A spunding valve is an adjustable pressure relief valve used in beer brewing that lets you set a pressure ceiling on a sealed fermenter or keg. When the pressure inside climbs above the level you dial in, the valve vents the excess; below that level, it holds everything in. That single trick unlocks pressure fermentation and, better still, lets your beer carbonate itself using the CO₂ it produces during fermentation — no gas bottle required.
This guide walks through exactly how to install and set one up, when to attach it for naturally carbonated beer, how to choose your pressure, and the small mistakes that trip people up. If you are weighing up your packaging options first, it is worth reading how spunding compares to kegging versus bottling and how it sits alongside force carbonation.
⚠️ The one thing to get right
A spunding valve does not pump gas into your fermenter. The yeast makes the CO₂; the valve simply holds pressure and vents the surplus. If you want naturally carbonated beer, you must seal the fermenter and fit the valve while fermentation is still active — not after it finishes. Attach it too late and there is no CO₂ left to carbonate the beer.
What a spunding valve actually does
Fermentation produces a steady stream of carbon dioxide. In an open or blow-off setup, that gas escapes and is lost. Seal the vessel and the CO₂ has nowhere to go, so pressure builds — quickly reaching levels that could deform equipment or, in a worst case, fail dangerously. The spunding valve is the safety release that also happens to be useful: set it to, say, 15 psi and the fermenter will pressurise up to 15 psi and hold there, venting anything above it.
That gives you two payoffs. First, pressure fermentation suppresses some fermentation by-products, letting you ferment ales a little warmer and faster while keeping them clean. Second, and most popular, is natural carbonation: because the beer is under pressure at the end of fermentation, the CO₂ dissolves straight into it. By the time the beer is done, it is already carbonated to your target — a genuine alternative to priming sugar or bottle conditioning.
Materials and tools you will need
- A pressure-rated fermentation vessel — a corny keg or a purpose-built pressure fermenter. A standard bucket or glass carboy is not safe under pressure.
- A spunding valve with a pressure gauge (the gauge is worth every cent — flying blind is how people over-pressurise).
- A gas disconnect (ball-lock or pin-lock to suit your keg) with a barb or flare fitting.
- PTFE (Teflon) tape or thread sealant for any threaded joints.
- A wrench or adjustable spanner — snug only, never gorilla-tight.
- Star-san or your usual no-rinse sanitiser.
Step-by-step: fitting and setting the valve
1. Clean and sanitise everything
Sanitise the fermenter, the gas post, the disconnect and the valve. Anything that touches the beer or seals the vessel needs to be clean — contamination under pressure is still contamination.
2. Pre-set your target pressure (recommended)
Before it goes on the fermenter, thread the spunding valve onto a gas disconnect, briefly connect it to a CO₂ bottle, and adjust the knob until the gauge reads your target. Setting it against a known gas source is far more accurate than guessing. No CO₂ bottle? You can still set it live in step 4 — it just takes patience.
3. Attach the valve to the gas post — at the right moment
Push the gas disconnect onto the fermenter's gas-in post and seal the vessel. Timing matters: for naturally carbonated beer, do this when there are still roughly 2–5 gravity points of fermentation left (SG around 1.010–1.014 for a typical ale). Seal too early and you trap unwanted early-ferment character; seal too late and there is not enough CO₂ left to carbonate. Snug the fitting by hand or with a light turn of the spanner — do not over-tighten.
4. Let fermentation build the pressure
This is the key correction to how spunding is often described: you are not feeding gas in. The active yeast produces CO₂, the sealed vessel captures it, and the pressure climbs on its own until it reaches your set point. From then on, the valve vents anything above that level with a periodic hiss. If you skipped step 2, dial the knob down until it just holds pressure and creeps up to your target.
5. Monitor and hold
Check the gauge over the next day or two. It should sit at your set pressure and vent occasionally. Position the vessel somewhere the vented gas can disperse safely — CO₂ is heavier than air, so avoid enclosed, unventilated spaces.
6. Finish and package
Once fermentation is complete and the gravity is stable, your beer is carbonated to your set pressure. Cold-crash if you can, then transfer under pressure to a serving keg to keep that carbonation locked in. If you are moving to a keg, keep an eye out for a corny keg quietly losing pressure through a tired relief valve or O-ring.
Choosing your pressure
Carbonation is usually described in volumes of CO₂. The pressure needed to reach a given volume depends heavily on temperature — warmer beer needs much more pressure than cold beer to hold the same fizz. Because spunding happens at fermentation temperature (often 18–22°C), you will need noticeably higher psi than the 10–14 psi typical of cold serving. Use the table below as a target for carbonation level, then set the actual pressure from a proper temperature-and-volume chart in the general carbonation guide.
| Beer style | Target carbonation (volumes CO₂) |
|---|---|
| British ales, stouts, milds | 1.5–2.0 (soft, low carbonation) |
| Pale ales, IPAs, ambers | 2.2–2.6 (standard) |
| Lagers, pilsners, wheat beers | 2.5–3.0 (lively) |
| Saisons, Belgian styles | 3.0–3.8 (highly effervescent) |
💡 Tips and tricks
Spund early for less waste. Sealing with a few gravity points to go lets the beer carbonate itself for free. If you seal too late, top up carbonation later with a gas bottle.
Ramp gently. If your gauge shoots past target, back the knob off and vent slowly. Sudden pressure swings can cause the beer to foam and push liquid into the valve.
Chill before you dial in fizz. Cold beer absorbs CO₂ far more readily, so a cold crash before final conditioning gives you tighter control over the end result.
Keep the valve clean. A little krausen or beer carried up into the valve can dry and clog it. Rinse it after every batch so it seats properly next time.
Troubleshooting
The valve is not venting / pressure keeps climbing. Either it is clogged or set too high. Vent it manually, then check the seat and diaphragm for dried krausen and reset the pressure. A valve that will not hold at all usually has a fouled or damaged seal.
Pressure will not build. You likely have a leak — check the gas post, disconnect and every threaded joint with a little sanitiser or soapy water and watch for bubbles. Re-seat O-rings and add PTFE tape where needed. If fermentation has already finished, there is simply no more CO₂ being produced.
Fermentation seems stuck under pressure. High pressure can slow yeast down. If things stall, this is often less about the valve and more about yeast health — see the notes on osmotic stress and stalled fermentations. For a broader rundown of brew-day gremlins, the homebrewer's first aid kit is a handy reference.
🛡️ Safety first
Only pressurise gear rated for it. Kegs and pressure fermenters are built for this; buckets, glass carboys and most plastic fermenters are not and can fail violently.
Never exceed your equipment's rated pressure, and always follow the manufacturer's limits.
Treat the spunding valve as a true safety device — keep it clear and unobstructed so it can always vent.
Wear eye protection when working on a pressurised vessel, and vent CO₂ somewhere well ventilated.
Watch it in action
If you prefer to see the setup before you build your own, this short video walks through a spunding valve installation.
Additional resources
For a DIY build, the HomebrewTalk spunding valve thread collects parts lists and tips from the forum, and BrewUnited's spunding valve write-up explains the theory behind pressure control. Closer to home, our own guides to force carbonation and carbonation in general pair naturally with everything above, and if you are still deciding how to condition your beer, the priming sugar guide and calculator covers the traditional route.
