I bought a brew belt today.  It was 16°C in the warmest room in the house(!).  A few hours later and my quince wine was up to temperature and bubbling nicely (and making a lovely fizz when you held your ear to the tub).  I had to find a timer switch to switch it on and off because after a few hours it was creeping up over 26°C.  You can also push the belt up and down, but a timer (that I had lying around) looked a better option until I can get the arduino going.