Airlocks and Bungs: Types and Proper Use

What Are Airlocks and Bungs?

Airlocks and bungs are simple but essential tools that protect fermenting wine from contamination while allowing carbon dioxide gas to escape. During fermentation, yeast converts sugar into alcohol and CO2. Without a way for this gas to exit, pressure would build inside the vessel and potentially cause it to crack or explode. At the same time, the wine must be shielded from outside air, bacteria, wild yeast, and fruit flies that could spoil the batch.

An airlock is a small device, usually made of clear plastic, that creates a one-way valve. Gas can exit the fermentation vessel, but outside air cannot enter. A bung (also called a stopper) seals the opening of the fermentation vessel and holds the airlock in place. Together, they form a closed system that is fundamental to successful winemaking.

These are among the least expensive pieces of winemaking equipment, yet they are arguably among the most important. A failed seal or a dry airlock can lead to oxidation, vinegar formation, or contamination that ruins an entire batch.

The Role of CO2 in Fermentation

During active fermentation, the constant stream of carbon dioxide exiting through the airlock actually provides additional protection. The CO2 is heavier than air and forms a protective blanket over the wine's surface, displacing oxygen. This is why active fermentation is relatively forgiving of imperfect seals. However, once fermentation slows and CO2 production decreases, a proper airlock seal becomes critical.

Types of Airlocks

Three-Piece Airlock

The three-piece airlock is the most popular type among home winemakers. It consists of a cylindrical chamber, a small floating cup, and a dust cap. The chamber is partially filled with water or sanitizer solution. As CO2 exits the vessel, it pushes through the liquid and bubbles out through the top. The floating cup prevents outside air from entering when gas pressure is not pushing outward.

Three-piece airlocks are easy to fill, clean, and monitor. The bubbling action provides a visible and audible indication that fermentation is active. They are also easy to disassemble for thorough sanitizing between batches.

Pros: Easy to clean, easy to monitor, widely available, inexpensive. Cons: Can be sucked back into the wine if temperature drops; liquid can evaporate. Price: $1 to $3 each.

S-Shaped (Twin Bubble) Airlock

The S-shaped airlock (also called a twin bubble or double bubble airlock) is a single molded piece of plastic with an S-curved channel that holds liquid in two chambers. CO2 travels through the liquid in both chambers before exiting. The design provides a more reliable seal than a three-piece airlock because even if some liquid is lost from one chamber, the other still maintains the barrier.

S-shaped airlocks are slightly more challenging to clean due to their narrow internal passages, but they offer excellent protection during long aging periods. They are less likely to be sucked back into the wine during temperature fluctuations because the S-curve design naturally resists reverse flow.

Pros: Reliable seal, resistant to suck-back, good for long aging. Cons: Harder to clean thoroughly, less visible bubble action. Price: $1 to $3 each.

Blow-Off Tube

A blow-off tube is not technically an airlock, but it serves the same purpose during vigorous primary fermentation when a standard airlock might clog or blow off. It consists of a length of food-grade tubing (usually 1 inch in diameter) attached to the bung or fermenter lid, with the other end submerged in a container of sanitizer solution.

During aggressive fermentation, foam, fruit particles, and yeast can push up through the tube and into the sanitizer container rather than clogging a standard airlock. Once fermentation calms down (usually after 3 to 5 days), the blow-off tube is replaced with a standard airlock for the remainder of fermentation and aging.

Pros: Handles vigorous fermentation, prevents clogging and blow-offs. Cons: Requires additional setup, takes up more space, not suitable for long-term use. Price: $3 to $8 for a complete blow-off assembly.

Silicone Vented Bungs

A newer option is the silicone vented bung, which combines the bung and airlock into a single piece. These have a slit or valve molded into the silicone that allows CO2 to escape while preventing air from entering. They are extremely easy to use and maintain, requiring no liquid and no separate airlock. However, they do not provide the visual feedback of bubble activity that traditional airlocks offer.

Pros: No liquid to maintain, compact, easy to use. Cons: No visual fermentation indicator, may not seal as reliably as liquid airlocks for long-term aging. Price: $3 to $7 each.

Types of Bungs (Stoppers)

Solid Rubber Bungs

Solid bungs have no hole and are used to completely seal a vessel when no airlock is needed, such as during short-term storage or transport. They are available in numbered sizes that correspond to different vessel openings. Always have a few solid bungs on hand for situations where you need a complete seal.

Drilled Rubber Bungs

Drilled bungs have a hole through the center that accommodates an airlock stem. They are the most common type used during fermentation and aging. The hole size is standardized to fit most airlocks and blow-off tube fittings. Drilled bungs come in the same numbered sizing system as solid bungs.

Silicone Bungs

Silicone bungs are more flexible and durable than traditional rubber. They resist cracking and deterioration over time and create an excellent seal even on slightly irregular vessel openings. Silicone bungs are also easier to insert and remove and are less likely to become stuck in the neck of a carboy.

Universal Bungs

Universal or tapered bungs are designed to fit a range of opening sizes. They are slightly conical in shape, so they can accommodate variations in vessel neck diameter. These are convenient for winemakers who use vessels from different manufacturers with slightly different opening sizes.

