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Pressure fermenting and spunding

Pressure fermenting and spunding explained: what pressure does to esters, the 10–15 psi advice, setting a spunding valve, vessel ratings, PRVs and safety.

Updated November 2026 · 7 min read

Stainless conical fermenters on a platform in a small brewery
Photo: Stephen Craven, CC BY-SA 2.0, via Wikimedia Commons
The short version
  • Pressure fermenting means fermenting in a sealed, pressure-rated vessel with a spunding valve that holds pressure and vents the excess.
  • Pressure suppresses esters and reduces diacetyl, which lets you ferment warmer with a cleaner result. BYO’s author recommends about 10–15 psi.
  • Higher is not better: at 30 psi (2 bar), BYO’s test found muted malt and harsher bitterness.
  • A spunding valve is not a safety device. The vessel needs its own pressure relief valve, and you must never exceed the maker’s rating.
  • Near the end of fermentation you can raise the setting to carbonate the beer naturally, then transfer under pressure without oxygen.

Pressure fermenting has moved from commercial breweries into homebrew garages thanks to affordable pressure-rated fermenters and corny kegs. The idea is simple: instead of letting CO2 escape through an airlock, you hold some of it in the vessel with an adjustable valve. That pressure changes how yeast behaves, lets you carbonate the beer as it finishes and makes oxygen-free transfers easy.

This guide draws on John Blichmann’s BYO article “Fermenting Under Pressure,” BYO’s pieces on spunding valves and the pressure ratings printed by vessel makers. Pressure vessels store energy, so the safety section matters as much as the recipe.

What does fermenting under pressure do?

In Blichmann’s BYO test, three batches fermented at about 68°F (20°C) at 0, 15 and 30 psi. The unpressurized beer had noticeable fruity esters, 15 psi had lower esters and 30 psi very low esters. Pressure also greatly reduced diacetyl. The pressurized beers finished within about two weeks, against about eight weeks for a traditional lager with a diacetyl rest and lagering.

10–15 psiPressure range BYO’s author recommends for the best resultsBYO: Fermenting under pressure
30 psiTested setting that gave muted malt and harsher bitternessBYO: Fermenting under pressure
~2 weeksRoom-temperature pressure batches vs about 8 weeks for the traditional lagerBYO: Fermenting under pressure

Blichmann’s conclusion is balanced: pressure fermentation at room temperature gets “pretty darn close” to a true lager, but the traditional lagers were still maltier and better balanced. Treat it as a useful tool, not a perfect substitute for cold fermentation.

Why pressure changes the flavor

Yeast responds to its surroundings. Dissolved CO2 rises as pressure rises, and higher CO2 levels slow yeast growth and the reactions that produce many fruity esters and fusel alcohols. That is the mechanism brewers use: instead of cooling the beer to keep esters down, they hold it at a moderate pressure and let the CO2 do part of the work.

The trade-off is that the same effect can slow fermentation if the pressure is high from the very first hours, when yeast is still multiplying. That is why many brewers start at low or no pressure and raise it once fermentation is underway, and why BYO’s spunding article advises letting pressure build gradually. Temperature still matters: pressure narrows the flavor gap between warm and cold fermentation, it doesn’t erase it.

What you need

Pressure fermenting gear and listed ratings

Vessel or partListed ratingNotes
KegLand FermZilla All Rounder 30L pressure kit2.5 bar (36.2 psi)PET; kit adds a spunding valve and PRV (per listing)
KegLand FermZilla Tri-Conical 27L Gen3Up to 2.4 barPET unitank with dump valve
VEVOR 30L stainless fermenter15 psiStay well below 15 psi
5 gal ball-lock corny keg (KegLand)130 psi operatingFerment about 4 gal to leave headspace
KegLand Kegmenter 50LNot listedCheck with the maker before pressurizing
Spunding valveAdjustable range per modelNot a safety device

Source: Maker listings; BYO: Fermenting under pressure

Ball-lock corny kegs and a larger keg stacked on a pallet
Corny kegs double as small pressure fermenters.Photo: Czeva, CC BY-SA 4.0, via Wikimedia Commons

PET pressure fermenters also have temperature limits that are lower under pressure; one FermZilla kit listing gives about 95°F under pressure. Read the manual for your exact model.

How to set a spunding valve

BYO’s method: connect a CO2 cylinder with its regulator set to your target pressure to the vessel, then adjust the spunding valve until it just stops venting. The valve now opens at that pressure. Some valves have their own gauge; others read on a separate gauge on the vessel.

  1. Inspect seals and the PRV

    Check O-rings, the lid seal and that the vessel’s own pressure relief valve moves freely.

