How big of a batch size before heating becomes unnecessary?

Pretty much like the subject line says. I know at commercial breweries cooling is all that's ever necessary on the fermenters, since the yeast in those huge batches generates so much heat. But, I generally find I need to give my carboys a little extra heat in the winter, when the ambient temp is ~50°F around the carboys. I'm starting to move up to 10-15 gallon batches in a plastic conical and I'm wondering if I'm going to need to worry about heating. Anyone have any experience here?

most ale yeast ferment best in the 62-68 degree range. So, the size of the batch shouldn't matter much, its the temp of the beer that you are concerned with. There might be a few more degrees generated by the activity in the larger batches, but the basics will be the same as far as keeping the wort at the right temps. If ambient temps are in the low 50s and you are producing ales, you certainly will want to provide some source of heat to bring the temperatures up into the ideal range. I don't think that with 10-15 gallons you'll generate enough heat that this becomes unnecessary.

I think you are going to have to heat still. Just ballpark, a doubling of volume increases surface area 2^(2/3) = 1.58. Since heat loss is related to surface area and heat generation is related to volume, the ratio goes up 2/1.58 = 1.25 or up 25%.

billl said:

I think you are going to have to heat still. Just ballpark, a doubling of volume increases surface area 2^(2/3) = 1.58. Since heat loss is related to surface area and heat generation is related to volume, the ratio goes up 2/1.58 = 1.25 or up 25%.

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Thanks, this puts things in much better perspective. I did some rough calculations taking into account changes in fermenter geometry and it looks like the SA/V ratio in a 15G conical (with 10G in it) is almost the same as in a 6G plastic carboy.