I'm currently doing 1.3 gal batches in carboys and I want to scale up to full 5 gal batches - I have a 30L fermenter but don't have the equipment to brew at that batch size (yet!) - I can probably manage ~2 gal max.

I have limited space, so I'd like to get rid of the carboys which is why I'm wondering if it's possible. If it makes any difference the 30L one is a pressure fermenter (fermzilla all rounder) (but I have no idea if you could even attain any kind of pressure at such a low volume..).

Is it OK to ferment such a small amount in a fermenter of that size?

  • I think in general it's fine to ferment in a vessel with plenty of headspace like this. However- don't secondary or store the beer long term like this. It will oxidize really, really fast. So if you do, don't take the bung out of the carboy until you're bottling.
    – rob
    Commented Sep 12, 2020 at 2:23

3 Answers 3


So far my brewing is all from kits, I don't use air locks, just 5gal buckets with lids, not sealed all round - so pressure doesn't build up. Then I prime and bottle the flat beer and it gets its fizz from there.

As the initial fermentation has co2 sealing the surface of the brew the free space above won't matter, then if you bottle condition it should all be fine.

Or, if you use pressure barrels, you should be able to fill any empty head space with C02 from a cartridge, as if you had just dispensed a load of beer already.


Correct me if I got this wrong, you want to ferment 2Gal (~10 litre) in a 30 Litre pressure vessel?

For 1st Ferment, you will probably get away with it as long as the vessel is left undisturbed then the CO2 produced by the yeast is heavier than air and should protect the beverage. the fermentation will probably not produce enough CO2 to bubble any/all air out through the airlock.

For 2nd Ferment, it will not reach any decent pressure way too much head room to give any sort of carbonation, the fermentation will not produce enough CO2 to pressurise the vessel and there will also be too much air in there that could make it go 'funny'.

The only way to do this would be to force carbonate the beverage, by pressurising and bleed the air out a couple of times with CO2, either using a cylinder (with a regulator and all the other trimmings) or with the little "Mini Keg Co2 Bulbs 16g" (you may need a few to fill it up, but google them you will see what I mean) and some fittings to attach it to your vessel. in either case this way you could skip the 2nd Ferment all together.

Hope this was of help.


yes it is possible, yes it should be fine.

Below is some maths (that I hope helps) -

1 mol of glucose weighs ~180g. You have 1.3 us gal ~ 5 litre (4.9....) of fermentable liquid, I will say 5 is much easier to work with here.

At gravity of 1070 you have roughly 1 mol/litre. (182.5g/l)

If we assume that during feremtation roughtly 1 mol of glucose yields 2 mol of CO2.

Then 5 litres @1070 fermenting down to roughly 1000 will yield 10 mol of CO2. And given that 1 mol of CO2 at room temp and 1 atmosphere of pressure fills ~24 litre or 6.3 USGal, our 10 mols of CO2 will fill 240l, or 63 US Gal.

I would say that any worry about headspace O2 vs CO2 is unfounded.

NB - if you did this in a pressure vessel of 30l... with 5 litres of uncompressable fuild(your fermenting must/wort), and never allowed any CO2 to escape ... the pressure would build and you would be (assuming the yeast ran to completion and diastatic pressure did not kill the yeast) end up with 240l in 25l headspace, or a very dangerous pressure vessel, unless you had a pressure regulator/saftey valve on there.



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