  2. Fill, pitch and seal

    Leave headspace (about 20% in fermenters; about 4 gal in a 5 gal corny keg). A drop of antifoam helps keep krausen out of the valve, as BYO’s spunding article suggests.

  3. Let it start (optional)

    Some brewers let fermentation start unpressurized for the first day, then attach the valve. BYO’s article on spunding notes that high pressure from the very start slows yeast growth.

  4. Hold 10–15 psi

    Set the valve in the range BYO’s author recommends, and keep fermentation temperature controlled; pressure is not a license to ignore temperature.

  5. Raise to carbonate near the end

    When about 1°P of gravity remains (roughly 0.004 SG), raise the valve setting to the chart pressure for your target volumes at the beer’s temperature. The yeast finishes carbonating the beer.

  6. Confirm final gravity

    Take samples through a picnic tap or sampling valve and look for two matching readings.

  7. Cold crash and transfer under pressure

    Chilling drops pressure; top up with CO2 to keep it positive. Then push the beer into a purged keg through the liquid posts.

Keg pressure (psi, sea level) by beer temperature and target CO2 volumes

Beer temp1.8 vol2.0 vol2.2 vol2.4 vol2.6 vol2.8 vol3.0 vol3.2 vol
32°F 0°C1.63.55.47.39.211.012.914.8
34°F 1.1°C2.34.36.38.210.212.114.116.0
36°F 2.2°C3.15.17.29.211.213.315.317.3
38°F 3.3°C3.96.08.110.212.314.416.518.6
40°F 4.4°C4.76.89.011.213.415.517.719.8
42°F 5.6°C5.57.710.012.214.416.718.921.1
44°F 6.7°C6.38.610.913.215.517.820.122.4
46°F 7.8°C7.19.511.814.216.619.021.323.7
48°F 8.9°C7.910.412.815.317.720.122.625.0
50°F 10°C8.711.313.816.318.821.323.826.3
52°F 11.1°C9.612.214.817.319.922.525.127.6
54°F 12.2°C10.413.115.718.421.123.726.329.0
56°F 13.3°C11.314.016.719.522.224.927.630.3

Formula: PSI = −16.6999 − 0.0101059·T + 0.00116512·T² + 0.173354·T·V + 4.24267·V − 0.0684226·V². Checked against the BYO grid (40°F and 2.4 vol: 11.19 psi by formula, 11.2 psi on the chart). Regulator gauges read relative to local air pressure. BYO notes that Denver needs about 3 psi over the sea-level chart; add roughly 0.5 psi per 1,000 ft. Sources: BYO: Carbonation priming chart; Brewer's Friend: Force carbonation chart; Brewers Association: Draught Beer Quality Manual.

Keg carbonation chart and PSI calculatorFind the pressure that gives your target carbonation at the beer’s temperature, then set the spunding valve to it.

Pressure safety

A sealed fermenter full of fermenting beer is a pressure vessel. BYO’s safety points are blunt, and they are the rules to follow:

Stainless conical fermenters above a row of oak barrels in a small brewery
Only vessels rated for pressure should be spunded.Photo: Scott Bugni, CC BY-SA 3.0, via Wikimedia Commons
  • Use a separate, dedicated pressure relief valve on any pressure vessel. A spunding valve is not a safety device.
  • Never exceed the maker’s rated pressure, and never tamper with relief devices.
  • Don’t vent through the PRV during fermentation; hop debris or yeast can plug it.
  • Be able to release all pressure to zero before opening the vessel, and always do so.
  • Use a CO2 regulator for any gas connection; never connect a cylinder directly to a vessel.
CO2 cylinders and pressureKeep cylinders upright and secured, and always use a CO2-rated regulator (US CO2 tanks use the CGA 320 valve fitting); never connect a cylinder straight to a keg. Store tanks cool and away from heat. CO2 displaces oxygen and collects low: ventilate, close the tank valve and leave the room if a tank leaks. Never exceed a fermenter’s or keg’s rated pressure.Source: Kegco: Everything you need to know about CO2; OSHA hazard bulletin on CO2
Glass carboys breakGlass carboys can break and cut. Use a good carboy handle and always support the bottom when you move a full one; a crate or carrier under the base helps on slippery floors. Never pour hot wort into cold glass. PET and stainless avoid the risk.Source: Craft Beer & Brewing: Homebrew safety

Never pressure-ferment in glass carboys or ordinary buckets. Only vessels sold with a pressure rating belong in this process. Check PET vessels for scratches, crazing or cracks before each use, and retire them if you find any.

Closed transfers: the other big benefit

Even if you never change your fermentation temperature, a pressure-capable fermenter lets you move finished beer into a keg without it ever touching air. Oxygen pickup during transfer is a main cause of dull, stale hoppy beers, so this alone justifies the gear for many IPA brewers.

