I'm reading all I can about this and I came across this: from the Tribe webpages about kombucha.
6.90 How can I keep my Kombucha warmer in the cool months?
Potting Pad-You can buy a "potting pad" at your local greenhouse supplier to use as a heater for your fermenting Kombucha Tea in the cool months. Simply set your jar of fermenting Kombucha Tea on the pad to give it some extra warmth. Potting pads are used by greenhouse growers to keep their seedlings at an even growing temperature.
Aquarium Heater-Some people set their jars of fermenting Kombucha Tea into a small glass 5-10 gallon fish aquarium, fill it up to near the top of the jars with water, then stick a submersible aquarium heater- set at the lowest temperature into the water in the tank. This will take the chill off the water that surrounds your fermenting Kombucha jars and keep the contents at more even temperature. You may wish to drape a towel over this "tea-tank" to help keep the warmth in.
Aquarium Heater #2-(Mike M., a subscriber to the Kombucha mailing list,) puts his one-gallon glass brew jar into a two-and-a-half gallon plastic bucket, then fills this with water up to about the level of the tea in the glass jar, and hangs a ten-watt aquarium heater over the side of the plastic bucket.
Space Heater-If you keep your fermenting jars of Kombucha in a separate small room that has a door, you may consider using a small space heater turned to the lowest temperature, just to take the chill of the room. However, keep in mind that this could be a fire-hazard and will run up your power bill!
Heating Panel-There is a heating device on the market called a "heating panel" you could buy, they are about 11 X 11 inches with a 72 inch cord. Just set your jars on top of this panel. They will keep four one gallon jars at a steady 82° F.
Hot Rock-You can purchase a hot rock in any pet shop that sells reptile supplies. They are made for use in reptile aquariums. You simply set the bowl or jar on top of it.
Brewing Belts-Some people are using "brewing belts" that can be purchased in home-brewing supply shops. These belts, according to John Novar of Kombucha Power in Florida, are designed to fit around a 5 gallon brew bucket. They plug into a 110 volt current.
Hot Water Closet-Some people put their jars of fermenting Kombucha Tea in the hot-water heater closet to keep them warm during the winter.
Toy Chest-A lady mentioned over the Kombucha list that she made a "brewing-box" using a sturdy old toy-chest that will hold up to 10 one gallon jars of Kombucha Tea at a time. She placed a water-bed heater on the floor of the chest and sits the jars on a shelf about 3" above this heating pad., closes the lid and lets the tea ferment for 7 days at 85°F.
Cardboard Box-Another lady says she wraps a cardboard box around her containers and installs a 15 watt light bulb. To keep in the heat, she covers the whole thing with a large towel.
Upside-down box-You can also make a similar box by flipping the box upside-down over your fermenting jars, and adding a small wattage bulb. Poke a few holes in the "bottom"-which is now the top- to let out excess heat. Unheated this setup will serve to help to keep the draft off your jars.
Ice-Chest-Another idea is to put a 15 watt light bulb inside an ice-chest turned up-side down. Poke a couple of holes in the sides and top for ventilation and to let out excess heat.
Reptile Heating Pad-A man in Kelowna BC said he came across a great device for heating/warming the pot of your choice. He bought a reptile heating pad in a pet store that is a very thin sheet of plastic that sticks to the underside of the reptile tank. He says they come in different sizes and are very low wattage. It could also be stuck to the bottom of a dish or plate to make a warming tray.
Incubator-One man constructed an incubator from a foam ice-chest filled with water. He covered this set-up with a foam lid and used an aquarium heater to keep the water temperature at 80°F.
Waterbed Heater-The flat low-wattage waterbed heaters are perfect to set your jars or bowls of Kombucha on during the cool months. You can set the thermostat at the lowest setting, cover it with a towel then set your jars on top of this mat.
Large Paper Bags-If you want to protect the jars from drafts etc., you can make "tents" out of heavy brown grocery bags. Fold the top two or three inches of the bag down to make a firmer base, then just "up-end" them over the jars or bowls. These heavy duty paper bags will also