12 years ago I installed an en-suite shower room in my son's house. A ceiling fan connected to the light takes air through a flexible duct approx 2.5 metres to an outside wall vent. There has been no problem up to now. As my son now works abroad the house is let and the tenant, who moved in last January, recently reported excessive mould. It appears on the tiles and ceiling and despite treatment with a spray product it soon re-appears. The tenant, who has respiratory problems, is adamant that she opens the bedroom window when at home and leaves the shower room door open at all times. I've removed the ducting in the loft to check for obstructions and the fan appears to be operating normally. Any ideas?