Airlock and Bung Guide UK: Best Choices for Homebrew Fermentation in 2026

Every homebrewer knows the feeling: you pitch the yeast, seal the vessel, and then wait for that first satisfying bloop from the airlock. It is the sound of fermentation working — and it is also the moment your precious beer, cider or wine is protected from the outside world.

The airlock and bung is arguably the most underrated piece of kit in the brewery. It is cheap, it is small, and yet it does a job that nothing else can: it lets carbon dioxide escape during fermentation while keeping oxygen, wild yeast and bacteria out. Get this part wrong and you risk oxidised beer, infected batches, or — as UK forum threads consistently document — a blown bung sailing across the room.

This airlock and bung guide UK covers everything homebrewers need to know in 2026: the main types of airlock, how to choose the right bung size using the UK numbering system, the best products to buy, how to fit and clean them, and where to pick them up. We have researched manufacturer specifications, analysed owner feedback across The Homebrew Forum and retailer reviews, and compared the options so you can buy with confidence.

Quick Picks

How to Choose an Airlock and Bung

Understanding the UK Bung Numbering System

The single most-Googled confusion point for UK homebrewers is bung sizing. Walk into any homebrew shop or browse an online retailer and you will see bungs labelled No. 3, No. 4, No. 5, No. 6, No. 7 and No. 8, with no immediately obvious pattern. The numbering is not metric or imperial — it is a standardised UK sizing convention that maps loosely to neck diameter, and getting it right is the difference between a snug, airtight seal and a stopper that either falls in or pops straight back out.

The key distinctions are straightforward once you know them:

• No. 4 drilled bung — the standard fit for 1-gallon glass demijohns. This covers most Young’s, Boots-era and generic UK demijohns. If you are fermenting in a traditional glass demijohn, this is almost certainly the size you need.
• No. 6 and No. 7 drilled bungs — these fit 25-litre plastic fermenting buckets, which typically have a neck or hole diameter of approximately 38–40 mm. Most standard UK bucket lids take one of these two sizes.
• No. 8 — designed for wider-neck vessels, less commonly needed but worth knowing about if you use non-standard buckets or jars.
• No. 3 and No. 5 — intermediate sizes that cover less common vessels; always measure your neck diameter before ordering.

Both Vigo Ltd and Harris publish neck-diameter-to-bung-number charts on their product pages, and these are the most reliable sizing references available to UK brewers. If you are unsure, measure the inner diameter of your vessel’s neck or lid hole with a ruler or calipers and cross-reference against the manufacturer’s chart rather than guessing.

Material Choice: Silicone vs Rubber

Bungs come in two main materials, and the difference matters more than most brewers realise. Silicone bungs — such as those in the Harris silicone range — tolerate repeated boiling sanitisation without warping or hardening. This matters because heat sterilisation is one of the most effective ways to ensure your bung is genuinely clean, and a material that survives the process will give you years of service.

Rubber bungs, by contrast, degrade faster when exposed to strong oxidising cleaners. Repeated use of bleach-based solutions causes rubber to harden, crack and lose its taper — meaning the seal weakens over time. The Homebrew Forum owner consensus is clear on this point: if you plan to use aggressive cleaners, silicone is the more durable investment.

Airlock Design: Three-Piece vs S-Type

There are two principal airlock designs you will encounter in UK homebrew shops, and each has distinct strengths.

The three-piece airlock consists of a central tube, an outer dome, and a cap that sits loosely on top. You fill the chamber partway with liquid, and CO₂ bubbles up through the central tube, out through the liquid, and escapes under the cap. Its principal advantage is ease of cleaning: each piece separates completely, so there are no awkward internal channels where dried krausen or yeast residue can hide. This matters because trapped residue can harbour bacteria that contaminate your next batch.

The S-type airlock is the classic design most people picture when they think of homebrewing. Shaped like an elongated letter S, it is filled with liquid so that the U-bend creates a seal. CO₂ pushes through one side and bubbles out the other. S-types are more compact than three-piece designs, making them useful when headroom is tight — under a shelf, for instance, or in a fermentation fridge. Their weakness is cleaning: the narrow internal bore is difficult to brush out thoroughly, and dried residue inside the curve can be stubborn to remove.

