What is the best kind of fermentation heater?
The two commonest types are electric mats and electric belts. The mat is easier to use. It is like a small electric blanket that you slip underneath the fermentation vessel. They can be quite useful, especially in the early stages when you want to help the process along, but later, when a sediment starts to form, applying heat directly below the sediment can release off flavours into the wine. The fermentation belt solves this problem, as it is wrapped round the outside of the vessel just above the sedimentation level. So, the belt is a better choice than the mat, but better still is simply to control the ambient temperature and use no heaters at all.
