Nothing. I do have central heating, but it's never switched on because I developed sinus issues a long time ago and decided to keep it off. To be fair, the first 4 weeks of a pretty bad winter were slightly difficult, however adjusting and climitasing soon followed and now I wouldn't have it any other way. I'm comfortable outside for lengthy periods wearing short sleeved t-shirts on what some people might consider really cold days in the middle of winter, whilst others are layered in clothing, gloves, wooly hats, the whole shebang whilst still shivering their backsides off.