As a continuation to my previous question about a very slow fermentation of sloe wine, I think my demijohn is just too cold where it is. I haven't taken readings - I'm still fuzzy on how - but judging by its activity I'm sure there's plenty of sugar left and not too much alcohol.

Can anybody suggest a safe way of warming it up, without it getting hot enough to harm the yeast? It's too big to fit in the microwave. :)

Once it's warmed up, I assume it will keep its heat for a while? I know after boiling the fruit up, it took until morning before it was even cool enough to add the yeast.

  • I hadn't heard the word 'carboy' before. Is that the american name? Commented Nov 16, 2010 at 11:40
  • a 'carboy' is a glass fermentation chamber, as opposed to a plastic chamber or a demijohn. It generally refers to 6.5 gallon chambers. Commented Apr 28, 2014 at 5:54

7 Answers 7


A few things on fermentation....

Airlock activity is not an indication of fermentation. Just because you don't see bubbles and a hear a gurgling airlock does not mean that your wine is not fermenting.

To take readings you need a hydrometer. A hydrometer measures a liquids gravity (or density). Liquid is more dense with sugar, and less with out. As yeast eats sugar and creates alcohol the gravity (or density) lowers.

When first making your wine you should have taken a gravity reading. Then, read it again week, 2 weeks, 3 weeks... whatever later and see how it's fermenting.

All of that being said yeast does not like sudden changes in temperature. It stresses them out and makes for unhealthy yeast. And unhealthy yeast makes for poor fermentation. If you want to warm them up then do it SLOWLY. Raise the ambient temperature of wherever you're fermenting by 1 degree a day. No more.

At what temperature are you fermenting? You really shouldn't need to be fermenting any higher than the 65 - 70 range.

I would advise against wrapping your carboy. Fermentation is an exothermic process. That means that it gives off heat. Inside your fermentation vessel it could be 5 - 10 degrees warmer than outside. Wrapping your vessel is a quick way to stress out the yeast as the vessel could heat up more on the inside than you intend.

Sorry for the book. Just wanted to be thorough.

  • Thanks. I'll make sure not to do anything sudden. Despite your warning, I might wrap it, but I'll be careful - I don't think it's active enough to heat up too much, but I'll keep my eye on it. Commented Nov 16, 2010 at 11:34
  • Btw, it's pretty wintery here - 65F (18C) would be nice Commented Nov 16, 2010 at 11:36
  • I've placed it near (but not too near) a radiator and I'll see if that helps. Commented Nov 17, 2010 at 8:35
  • Keeping it near a radiator helped a little, as does shaking it around every so often. It's still slow, but not dead yet. Commented Nov 22, 2010 at 10:29

I took the Red Green approach to keeping a carboy warm when I was brewing last winter and had temperatures in my brewing area that were too low (perhaps not for lagering, but certainly for what I was doing). I took a large corrugated cardboard box (I used the one my 21 gal. air compressor came in) and placed it over the carboy. I cut a hole at the bottom of the box. I used a programmable outlet thermostat attached at the neck of the carboy to operate a small space heather outside of the box that blew warm air in through the hole at the base. I put a piece of corrugated cardboard against the carboy where it was exposed to the hole so the heat wasn't blowing directly on it. Assuming you can get a thermostat that'll operate in your desired range, this could work. Given the small volume of air in the box and the thermal mass of the carboy + 5 gallons of liquid nothing was ever experiencing a huge thermal shock, so I can't imagine the yeast we're unduly harmed. I'm sure there are some purists who are cringing at the thought of using a space heater on the fermenter, but it turned out well for me.


One suggestion I've heard is just to insulate the demijohn/carboy, since the yeast action should give off some amount of heat over time. You can try wrapping it in towels and/or blankets, for example.

  • That would be true if the yeast were active enough, but I'm not sure it is. Commented Nov 16, 2010 at 1:17

A buddy has successfully heat up some bottles for carbonation using a microwave heat pad and blankets for insulation. I have a feeling it may hurt the yeast if you provide too much heat at once while fermenting.


I have a carboy heater. I got it from the local homebrew store. But I agree with Matt measure first and get some data.

Carboy heater

  • Curious. It looks like it would only heat a specific section of it. Is that designed to make the warm bits rise and stir the contents for you? Commented Nov 17, 2010 at 8:32

Use a Fermwrap and check this question I asked.


If your wine or whatever has slowed down, you could warm a little of the liquid up and put in a couple of teaspoons of sugar. Melt the sugar and then add to the wine and this should increase the fermenting process. Leave for twenty four hours and then check. If there are no bubbles within half an hour the fermentation is finished. It can be complete in three weeks. Good luck

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