In the very cold weather I have water coming back through the cooker hood ,condensation, should this happen. It is a 2 1/2 year old Bungalow and the extractor pipes exit through the lofft and roof The same is happening in the cloak room. we have stains on the ceiling and the fan stayed on when we turned of the combined light switch. I had to switch off by the isolation fan switch.,Both these events happen without even using the said extractor fans.would lagging solve this?