Why is my kegerator beer flat? The 5 causes and the timeline that diagnoses them

Flat beer from a kegerator is almost always one of five things, and the timeline tells you which. When did the flat pours start? Today, with a brand new keg? Probably an unequilibrated keg, no fix needed beyond 24 hours. A few days ago? Probably a slow CO2 leak. A week ago, with no obvious change? Probably the tank has been emptying without you noticing. Below: how to diagnose by timeline, the 5 actual causes, and the order to check them.

Flat beer is a different problem than “no CO2”

Most kegerator owners conflate two things when troubleshooting flat beer:

1. CO2 isn’t reaching the line. Tank empty, regulator off, gas line disconnected. The beer wouldn’t pour at all, or would pour as a slow trickle. Easy to spot.
2. CO2 isn’t dissolved in the beer. Pressure is reaching the keg, beer is pouring fine, but the carbonation level in the beer itself has dropped below where it should be. Pours look normal except no bubbles, no head, flat mouthfeel.

The second one is what most “flat beer” complaints actually are. The fix isn’t “add more CO2 right now,” it’s “fix what made the beer lose carbonation, then wait for it to re-absorb gas over the next day or two.”

This distinction matters because the instinct to crank pressure up to 18-20 PSI to “force-carbonate” usually causes over-carbonation (and then foam) by the time you notice. Set the pressure correctly. Be patient.

The diagnostic question that solves it 80% of the time

When did the flat pours start?

The timeline narrows the cause faster than any other check.

Timeline	Most likely cause	Verify first
Started today, brand new keg	Keg hasn’t had time to equilibrate at set pressure	Give it 24-48 hours
Started today, established keg	Pressure changed (intentionally or via leak)	Check regulator setting + tank weight
Started 2-3 days ago	Slow leak (downstream of regulator)	Spray-test all joints
Started a week ago	Tank emptying without your notice	Weigh the tank
Gradual decline over weeks	Temperature drift or kegerator running warm	Check thermometer + cycle behaviour
First pint of a session is flat, later pints are fine	Beer near the dip tube bottom degassed during dormancy	Pour a pint, wait 30 min, retest

This single question routes you to the right diagnostic in under a minute. Don’t skip it.

Cause 1: Slow CO2 leak (most common after the first few weeks of kegerator ownership)

A 1 PSI/hour leak in the gas line will empty a 5lb tank in about a week and produce gradually flatter beer over the same period. For a primer on CO2 tank sizing and refill economics, see the 5 lb vs 10 lb vs 20 lb cornerstone. The owner doesn’t notice because:

• The regulator gauge reads “correct” as long as the tank still has any pressure
• Beer keeps pouring normally
• Carbonation drops in tiny daily increments

By the time the gauge starts visibly dropping or the pour quality changes, the tank is mostly empty and the beer has been progressively under-carbonated for days.

A US-based kegerator owner I worked with reported his 5lb CO2 tank lasting three weeks instead of six. He’d been chasing flat-beer fixes for ten days before he thought to check the tank weight. The cause turned out to be a hairline crack in a plastic disconnect coupler that cost $8 to replace. Tank life went back to normal the next week.

How to find a slow leak:

1. Spray Star San (or dish soap + water) on every joint between the tank and the keg.
2. Watch for bubbles. They’ll form within 5-10 seconds at the leak point.
3. The most common leak spots, in order: the regulator-to-tank seal (PTFE tape wears out), the gas line clamps, the disconnect couplers, the Sankey coupler’s gas side.
4. Tighten what you can; replace what you can’t.

A leak that survives 30 seconds of spraying without bubbling is below the rate worth worrying about. But anything visible should be fixed; even slow leaks add up.

Cause 2: Pressure was lowered “to fix foam” (the self-inflicted version)

This pattern is common with newer owners who hit a foam problem and try to fix it by turning pressure down. The foam might improve briefly, but a week later the beer is flat because the lower pressure no longer maintains enough CO2 in solution.

The fix isn’t “crank the pressure back up immediately.” The fix is:

• Set pressure to the correct level for the beer style (12-14 PSI for most American beers, 8-10 PSI for British cask-style, 30-35 PSI for nitro)
• Wait 24-48 hours for the beer to re-equilibrate
• If foam returns at correct pressure, the original foam cause was something else (lines, temperature, regulator drift), and you should diagnose that instead

The original foam was caused by something other than pressure 80% of the time. Lowering pressure was treating a symptom and creating a new one. The foamy-after-CO2-swap cornerstone walks through the actual diagnostic order for foam.

