The optimum temperature for fermentation if one is thinking of speed rather than flavour is about 33°C. Fermentation itself generates some heat but probably insufficient to maintain this temperature, particularly if the room is cool. An external heat source, therefore, should be provided and since only 100 watts or so are required an immersion heater such as used in an aquarium is ideal. If it does not contain its own thermostat, or if you use a different type of heater, an ordinary light dimmer switch works very well. They are inexpensive and can take up to 600 watts.
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