How to Select the Right Size

Measuring Your Vessel Opening

To select the correct bung size, measure the inside diameter of your vessel's opening. Standard carboy openings range from about 28mm to 38mm. Fermentation bucket lids typically use a grommet with a standard hole size. Demijohns and specialty vessels may have non-standard openings that require specific bung sizes.

Bungs are numbered (typically #2 through #10 for home winemaking), with each number corresponding to a range of diameters. If you are unsure of the right size, bring your vessel (or measure the opening precisely) to a homebrew supply shop for a proper fitting.

Ensuring a Proper Seal

A good bung should fit snugly but not so tightly that it cannot be removed. It should not wobble or feel loose. When properly seated, the bung should require a firm twist or pull to remove. Test the seal by pressing gently on the airlock. If you see bubbles coming around the bung rather than through the airlock, the seal is inadequate and you need a different size or a new bung.

How to Use Airlocks and Bungs Properly

Filling and Installing an Airlock

Fill the airlock to the indicated line (or about halfway) with clean water, sanitizer solution, or cheap vodka. Vodka has the advantage of not supporting microbial growth and not leaving residue. Insert the airlock stem into the drilled bung, then press the bung firmly into the vessel opening. Check that the airlock is vertical and the liquid levels are even on both sides.

Maintaining the Liquid Level

Over weeks and months of aging, the liquid in your airlock will evaporate, especially in dry or warm environments. Check your airlocks weekly and top off as needed. A dry airlock provides no protection against oxygen and contamination. Set a regular schedule to inspect all vessels, and consider using S-shaped airlocks for long-term aging as they hold more liquid and lose it more slowly.

Troubleshooting Suck-Back

If the temperature in your fermentation area drops, the gas inside the vessel contracts and can suck liquid from the airlock back into the wine. If you use sanitizer solution, a small amount entering the wine is generally harmless. To prevent this, use water or vodka in your airlock, maintain a stable temperature, and consider S-shaped airlocks that resist suck-back more effectively.

When to Replace

Rubber bungs eventually dry out, crack, and lose their elasticity. Replace them when you notice cracking, a chalky texture, or difficulty achieving a good seal. Silicone bungs last significantly longer and resist degradation. Airlocks are inexpensive and should be replaced if they crack, become cloudy and impossible to see through, or develop residue that won't clean off.

Cleaning and Sanitizing

Cleaning Airlocks

Disassemble three-piece airlocks after each use and soak in warm water with a cleaning agent such as PBW. Use a small brush or pipe cleaner to clean inside the chambers and passages. For S-shaped airlocks, soak thoroughly and flush with clean water. Inspect for residue and repeat if necessary.

Sanitizing Before Use

Before each use, soak all airlocks and bungs in a no-rinse sanitizer for the recommended contact time. Shake off excess sanitizer but do not rinse. Fill the airlock with sanitizer solution or the liquid of your choice and install immediately.

Storing Between Batches

Store clean, dry airlocks and bungs in a sealed plastic bag or clean container to protect them from dust and contamination. Keep them away from heat and direct sunlight, which can degrade plastic and rubber over time.

Frequently Asked Questions

What liquid should I put in my airlock?

The most common options are clean water, sanitizer solution (like Star San), or cheap vodka. Vodka is preferred by many winemakers because it does not support microbial growth and won't harm the wine if sucked back into the vessel.

How do I know if my airlock is working?

During active fermentation, you should see regular bubbling (every few seconds to every few minutes depending on the stage). During aging, bubbles may be very infrequent or absent, which is normal. The key indicator is that the liquid levels in the airlock remain stable and the seal is intact.

Why did my airlock stop bubbling?

Bubbling slows and stops as fermentation finishes. This is normal. However, if bubbling stops very early (within a day or two of starting), check for a stuck fermentation, a leaking seal, or a temperature problem. Use a hydrometer to verify whether sugar is still present.

Can I use one airlock for multiple batches?

Yes, as long as you thoroughly clean and sanitize the airlock between batches. Airlocks are inexpensive, so many winemakers keep several on hand and dedicate one to each vessel.

What do I do if my airlock gets clogged?

Remove the clogged airlock carefully and replace it with a blow-off tube until the vigorous phase of fermentation passes. Clean the airlock thoroughly before reinstalling. Clogging is most common during the first few days of active fermentation when foam and fruit particles are at their peak.

How often should I check my airlocks?

During active fermentation, check daily. During secondary fermentation and aging, check at least once per week to ensure the liquid level is adequate and the seal remains intact. More frequent checks are needed in warm, dry environments where evaporation is faster.

Do I need different airlocks for primary and secondary fermentation?

Not necessarily, but many winemakers use a blow-off tube during primary fermentation (when activity is vigorous) and switch to a standard three-piece or S-shaped airlock for secondary fermentation and aging (when activity is calm).

Can I skip using an airlock entirely?

During active primary fermentation, some winemakers simply place a loose lid or cloth over the fermenter, relying on the blanket of CO2 to protect the wine. This is acceptable for the first few days of vigorous fermentation. However, once activity slows, an airlock is essential to prevent oxidation and contamination.