What Actually Fails: Blown Bungs, Not Cracked Glass

A common misconception among new brewers is that trapped CO₂ pressure will crack a glass demijohn. In practice, the real-world failure mode is different and far less dramatic — but still messy. The Homebrew Forum threads consistently cite “blown bung” as the actual problem. CO₂ pressure builds until it pops the stopper out of the neck, sometimes with surprising force. The demijohn itself remains intact.

In plastic fermenting buckets, the failure modes are similarly stopper-related: the lid bulges, the bung blows out, or an imperfect lid seal weeps liquid rather than the vessel rupturing. The practical takeaway is that your bung fit and lid seal matter more than the structural integrity of your vessel. A well-fitted No. 4 or No. 6/7 bung with a properly seated airlock is your best defence against pressure-related failures.

Product Reviews

Young’s Bubbler Airlock

Best for: All-round demijohn fermentation

The Young’s Bubbler Airlock is the UK’s default airlock for 1-gallon glass demijohn work. It features an integral bung moulded directly onto the airlock body, sized to fit a No. 4 neck — the standard demijohn opening found on most Young’s, Boots-era and generic UK vessels. Because the bung is integral rather than a separate component, there is no risk of misalignment between stopper and airlock, and no separate bung to lose.

The bubbler design uses an internal chamber that creates a one-way valve when filled with liquid. CO₂ escapes through the liquid while air is prevented from flowing back in. The design is simple, reliable and has been a staple of UK homebrewing for decades.

Key specifications:

• Integral bung fitted for No. 4 demijohn necks
• Single-piece airlock body with removable cap for filling
• Bubbler valve mechanism requiring approximately 15–20 ml of liquid

Standout strength: The integral bung eliminates the most common fitting error — mismatched airlock and bung sizes — making it the most foolproof option for demijohn brewers.

Caveat: Because the bung is moulded on, you cannot swap it for a different size if you change vessels. Brewers using both demijohns and buckets will need a separate airlock solution for each.

Verdict: The simplest, most reliable demijohn airlock on the UK market — ideal for beginners and anyone who wants a fit-and-forget solution.

Where to buy: Widely stocked at Wilko (limited stock in 2026), Brew2Bottle and Amazon UK, typically priced from a few pence to around £2 depending on retailer.

Harris 3-Piece Airlock

Best for: Brewers who prioritise thorough sanitisation

The Harris 3-Piece Airlock is the go-to choice for brewers who want no compromises on cleaning access. Unlike single-piece or S-type designs, this airlock fully disassembles into its three component parts — central tube, outer dome and cap — allowing you to sanitise every surface individually. This matters because fermentation residue (krausen, yeast sediment, dried foam) can accumulate inside any airlock, and trapped organic material in an inaccessible internal channel is a leading cause of batch contamination.

The three-piece design also makes it easier to monitor fermentation activity: individual bubbles are visible travelling up the central tube, and the liquid level in the outer dome is easy to read at a glance.

Key specifications:

• Three fully separable components: central tube, outer dome, floating cap
• Standard fit for a drilled No. 4 or No. 6/7 bung (sold separately)
• Transparent construction for visual fermentation monitoring

Standout strength: Complete disassembly means every surface is accessible for cleaning — no blind channels, no trapped residue, no guesswork about whether the interior is sanitary.

Caveat: The three pieces must be reassembled correctly before each use, and the loose cap can be misplaced during cleaning. It is marginally more fiddly than a single-piece design.

Verdict: The best choice for meticulous brewers who treat sanitisation as non-negotiable — the extra assembly step is a small price for full cleaning access.

Where to buy: Stocked at The Malt Miller, BrewUK and Crossmyloof Homebrew, typically priced around £1–£2.

Harris S-Airlock

Best for: Budget-conscious brewers and tight-headroom setups

The Harris S-Airlock is the cheapest airlock option on the UK market, typically priced under £1. It uses the classic S-shaped design, where a U-bend filled with liquid creates the one-way seal. CO₂ pushes through one side of the S and bubbles out the other, while the liquid barrier prevents air from flowing back into the vessel.

