When To Remove Blow Off Tube & Use Airlock

I have a batch of steam beer that I brewed yesterday and all appears to be going well. Used the 6.5 gal glass carboy (1st time) to primary this batch. Nice head of foam forming the top of which has risen to about 3" or so from the bottom of the carboy neck. I can see that CO2 from the 6.5 gal glass carboy is bubbling into the 1 gal milk jug with sanitizer at the other end.

My question is this - it has been about 17 hours since the brew concluded so am I safe to assume that the time for a major blow off has passed? Can I now switch to the airlock at this point or should I continue to use the blow off tube and if so, for how much longer? Thanks, Montanaandy

Montanaandy said:

I have a batch of steam beer that I brewed yesterday and all appears to be going well. Used the 6.5 gal glass carboy (1st time) to primary this batch. Nice head of foam forming the top of which has risen to about 3" or so from the bottom of the carboy neck. I can see that CO2 from the 6.5 gal glass carboy is bubbling into the 1 gal milk jug with sanitizer at the other end.

My question is this - it has been about 17 hours since the brew concluded so am I safe to assume that the time for a major blow off has passed? Can I now switch to the airlock at this point or should I continue to use the blow off tube and if so, for how much longer? Thanks, Montanaandy

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If the krausen has dropped, and it's quiet, you can switch to an airlock if you want. You can continue to keep the blow off on if you want, though. Really, a blow off tube is just a giant airlock.

I never use a blow off tube, but it's because I ferment in 7.5 gallon buckets. If you have enough headspace you don't need one, but if you're concerned that you may have some blow-off, it's better to be safe than sorry! Usually, the most vigorous part of fermentation ends in 3-5 days.

yes you are way safe by now. a blow off tube is really only needed for over active fermentation. typically if the fermentation blows off the airlock or pushes itself up into and out of the airlock then i switch to a blow off till it subsides. then switch back to the airlock when it calms down. but if like you say the fermentation has passed it should be safe to witch to an airlock.

I think you're fine to use an airlock. I almost never use blow off tubes, only on beers with really high OGs, some strains of yeast, or stouts. I use 6.5 gallon carboys. Normally you'll know its gonna blow within the first 24 hours. If the krousen is about 2'' thick and holding I'd say swap it out.

Also with the couple of batches that did blow, maybe about 5, I had no contamination and they tasted great. With all that yeast activity it's a bit hard for anything to take hold in your beer.

The way you say "nice head of foam forming on top", and the fact that it has only been 17 hours, makes me think that the most active part of the fermentation is still to come. I remember watching one inch its way up the glass, until it started coming out after about 36 hours. If the foam is still rising, leave the blow off on. Unless you have some pressing need to take it off, I'd leave it on until you are SURE the chance of a blow off has passed.

Is there a reason to switch back to the airlock? Space?

I had a porter that I started last Sunday. I kept a close eye on it for 48 hours, and it seemed to have leveled off in its fermentation, (about 2 inches krausen). I figured I was safe.

It blew off a day and a half later, gunk everywhere....

Now it's blow off tubes until the krausen falls.

The only reason I switch to airlocks is to be able to move the carboys easier.

Definitely keep the blowoff on until you see the krausen fall. I had an irish red blow off on me the other day after 48 hours of active fermentation.

It was my first blow off and it managed to make quite a mess in the few minutes it was blowing off before I was able to get the tube attached. I can't imagine how bad it would have been if it had been going all night before I noticed it.

Had trouble posting to this thread earlier but here it goes...

I returned this afternoon and the krausen has increased quite a bit and I can see residue on the blow off tube near the neck of the carboy. 1 gal jug partially filled with sanitizer is bubbling along pretty good. I think that the blow off tube will remain in for the time being given all of the activity.

Re: switching to the airlock from the blow off tube - I just assumed that the airlock with the stopper placed into the top of the carboy afforded a bit of a tighter seal than sticking one end of the 1" tubing down the neck of the carboy and the other into a 1 gal milk jug. I will say though that the seal at both ends seems pretty tight given the activity going on.

This reminds me of another question I had related to the blow off tube - how far down into the carboy to insert it. Right now I have it where there are 2"" extending into the carboy below the base of the neck of the bottle. Is this sufficient? Thanks, Montanaandy

Montanaandy said:

Had trouble posting to this thread earlier but here it goes...

I returned this afternoon and the krausen has increased quite a bit and I can see residue on the blow off tube near the neck of the carboy. 1 gal jug partially filled with sanitizer is bubbling along pretty good. I think that the blow off tube will remain in for the time being given all of the activity.

Re: switching to the airlock from the blow off tube - I just assumed that the airlock with the stopper placed into the top of the carboy afforded a bit of a tighter seal than sticking one end of the 1" tubing down the neck of the carboy and the other into a 1 gal milk jug. I will say though that the seal at both ends seems pretty tight given the activity going on.

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A blow off tube into liquid is essentially identical to an airlock. The only potential problem with a blowoff tube into a pot of liquid is that, if the carboy cools significantly post-fermentation, it could suck liquid back into the vessel. This could happen with a one piece air-lock, but not a 3 piece.

This reminds me of another question I had related to the blow off tube - how far down into the carboy to insert it. Right now I have it where there are 2"" extending into the carboy below the base of the neck of the bottle. Is this sufficient? Thanks, Montanaandy

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Doesn't matter. Just stick it in till it holds. I put a piece of tubing over the middle stalk of my 3-piece, so it is barely down into the vessel at all. [If you do this, make sure to cut off the cross piece on the bottom of the air lock.] All you are doing is giving the thick krausen an escape route that doesn't involve explosively removing a clogged airlock.

BioBeing said:

Doesn't matter. Just stick it in till it holds. I put a piece of tubing over the middle stalk of my 3-piece, so it is barely down into the vessel at all. [If you do this, make sure to cut off the cross piece on the bottom of the air lock.] All you are doing is giving the thick krausen an escape route that doesn't involve explosively removing a clogged airlock.

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I was trying to do this, (stick tubing onto my 3-piece airlock), and discovered that the tube inside my 3-pieces is probably around 1/2" OD, and my tubing was 5/16" siphon tubing.

Fun fact, 5/16" siphon tubing fits perfectly inside a "universal bung"....no need for the airlock at all, and great reuse for old siphon tubing.

shortyjacobs said:

I was trying to do this, (stick tubing onto my 3-piece airlock), and discovered that the tube inside my 3-pieces is probably around 1/2" OD, and my tubing was 5/16" siphon tubing.

Fun fact, 5/16" siphon tubing fits perfectly inside a "universal bung"....no need for the airlock at all, and great reuse for old siphon tubing.

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This might be true. I think I had some 1/2 " tubing lying around that does fit.

When the airlock or blowoff is removed to rack or check gravity, etc., how is this done without letting gunky overflow that is sitting in the carboy bung grove from falling into the beer?

BioBeing - thanks for the explanation & quick response. Montanaandy.

Even though this 6.5 gal carboy probably doesn't need a blow-off tube, I use this one (easily taken apart and cleaned):


Click for larger image.

For 10 gal. batches I use the other fermenter, obviously , which has no airlock/blow-off at all, just a non-airtight screw-on lid.