Cause 3: Temperature drift

Beer holds CO2 better at colder temperatures. The carbonation curve says:

• 38°F at 12 PSI: ~2.4 volumes of CO2 (normal lager carbonation)
• 42°F at 12 PSI: ~2.1 volumes (slightly flat)
• 46°F at 12 PSI: ~1.8 volumes (noticeably flat)

A kegerator drifting from 38°F to 44°F over a few weeks will produce progressively flatter beer at the same pressure. Causes include:

• Door seal degradation (cold air leaking out)
• Refrigerant level low (compressor running but not cooling efficiently)
• Ambient temperature change (kegerator in a garage that just hit summer)
• Dirty condenser coils (impeding heat exchange)

Verify with an independent thermometer placed inside the kegerator at keg height (not in the door, which reads warm). If actual temperature is 4°F or more above your set point, fix the unit before adjusting pressure.

Cause 4: New keg hasn’t equilibrated yet

A freshly tapped keg, especially from a homebrew batch or a transported commercial keg, may need 24-48 hours at the right pressure to reach proper carbonation. Pouring within the first 12 hours of tapping often produces flat-ish beer that resolves itself.

Diagnostic: did you tap the keg in the last 48 hours? If yes, give it more time before adjusting anything.

Some commercial kegs ship pre-carbonated to specific volumes and pour correctly within an hour of tapping. Some homebrew kegs are tapped right after a cold-crash with minimal carbonation; those need a full force-carbonation cycle. Know which type you’re working with.

Cause 5: The keg is old and has degassed (occasional)

A keg that’s been tapped for 6+ weeks, especially if there have been periods of low pressure or open headspace, can lose carbonation that won’t return even at correct pressure. The CO2 has escaped over the keg’s lifetime; what’s left in solution is the maximum the beer will hold without re-carbonating from scratch.

This is rare on home setups because most owners empty a keg before it gets to this point. But it can happen with slow-pour kegs that sit for 2-3 months between events.

The fix: live with it for the remaining pints, or pop the lid (if it’s a homebrew corny keg) and force-carbonate from scratch. For commercial kegs, there’s no realistic recovery; just plan to drink the rest faster.

Diagnose flat beer in 30 minutes

The order that gets you to the cause without burning a tank of gas or a keg of beer. Run these sequentially.

1. Identify when the flat pours started. Was it today? Two days ago? A week ago? The timeline narrows the cause faster than any other check. Yesterday = leak or pressure change. A week ago = old keg or temperature drift. Yesterday with a new keg = unequilibrated keg.
2. Check the gas tank weight. Lift the tank. If it's much lighter than it was a week ago, you have a leak (or you've poured a lot). Weigh it on a bathroom scale if you're unsure: 5lb tank should weigh 12-14 lbs when full.
3. Spray-test for leaks. Mix Star San or any soapy water. Spray every joint between regulator and keg, including the gas line connections, the regulator inlet, and the Sankey coupler. Bubbles = leak. Tighten or replace the affected component.
4. Check delivery pressure on a test gauge. A primary regulator can show 12 PSI on its gauge while actually delivering 6 PSI if the gauge has failed. Use an inline test gauge between regulator and line, or borrow a known-good regulator, to verify.
5. Wait 24-48 hours after correcting pressure. Once you've fixed the leak or restored pressure, the beer needs time to re-absorb CO2. 24-48 hours at 38°F. Don't pour during this window to test (it slows the equilibration). Patience is the last step.

When to give up and accept the beer is gone

Three signs the keg is past saving:

1. Carbonation has been low for more than 2 weeks despite correct pressure.
2. The beer also tastes oxidised (cardboard, sherry-like) along with being flat.
3. The keg has been on tap for more than 6 weeks at slow pour rates.

In any of those cases, the dissolved CO2 isn’t coming back without re-fermentation, which isn’t practical on a commercial keg. Drink what’s left, replace the keg, prevent the underlying cause from happening again.

If your kegerator chronically produces flat beer despite correct pressure and no leaks, the issue is upstream of “wrong setting” and into “wrong setup.” The most common culprit at that level is the unit running 2-4°F too warm. See the setup pillar for kegerator temperature calibration.

Carbonation theory in plain English

The reason flat beer is more than just “no CO2 in the line” comes down to physics most owners never bother learning. Here’s the version you actually need.

CO2 dissolves into beer at a rate determined by temperature and pressure. Colder beer absorbs more gas. Higher pressure pushes more gas in. The equilibrium between gas in the headspace and gas in the liquid is governed by Henry’s Law, a 19th-century chemistry rule that hasn’t changed and isn’t going to.