The compact S-shape makes this design particularly useful where vertical headroom is limited — in a fermentation fridge, under a shelf, or in a cramped brewing cupboard. The overall height is lower than a three-piece airlock, which can make the difference between fitting a vessel into a tight space and not.

Key specifications:

• Single-piece S-shaped body with two liquid chambers
• Narrow internal bore throughout the S-curve
• Fits standard drilled bungs (No. 4, No. 6 or No. 7 depending on vessel)

Standout strength: The lowest price point of any airlock in this guide, combined with a compact profile that suits confined fermentation spaces.

Caveat: The narrow bore makes cleaning genuinely fiddly. Dried krausen or yeast sediment inside the S-curve is difficult to reach with a brush, and the internal channel cannot be fully inspected visually. Brewers who reuse S-airlocks across multiple batches should be aware of this limitation.

Verdict: A perfectly functional airlock at a rock-bottom price — ideal for budget setups or tight spaces, provided you are diligent about cleaning.

Where to buy: Available from UK homebrew retailers including The Malt Miller and BrewUK, typically priced under £1.

Hambleton Bard Bung & Airlock Set

Best for: New brewers who need a complete, matched package

The Hambleton Bard Bung & Airlock Set solves a problem that catches many first-time buyers: purchasing an airlock and then realising they do not have a compatible bung. This set bundles a drilled bung and an S-type airlock together, ensuring the two components are correctly matched and ready to use straight out of the packet.

Hambleton Bard is a long-established UK homebrew brand, and their guidance materials are referenced across the homebrewing community — including their recommendation of glycerine as an airlock liquid for long-term conditioning, which prevents evaporation during extended secondary fermentation.

Key specifications:

• Bundled drilled bung and S-type airlock as a matched pair
• S-airlock with classic U-bend liquid seal
• Available in bung sizes to suit both demijohn and bucket fermenters

Standout strength: The bundled format removes all guesswork from the purchasing decision — you get a guaranteed-compatible bung and airlock in one transaction, which is particularly valuable for brewers new to the hobby.

Caveat: Because the set uses an S-type airlock, it inherits the same cleaning difficulty as the standalone Harris S-Airlock — the narrow bore is difficult to brush out thoroughly.

Verdict: The most convenient purchase for beginners — a matched pair at a fair price, albeit with the cleaning limitations inherent to S-type designs.

Where to buy: Available from Amazon UK and homebrew specialists, typically priced around £1.50–£3.

Vigo 3-Piece Airlock

Best for: Wine and cider fermenters

The Vigo 3-Piece Airlock is a food-grade three-piece design that has built a strong following in UK wine and cider making circles. Vigo Ltd is a respected UK supplier to the cider and wine-making community, and their 3-piece airlock reflects that focus: it is built to the same disassembling principle as the Harris version, with the same cleaning benefits, but is frequently recommended in wine and cider contexts where longer fermentation periods make thorough sanitisation especially important.

Wine and cider fermentations often run for weeks or months rather than days, meaning the airlock is in place for extended periods. A three-piece design that can be fully dismantled and sanitised between batches is a significant advantage here, as residue has more time to bake onto internal surfaces during long fermentations.

Key specifications:

• Food-grade construction
• Three fully disassembling components: central tube, outer dome, cap
• Compatible with standard drilled bungs (sold separately)

Standout strength: The food-grade specification and strong reputation among wine and cider makers, who value both material safety and the ability to sanitise thoroughly between long-running fermentations.

Caveat: Like all three-piece designs, it requires correct reassembly before each use, and the bung is sold separately — you will need to select the correct size (No. 4 for demijohns, No. 6 or No. 7 for buckets) at the point of purchase.

Verdict: A trusted, food-grade option that earns its place in wine and cider setups where fermentation duration makes cleaning access paramount.

Where to buy: Available direct from Vigo and from The Malt Miller, typically priced around £1–£2.

Airlock Liquid: What to Put Inside

The liquid you fill your airlock with is not a trivial choice — it is the barrier that separates your fermenting brew from the ambient air. If that liquid is sucked back into the vessel (which can happen when temperature drops cause a pressure differential, or when the vessel is moved), it ends up in your beer, wine or cider. What you put in the airlock therefore matters.

