I’ve seen many posts on the various homebrewing forums and subreddits showing fermenters with minimal headspace and inadequate airlock setups, often with liquid foaming up and out of the vessel. I, and many others, are constantly posting warnings and how to avoid such messes. There’s a much better way of doing it: blowoff tubes!

Blowoff tubes can be extremely useful alternatives to standard airlocks. Not only do they help prevent blowouts and messes during active fermentation, they can be made from things you already have around your kitchen, making a blowoff tube the best choice for the budget-conscious homebrewer.

A blowoff tube is an awesome alternative to a standard S-shaped or 3-piece airlock for many brewers.

For one thing, it’s really useful during the most active, vigorous part of fermentation – when your brew is bubbling up aggressively and threatening to push out of the top. A blowoff tube, in combination with a wide-mouthed vessel that has plenty of headspace, will help to prevent any chance of such a thing making a mess all over your walls and ceiling.

But there’s more!

On the various homebrewing forums and subreddits (particularly /r/prisonhooch) there are a bunch of suggestions for “hoochers” (meaning brewers who are going for a bare-minimum, no-cost brewing setup) to avoid having to purchase extra equipment like an airlock.

But in comparison to balloons, condoms, or just leaving the cap loose, a blowoff tube isn’t costing you any more money, since you already need siphon tubing, anyways. And it’s a whole lot easier and more reliable.

And you can always swap out the blowoff tube for an airlock when fermentation is near-completion for its convenience and compactness, and get the best of both worlds.

What Is a Blowoff Tube and How Do You Use One?

A blowoff tube is a cheap alternative to standard airlocks.

Not that airlocks are expensive or hard to get – but everything you need to make a blowoff tube is either found in your kitchen already or should be purchased anyways for other steps in the homebrewing process.

What Equipment Do You Need to Make a Blowoff Tube?

To make a blowoff tube, you don’t need any extra equipment.

The only item that you’ll need to purchase, period, is a bit of food-grade silicon or vinyl siphon tubing. This is the same tubing that you’d be using for siphoning the liquid (when racking or bottling) as well as for beer line if you have a kegging setup.

I find 7⁄16″ OD (outer diameter) to be the optimal size for a blowoff tube. ID (inner diameter) does not matter for this purpose – it just needs to fit snugly into the airlock hole.

In fact, you can purchase one short length of tubing and reuse it for every stage of the process!

Besides the tubing, you’ll need some kind of extra container to run the tubing into. This can be just about anything that will hold some liquid. I generally use mason jars – just because we have so many in our house – but a drinking cup or even a bowl will work just fine.

Nearly any actual fermenter will work with a blowoff tube – you’ll be using the same hole in the lid or bung that you would with a standard airlock.

Those of you from /r/prisonhooch, who are fermenting in milk jugs (or simply the container that the juice you plan to ferment came in), may need to rig up something special to make this work. I’ve seen images of hoochers duct taping the tubing to a hole punched into a plastic lid – and while this works, I’d suggest taking it just one step further by drilling a clean hole and using a grommet to create a clean seal.

Look out for a future article or video showing exactly how to do that!

How to Make a Blowoff Tube

Making a blowoff tube is simple:

1. Fill the secondary container (jar, glass, etc.) with liquid. The liquid shouldn’t be able to make it back up the tube and into your fermenter, so anything will do, including just water. However, I usually use a sanitizing solution like Star-San just to be sure.
2. Attach one end of the tubing into the fermenter. The tubing should fit into the hole in the lid or bung where a standard airlock should go. Push it just through the hole so that it stays attached snugly but does not drop down into the wort or must.
3. Insert the other end of the tubing into the secondary container. The end of the tube should remain under the surface of the liquid at all times to create an airlocked seal. Note: leave the secondary container open and uncovered!

This setup will create a perfect airlocked seal – in fact it works just like a standard S-shaped or 3-piece airlock, it’s just much bigger.

During fermentation, gas will build up in the headspace of the fermenter, creating external pressure. That pressure will force the gas out of the only exit available – the siphon tubing. Pressure forces the gas out into the liquid on the other end, where it bubbles up and escapes into the atmosphere.

Outside air cannot push through the liquid, so it can’t ever make its way into the fermenter.

The following infographic shows exactly how to perform these steps.

Why Is a Blowoff Tube a Better Alternative?

Blowoff tubes aren’t always better alternatives, and there are times when a standard airlock is more functional (which we’ll discuss further down).

But using a blowoff tube instead has two huge benefits under specific circumstances in homebrewing.

1. Blowoff Tubes Can Help Prevent Blowouts During Vigorous Fermentation

I’ve said it many times: fermentation can get messy.

I don’t know how many times I’ve seen brewers posting images of their foamy, mid-fermentation beer, wine, or mead gushing out of a standard airlock on the various homebrewing forums and subreddits.