At a typical home setting of 12 PSI and 38°F, beer holds about 2.4 volumes of CO2: meaning 2.4 litres of dissolved gas per litre of beer. Bump the temperature to 42°F at the same pressure, and the beer can only hold 2.1 volumes. The “missing” 0.3 volumes don’t disappear; they migrate to the headspace, where they don’t taste like anything. The result: a beer that pours, looks normal, and tastes flat because the dissolved CO2 was the part that gave it the mouthfeel.

The same principle works in reverse. Crank pressure to 18 PSI at 38°F, leave it for a week, and the beer absorbs roughly 3.0 volumes: well over-carbonated for most styles. The pour becomes foam-heavy because the dissolved gas wants out at atmospheric pressure.

The equilibrium time matters. The beer doesn’t move from 2.1 to 2.4 volumes instantly when you correct the temperature. CO2 absorbs into the bulk of the liquid over 24-48 hours at home temperatures, faster if you agitate the keg (which most owners shouldn’t because it’s easy to overdo).

Why this matters for the flat-beer diagnostic. If you find a cause (leak, wrong pressure, temperature drift) and fix it, you can’t immediately pour-test the result. The beer needs time to re-equilibrate. Pour the first day after correction, and the keg might still taste flat: not because the fix didn’t work, but because the gas hasn’t fully redissolved yet. Wait 48 hours before declaring a fix failed.

Temperature	At 12 PSI: volumes of CO2	At 14 PSI: volumes of CO2
34°F	2.6	2.8
38°F	2.4	2.6
42°F	2.1	2.3
46°F	1.8	2.0
50°F	1.6	1.7

The table above is the working reference. If your beer is meant to be at 2.4 volumes (typical American lager) and your kegerator runs at 42°F, you’re never going to hit that volume at 12 PSI. Either drop the temperature or raise the pressure. The maths doesn’t negotiate.

---

Part of the flat beer cornerstone series. Follow-ups coming on CO2 leak-finding techniques and force-carbonation maths.

People also ask

How can I tell if my CO2 tank is leaking without specialised equipment?

Soapy water on every joint, run a finger along each connection. Bubbles form wherever there's an escape. The most-common leak points: the regulator-to-tank seal (PTFE tape wears out), the disconnect couplers, the Sankey coupler's gas-in port. A 1 PSI/hour leak halves a 5lb tank's life and produces gradually flatter beer.

Why is my homebrew flat after force-carbonating for 24 hours?

24 hours is enough for the headspace to fill but not for the beer to absorb all the CO2 it needs. Give it a full 48-72 hours at correct pressure (12-14 PSI for most styles at 38°F). Shaking the keg with gas on speeds this up but risks over-carbonation; patience is safer.

Does cold beer hold more carbonation than warmer beer?

Yes, by a meaningful amount. At 12 PSI, beer at 38°F holds ~2.4 volumes of CO2; at 46°F the same beer holds only ~1.8 volumes. A kegerator drifting warmer over time produces progressively flatter beer at the same pressure setting.

Should I increase pressure to fix flat beer?

Only after confirming the actual cause. If the cause is a leak, raising pressure just leaks faster. If the cause is temperature, fix the kegerator. If the cause is genuinely under-carbonation at correct pressure, give the keg 24-48 hours at the correct setting before raising. Pressure-cranking creates over-carbonation problems within a few days.

Can I rescue a keg that's been on tap too long and gone flat?

If the keg's seal has been intact and pressure has been maintained throughout, yes: top up pressure to 12-14 PSI, wait 48 hours, retest. If the keg has had its headspace pressure repeatedly relieved (bleeding), the dissolved CO2 won't come back without re-carbonating the beer from scratch, which isn't practical on a commercial keg.

Why does the first pint after a long break taste flat but the rest are fine?

Beer near the bottom of the dip tube degasses slightly during dormancy. Pour 1-2 pints to clear the line, then the rest of the keg pours normally. If the entire keg is flat (not just the first pints), the issue is pressure, leak, or age.

Sources & references

Claims in this article cross-check against the following. We link out so you can verify.

1. Carbonation Volume Tables — Brewers Friend reference
2. CO2 Tank Sizing & Leak Detection — Taprite manufacturer
3. Draft Beer Carbonation Standards — Brewers Association standard
4. Star San Sanitiser MSDS + Usage — Five Star Chemicals manufacturer
5. Flat keg troubleshooting megathread — HomeBrewTalk community