Water

Plain water works as an airlock liquid — it creates the physical barrier needed to block air. However, if water is sucked back into the vessel, it can introduce whatever bacteria or wild yeast are present in your tap water. This is a low-probability risk, but it is a real one, and many brewers consider it an unnecessary gamble when better alternatives exist at minimal cost.

Star San Solution

Star San, diluted at approximately 1 ml per 5 litres of water, is the consensus upgrade from plain water among UK homebrewers. The Homebrew Forum and r/Homebrewing UK threads strongly favour this approach. The well-worn phrase “don’t fear the foam” captures the key advantage: Star San is a no-rinse acid sanitiser, so if it is sucked back into your fermenter, it will not harm your beer. There is no sanitising downside, and the protective upside is significant — the liquid itself is actively hostile to the bacteria and wild yeast that plain water might carry.

Glycerine

Glycerine is used in airlocks during long secondary fermentation or conditioning, where a vessel may sit undisturbed for weeks or months. Water-based airlock liquids evaporate over extended periods, potentially dropping below the level needed to maintain a seal. Glycerine’s low evaporation rate means the airlock stays filled and functional for far longer. This approach is mentioned in Hambleton Bard’s own guidance materials, reflecting its established use in conditioning contexts.

Vodka

Vodka is a popular airlock liquid choice, particularly among brewers who reason that high-proof alcohol must be self-sanitising. In practice, this is wasteful — vodka offers no sanitising benefit over Star San solution, and it is considerably more expensive per fill. The alcohol content does inhibit microbial growth in the liquid, but so does Star San, at a fraction of the cost.

What to Avoid

Avoid using undiluted bleach in your airlock liquid. Bleach-based cleaners degrade silicone seals over time, and the same applies to rubber bungs — repeated exposure causes hardening, cracking and loss of taper. The Homebrew Forum owner consensus is consistent on this point. If you use bleach elsewhere in your cleaning routine, keep it away from your airlock and bung.

Cleaning and Sanitising Airlocks and Bungs

Cleaning your airlock and bung between batches is not optional — it is the difference between consistent, clean fermentations and a recurring contamination problem that ruins beer after beer. The good news is that the process is straightforward, provided you use the right products and follow the correct contact times.

VWP (Brew Craft)

VWP is a widely available UK alkaline cleaner. To use it effectively on airlock components, soak the disassembled parts for 10–15 minutes, then rinse thoroughly. The alkaline formulation breaks down organic residue — dried krausen, yeast sediment, dried foam — that water alone will not shift. Thorough rinsing is essential because alkaline residue left on surfaces can affect head retention in beer and may interfere with yeast performance in subsequent batches.

VWP is stocked in most UK homebrew shops and is a staple of many brewers’ cleaning routines. Its availability and effectiveness on stubborn residue make it a practical choice for regular airlock maintenance.

Chempro SDP

Chempro SDP is a combined cleaner and sanitiser, meaning it both removes residue and sanitises in a single step. The manufacturer recommends a dilution of approximately 5 g per 5 litres of water with a 10-minute contact time. This dual-action formulation saves time compared to separate cleaning and sanitising stages, which is useful when you are processing multiple pieces of equipment on brew day.

Star San

Star San is a no-rinse acid sanitiser with a manufacturer-specified contact time of just 30 seconds, per the Five Star Chemicals spec sheet. This makes it the fastest-acting sanitiser in this guide. After cleaning your airlock components with an alkaline cleaner to remove visible residue, a brief Star San soak sanitises the surfaces without requiring a subsequent rinse — the residue is food-safe and will not affect your brew.

The 30-second contact time is a significant practical advantage on brew day, when you are juggling multiple tasks and equipment pieces. It means you can sanitise your airlock and bung immediately before assembly without building in extra waiting time.

Bleach-Based Cleaners: A Word of Caution

Bleach-based cleaners will clean your airlock and bung effectively, but they come with a long-term cost. Repeated use degrades rubber bungs and silicone components, causing them to harden, lose their taper and eventually fail to seal properly. This is consistent owner consensus across The Homebrew Forum. If you choose to use bleach, be aware that you are shortening the lifespan of your bungs and seals, and inspect them regularly for signs of hardening or cracking.