  1. Fill a clean keg with sanitizer and push it out with CO2 so the keg is full of CO2.
  2. Connect a liquid line from the fermenter’s beer-out port to the keg’s liquid-out post.
  3. Connect the keg’s gas post to a spunding valve set a few psi below the fermenter pressure, or vent it slowly by hand.
  4. Let the pressure difference push the beer across; keep the fermenter topped up with CO2 so its pressure stays positive.
  5. When gas spurts through the line, the transfer is done. Close the valves before disconnecting.

Stay under both vessels’ ratings throughout, and keep the PRVs free.

Styles and recipes that benefit

  • Lager-style beers without a long lagering schedule: lager yeast warmer than usual under pressure, for example W-34/70 or NovaLager.
  • Clean American ales in a warm space: pressure keeps esters down when temperature control is limited.
  • Hoppy beers: closed, oxygen-free transfers protect hop aroma.
  • Not ideal: styles that depend on yeast esters, such as hefeweizen and many Belgian ales.

Beer styles by the numbers (BJCP 2021 vital statistics)

StyleOGFGIBUColor (SRM)ABV
4A Munich Helles1.044–1.0481.006–1.01216–223–54.7–5.4%
5D German Pils1.044–1.0501.008–1.01322–402–44.4–5.2%
18B American Pale Ale1.045–1.0601.010–1.01530–505–104.5–6.2%
21A American IPA1.056–1.0701.008–1.01440–706–145.5–7.5%

Source: BJCP 2021 Beer Style Guidelines. Ranges are for judging; your recipe can sit anywhere inside them. Color swatches are an on-screen approximation.

Mistakes to avoid

  • Treating the spunding valve as a safety valve. Keep a PRV on the vessel.
  • Going high to be “safe” on esters. BYO found 30 psi hurt malt and bitterness.
  • Overfilling. Krausen clogs valves; leave headspace and use antifoam.
  • Ignoring temperature limits on PET. Lower limits apply under pressure.
  • Opening a pressurized lid. Vent to zero first.
  • Cold crashing without CO2 backup. Pressure drops and can pull in air or sanitizer.
  • Using vessels without a listed rating. If the rating isn’t listed, don’t pressurize it.

Who should try pressure fermenting

  • Already kegging: a corny keg plus a spunding valve is the lowest-cost entry.
  • Want lager-like beer without months of lagering: a pressure-rated fermenter and lager yeast.
  • Care about hop freshness: closed transfers are the main win.
  • Brand new to brewing: learn standard fermentation first; see the fermentation temperature guide.

For carbonation background, read carbonation explained; for keg handling, how to keg homebrew. Record pressure settings and temperatures in the brew-day log.

Gear picks for this guide: best pressure fermenters and best CO2 regulators for kegging.

VerdictPressure fermenting at about 10–15 psi trims esters and diacetyl and makes oxygen-free transfers easy, but it isn’t magic and it isn’t a reason to skip temperature control. Use only pressure-rated vessels with their own relief valve, stay under the maker’s rating, and vent to zero before opening.
Keg carbonation chart and PSI calculatorKeg carbonation chart and calculator: regulator PSI by beer temperature and CO2 volumes, with an altitude…Printable brew-day log and recipe sheetFree brew-day log: record recipe, water and salts, mash, gravities, yeast, fermentation temperatures,…

Questions

What PSI should I pressure ferment at?
BYO’s author recommends about 10–15 psi. In his test, 30 psi muted malt character and made bitterness harsher.
What is a spunding valve?
An adjustable valve that holds a set pressure in a fermenter or keg and vents CO2 above it. It is not a safety device; the vessel needs its own pressure relief valve.
Can I ferment lagers warm under pressure?
Many brewers do, and BYO’s test got close to a traditional lager at about 68°F. Traditional cold fermentation still gave a maltier, more balanced beer in that comparison.
Can I ferment in a corny keg?
Yes. A corny keg has a high pressure rating (KegLand lists 130 psi operating for its 5 gal keg). Ferment about 4 gal to leave headspace, and fit a spunding valve on the gas post.
When should I attach the spunding valve?
Either from the start at a moderate setting, or after the first day of unpressurized fermentation. For natural carbonation, raise the setting when about 1°P (about 0.004 SG) remains.
Is pressure fermenting dangerous?
It is safe with pressure-rated vessels, a working pressure relief valve and settings below the maker’s rating. Never pressurize glass carboys or ordinary buckets.

Sources

  1. BYO: Fermenting under pressure (John Blichmann)
  2. BYO: Using spunding valves
  3. BYO: Pressurizing your fermenter, tips from the pros
  4. BYO: Carbonation priming chart (pressure grid)
  5. Kegco: Everything you need to know about CO2
  6. Fermentis: SafLager W-34/70 product page