What a blow-off tube does and when you need one

A blow-off tube is a wide airlock that foam cannot block

During the first two or three days of fermentation, yeast throws up a thick head of foam called krausen. In a fermenter with little headspace that foam climbs into the neck. A standard three-piece or S-shaped airlock has a passage only a few millimetres wide. Krausen full of yeast and hop matter plugs it in minutes.

Once the airlock is blocked, carbon dioxide keeps coming with nowhere to go. Pressure builds until something gives: the airlock shoots out of the grommet, the bucket lid bows and pops or foam sprays across the ceiling. A blow-off tube replaces the airlock with a length of food grade tubing about 2.5 cm (1 inch) across, running from the fermenter down into a jar of sanitiser. Foam and gas travel down the tube together. The liquid in the jar keeps air and fruit flies out, which is all an airlock was doing anyway.

Headspace decides whether you need one

The rule we teach is simple: leave about 20 to 25% of the fermenter empty above the wort. A 20 litre batch in a 25 litre bucket has roughly 5 litres of headspace, which is usually enough for a clean ale yeast at a steady temperature. Fill a 23 litre carboy with 21 litres and you have almost nothing, so plan on a blow-off tube from day one.

Glass carboys are the usual culprit because their narrow neck funnels the krausen straight up. Buckets with wide lids tolerate more, but a bucket filled to the brim behaves just like a carboy.

The brews that climb out of the fermenter

Some fermentations are reliably more violent than others. Fit the tube without thinking twice for:

• Wheat beers and witbiers. Wheat proteins make a dense, sticky krausen.
• Belgian and saison strains. Many are fermented warm and get going fast.
• Kveik. At 30 °C and above it can finish in two or three days, most of it in the first day.
• High gravity beers. More sugar means more gas and more foam for longer.
• Big starters or repitched slurry. A large, healthy pitch starts hard.

Temperature makes all of these worse. In an Indian summer an unchilled fermenter can sit well above the yeast's ideal range. That warm start is the classic recipe for a lid on the floor at three in the morning.

How to rig one on a bucket or a carboy

On a carboy, 2.5 cm outside diameter tubing pushes snugly into the neck and seals there with no bung needed. On a bucket, the simplest route is a drilled bung or the airlock grommet with a short length of narrower tubing, though narrow tube clogs more easily. Some brewers take the centre post out of a three-piece airlock and fit tubing over the stem, which works for moderate fermentations.

Run the other end into a jar or bottle half full of no-rinse sanitiser, with the tube end a couple of centimetres below the surface. Use a jar big enough to hold a litre or more of foam. Put it in a tray, because it will overflow on a lively batch. Sanitise the tube inside and out before fitting it, since wort will touch it.

Swap back to an airlock once the krausen falls

The tube earns its place for the first two to four days. Once the krausen drops and bubbling slows, you can swap to a normal airlock, which is easier to live with in a small flat. Leave it in place if you are not sure. There is no harm in a blow-off tube staying put for the whole fermentation.

The one moment it causes trouble is cold crashing. As the beer cools, the gas in the headspace contracts and pulls liquid back up the tube into the fermenter. A wide tube with a deep column of sanitiser can draw a surprising amount. Pull the tube end out of the jar or switch to an airlock before you drop the temperature.

A blow-off is a symptom worth reading

A tube full of foam tells you the fermentation was vigorous. Whether that was a good thing depends on the temperature. A hard, warm start drives fusel alcohols and fruity esters well past what most styles want. Read the tube alongside your thermometer, as we explain in our guide to how long beer takes to ferment.

That link between temperature, pitch rate and yeast behaviour is the part most new brewers learn by trial and error. Learning it on purpose saves a lot of dumped batches. If you are still choosing kit, our look at all-in-one brewing systems covers the brew day side.

Your first step: before your next brew day, buy a metre of 2.5 cm silicone or vinyl tubing and a wide jar. Store them with your sanitiser so they are ready on the day the foam arrives.