It’s happened to me!

One of my first batches, way back in 2009, was a particularly vigorous beer. Brew day went on until late in the day, and once the yeast was pitched, I popped the bucket into our apartment’s coat closet to do its thing.

Early the next morning, my roommate (who had class earlier than me) woke me up to show me what had happened: the beer was actively spraying out of the airlock and all over the walls (and various items we had stored in there)!

After having to skip class and clean up that ridiculous mess, I learned something. Since then, I never use a standard airlock at the beginning of fermentation. While many batches might be fine, you can’t know which ones are going to go nuts. It’s better to be safe than sorry and just use a blowoff tube – at least for the first few days.

Blowoff tubes help to prevent blowouts like that from occurring.

There’s a whole lot more space inside of the long tube than there is in a standard airlock. That’s a lot of space (and distance!) for the foam and liquid to get pushed before it comes out of the other side and starts gushing everywhere.

Plus, once it’s come out the other side, it still has to fill the secondary container. With the 32oz mason jars that I use, that gives me plenty of time to catch it before it becomes a problem.

(With my recent obsession with Verdant IPA yeast, that’s really helpful! I had one batch of beer that kept filling the mason jar with foam, and I had to clean it out and replace the Star San roughly once per day. But that gave me a full day to check in on it before it became an issue!)

They really make managing the mess a non-issue.

2. Blowoff Tubes Are Great for Brewers on a Budget

Airlocks aren’t expensive – and you really oughtta have a bunch of them. After over a decade of brewing, I have a drawerful.

But there are plenty of brewers (you can find a concentrated number of them over on /r/prisonhooch) who will go to great lengths to save a dollar. And one of these methods is to use balloons (or even condoms) over the opening of the fermenter to act as an airlock.

I tried that, once, out of curiosity – and it sucked. Too small of a hole, and the balloon pops; too large, and what’s the point? Plus, what do you do when fermentation is complete?

If you want to truly go the extreme budget route, why wouldn’t you just try open fermentation? Seems like there is an equal contamination/oxidation concern with far less effort.

But that’s not the point of this article, just a bit of story to add context. The real answer, if you do not want to buy a “specialty” item like a standard airlock, is to create a blowoff tube.

You will already need the siphon tubing to rack and bottle your batch. I’ve already written about the bare minimum homebrewing setup for a budget-minded beginner, and one of the items you need is something to siphon with.

So if you’re already buying that, why not just use it for a makeshift airlock?

When to Use an Airlock Instead (Swapping it Out)?

Blowoff tubes are great – for what they’re useful for.

If you’re on a tight budget, then you can go ahead and use your siphon tube to make a blowoff tube for every batch and avoid having to spend any extra money on an airlock.

And for the most vigorous part of every fermentation, your blowoff tube will help keep the mess inside the fermenter and prevent blowouts.

But what if you do have an airlock? Is there value in using it?

(Of course there is!)

To be honest, while I always start with a blowoff tube on every batch, I always swap it out for a standard (usually S-shaped) airlock as fermentation nears completion and the krausen dies down.

Once the krausen starts dying down, you can rest assured that it won’t foam up and out of your airlock anymore. Krausen builds up as fermentation becomes more active, but I’ve never seen it die down and then build back up. There just won’t be enough sugar left to continue fueling fermentation.

At this stage, a standard airlock becomes more useful. It’s just more compact and less unwieldy. It may not be an issue if you leave your fermenter stationary until it’s emptied, but I tend to move mine around – carrying it to a counter or table for racking, bottling, etc.

And it’s a lot easier to carry just the fermenter rather than the fermenter and the secondary container (all while making sure that the siphon tube stays in the container and under the surface of the liquid!).

You might ask: is it safe to swap the blowoff tube for an airlock? Won’t oxygen and other contaminants get in when I do so?

The key is: swap the blowoff tube out for an airlock while fermentation is still ongoing! If you wait until the krausen starts to die down, but perform the swap while there’s still a bit of bubbling (and off-gassing) then the CO₂ being produced will create enough external pressure to protect the batch (and keep the headspace filled and pressurized) for the short moment that it is exposed during the swap.

After all, while fermentation is ongoing, there is no danger of oxidation.

Once I’ve swapped for an airlock, I usually stick with it until the end of the process: I’ll leave the airlock installed until racking to secondary or bottling (and I’ll use a standard airlock in secondary, as well).

They are convenient. After all, it is the de facto standard amongst homebrewers.

Conclusion

Standard airlocks are, of course, useful – and you should probably own at least a few of them!

But using a blowoff tube instead can be a great idea for new brewers or those on a budget who don’t want to purchase any more equipment than they absolutely need to.

Plus, you should probably be using a blowoff tube during the most active stages of fermentation, anyways, in order to avoid having to clean up a horrific mess from a blowout.