fan you may be able to see which way the air is flowing.
