I'm using a reptile heating mat under my 1 gallon jars. It of course adds some heat and helps lessen temperature swings to some degree, but I don't monitor the temperature so I'm not sure how much. Subject to room temperature there will still be temperature swings, but the mat will lessen those slightly. It is of course not the same as having a heater with a thermostat. The warmer the water gets the higher the evaporation rate will be though, which helps cool it slightly and might help prevent overheating if the room is hot and the mat is adding heat in any case.