When is controling ambient air temp not enough? (Batch size)

Hi gang....

Is there a ballpark guess as to at what batch size, controlling the ambient air temperature is no longer enough for temp controlling a fermentation well?

I used to temp control 5 gallon batches in a fridge with a temp controller and that worked great. Now my brother and i are doing 10 gallon batches fermenting in sanke kegs (discarded legally purchased kegs by the way :rockin: ) in a temp controlled box i built using a window AC and CoolBot and a ceramic pet heater, and that still seems to be doing a good job yet.

Just wondering at what size, might that not be enough with more heat being generated internally from a bigger batch and perhaps some kind of internal cooling coil might be in order. This post here is what got me wondering about it...

https://www.homebrewtalk.com/f51/internal-fermentation-chiller-236079/

i thought that was a pretty nifty idea...

Thanks guys...
Kyle...

Plenty of breweries that measure their batch size in tens or hundreds of barrels do fine with external cooling only. Obviously you want to be measuring the temperature of the wort and not the ambient air, but that's true for any size batch.

That is a good point.... Though, i'd guess most use glycol jackets as their external temp controller to individually control multiple fermenters and not just controlling the air temp around the fermenters? I could be wrong on that though. I kind of assumed the glycol jackets would have more cooling power than just the outside air, which is what got me wondering about the internal coil to give it a bit of a boost during the initial aggressive phase, but I could be wrong on that.

I suppose the tricky part might be if you have 2 or 3 going at once that might need slightly different requirements between them. When we have done that, we would just measure the air temp, which, yea, isn't ideal, but figured with smaller batches, the air temp, with a small fan to move the air around the box would be close. I figured with larger batches, it would be less "close enough".. heh..

for hot ferments like my saison I temp control with a water bath heated by an aquarium heater. Otherwise to cool it just toss a wet cool towel over it.