The practical alternative is straightforward: use VWP or Chempro SDP for cleaning and Star San for sanitising. This combination avoids the degradation risk entirely while delivering equivalent or better results.

UK Retailers and Availability

Airlocks and bungs are low-cost, high-turnover items, and availability can fluctuate. The following retailers are the principal UK stockists as of 2026:

• The Malt Miller — stocks the Harris 3-Piece Airlock, Harris S-Airlock and Vigo 3-Piece Airlock, alongside a range of drilled bungs in standard sizes.
• BrewUK — stocks the Harris 3-Piece Airlock and Harris S-Airlock, with bungs available separately.
• Crossmyloof Homebrew — stocks the Harris 3-Piece Airlock and associated accessories.
• Brew2Bottle — stocks the Young’s Bubbler Airlock and a range of bungs.
• Amazon UK — stocks the Young’s Bubbler Airlock and the Hambleton Bard Bung & Airlock Set, among other options.
• Wilko — limited stock in 2026; historically a convenient high-street source for the Young’s Bubbler Airlock, but availability has become inconsistent. Check online before making a special trip.
• Vigo direct — the Vigo 3-Piece Airlock is available direct from Vigo Ltd, who also publish neck-diameter-to-bung-number sizing charts on their product pages.

Because airlocks and bungs are inexpensive (typically under £3 for any single item in this guide), it is worth ordering spares. A backup airlock and a couple of spare bungs in different sizes will save you a mid-fermentation panic if a component cracks, goes missing or needs emergency replacement.

FAQ

What size bung do I need for my demijohn?

The vast majority of UK 1-gallon glass demijohns — including Young’s, Boots-era and generic brands — take a No. 4 drilled bung. This is the standard size for demijohn fermentation. If your demijohn is non-standard, measure the inner neck diameter and cross-reference against the sizing charts published by Vigo Ltd or Harris on their product pages.

What size bung fits a 25-litre fermenting bucket?

Most standard UK 25-litre plastic fermenting buckets, which typically have a neck or hole diameter of approximately 38–40 mm, take either a No. 6 or No. 7 drilled bung. The exact fit depends on the bucket manufacturer. If you are unsure, measure the hole diameter in your bucket lid and check it against a published bung-sizing chart before ordering.

Can a glass demijohn crack from fermentation pressure?

In practice, no — the bung will blow out first. The Homebrew Forum threads consistently report “blown bung” as the real-world failure mode, not vessel rupture. CO₂ pressure builds until it pops the stopper out of the neck. The demijohn itself remains intact. This is why a well-fitted bung is more important than worrying about the structural integrity of the glass.

What should I put in my airlock — water or something else?

Plain water works but can introduce contaminants if sucked back into the vessel. The UK forum consensus strongly favours Star San solution diluted at approximately 1 ml per 5 litres, because it is actively sanitising and will not harm your brew if sucked back. For long secondary fermentation or conditioning, glycerine prevents evaporation over extended periods. Vodka works but is wasteful and offers no sanitising advantage over Star San.

Are silicone bungs better than rubber bungs?

Silicone bungs, such as those in the Harris silicone range, tolerate repeated boiling sanitisation without degrading, making them more durable for brewers who heat-sterilise their equipment. Rubber bungs degrade faster when exposed to strong oxidising cleaners, particularly bleach-based products, which cause hardening and cracking over time. If you use aggressive cleaners, silicone is the longer-lasting investment.

How do I clean a three-piece airlock?

Disassemble the airlock into its three components — central tube, outer dome and cap — and soak them in an alkaline cleaner such as VWP for 10–15 minutes, then rinse thoroughly. Follow with a 30-second soak in Star San solution (approximately 1 ml per 5 litres) to sanitise without rinsing. The advantage of a three-piece design is that every surface is accessible, unlike S-type airlocks with narrow internal bores that are difficult to brush out.

Can I use bleach to clean my airlock and bung?

Bleach will clean effectively but degrades rubber bungs and silicone seals over repeated use, causing hardening, cracking and loss of seal. The Homebrew Forum owner consensus is consistent on this point. VWP or Chempro SDP are safer alternatives for cleaning, followed by Star San for sanitising. If you do use bleach, inspect your bungs regularly and replace them at the first sign of deterioration.