A member posted this back in 2014 -
Also see Terrys version here -
How to Set up a Water Bath for your FV
A good and cheap solution to setup is a storage container, from Wilkos or Ikea etc, to stand the FV in, add some water to create a water bath and heat that water bath using a 100watt Aquarium heater.
The aquarium heater can be set using the dial to hold a given temp,
I have 2 of these setups in my garage and they work really well at heating above ambient (cool) temperature and holding a brew at a constant temp (20°C or whatever) during fermentation.
A few things to point out:
1. You need a FV without a tap - just a bucket type.
2. Make sure you syphon out the water bath before syphoning your beer out of the FV, otherwise as you syphon the beer the FV floats and tips over.
3. You only need around 200mm deep of water in the bath - no need to completely submerge the FV in deep water.
Storage container will be £6 or £8 and about the same for the aquarium heater off Ebay.
Here is my setup in the garage - I have covered the water bath to get a bit better power efficiency but no need to do that really in a spare room (my garage gets very cool in the winter).