Flat, Foamy, or Off? How to Diagnose Common Draught Beer Problems in Under 5 Minutes

A customer is waiting. The pint looks wrong. What do you do?

In a busy pub, you don’t have time to call an engineer every time a pour goes sideways. The difference between a quick fix and a ruined round often comes down to how fast you can diagnose the problem.

The good news? Most draught beer issues fall into three categories—flat, foamy, or off—and nearly all of them can be traced to one of four root causes: temperature, pressure, cleanliness, or gas.

Here’s your five-minute diagnostic toolkit.

Start Here: The 30-Second Visual Check

Before you touch a spanner or a cleaning brush, look at the pint.

What You Seemix/nitrogencontamination/beer	What It Usually Means
No head, lifeless appearance	Under-carbonation / low applied pressure
Excessive foam, half-pint overflow	Over-carbonation / high applied pressure / warm lines
Cloudy or hazy beer	Temperature fluctuation / old stock / dirty lines
Sour, metallic, or stale taste	Line contamination / beer stone / bacterial growth
Large bubbles instead of creamy foam	Wrong gas mix / nitrogen deficiency

Now you know what you’re dealing with. Let’s dig into the fix.

Problem 1: The Flat Pint

Symptoms: No head, thin mouthfeel, beer looks tired before it hits the glass.

Quick Diagnosis (2 Minutes)

1. Check the temperature. Beer should leave the cellar between 5–7°C. If it’s warmer, CO₂ escapes before it reaches the glass. If it’s colder, carbonation stays locked in the liquid and won’t form a head.

2. Check the gas pressure. Too low, and there’s not enough force to push CO₂ into solution. Look at your regulator gauge. Has it drifted since yesterday?

3. Check the gas type. Are you running pure CO₂ on a beer that needs a mixed gas? Stouts and smooth-flow beers need nitrogen to create that tight, creamy head. Without it, they fall flat—literally.

4. Check for leaks. Spray soapy water on gas line connections. Bubbles mean you’re losing pressure before it ever reaches the keg.

The Fix

• Contact your cellar maintenance company; they have the formulas and equipment to check and confirm.

• Verify your gas blend matches the beer style. White or grey tubes for lager and ciders, green tubes for heavies and stouts

• If you’re using cylinders and pressure is inconsistent, you’re fighting a losing battle. Consider an onsite system that maintains steady output.

  
  

Problem 2: The Foamy Pint

Symptoms: Glass fills 70% foam, slow pour, frustrated bartender, thirsty customer.

Quick Diagnosis (2 Minutes)

1. Check the temperature again. This is the #1 cause of foam. If your cellar is running warm—or if the beer line runs through a hot spot above the ceiling—CO₂ breaks out of solution and foams violently.

2. Check for over-pressurisation. Too much applied pressure forces CO₂ out of the beer before it reaches the tap. If your gauge reads high, dial it back gradually.

3. Check the faucet. A dirty or partially obstructed tap nozzle disrupts laminar flow and creates turbulence. A quick clean can solve it instantly.

4. Check for frozen lines. In winter, lines running near exterior walls can drop below 2°C, causing ice crystals that nucleate foam on contact.

The Fix

• Stabilise cellar temperature. Even a 1°C swing can turn a good pint into a frothy mess.

• Clean the faucet and nozzle with a brush. It takes 30 seconds and solves more foam issues than you’d expect.

• If the problem persists across multiple taps, your gas system is likely the common denominator. Contact your gas provider or your Cellar tech.

  
  

Problem 3: The “Off” Pint

Symptoms: Sour, vinegary, cardboard-like, or metallic taste. Smells wrong. Customer pulls a face.

Quick Diagnosis (2 Minutes)

1. When were the lines last cleaned? If the answer is “I’m not sure” or “last month,” you’ve found your problem. Biofilm, bacteria, and beer stone build up fast. Industry standard is every 7 days for high-volume venues.

2. Check the date on the keg. Old beer oxidises. A keg tapped three weeks ago is not the same product it was on day one.

3. Check for cross-contamination. Did someone tap a new keg without flushing the line? Residual cleaning chemicals or old beer style can taint the next product.

4. Smell the gas lines. Yes, really. Contaminated gas lines can push odours and bacteria straight into your beer.

5. 
   

The Fix

• Clean your lines. Immediately. If you don’t have a scheduled cleaning protocol, implement one today.

• Rotate stock properly—first in, first out.

• If off-flavours persist after cleaning, your lines may have stubborn beer stone or biofilm that requires a professional deep clean.

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The 5-Minute Diagnostic Cheat Sheet

Print this and stick it in your cellar.

Symptom	Check First	Likely Culprit
Flat, no head	Temperature / Pressure	Under-pressurisation or wrong gas blend
Foamy, overflowing	Temperature / Pressure	Over-pressurisation or warm lines
Cloudy appearance	Temperature / Age	Fluctuation or old keg
Sour / vinegary taste	Line hygiene	Bacterial contamination
Metallic / stale taste	Line hygiene / Gas	Beer stone or gas contamination
Large, loose bubbles	Gas type	Wrong CO₂/N₂ ratio

Why These Problems Keep Coming Back

If you’re diagnosing the same issue every week, you don’t have a staff problem. You have a system problem.

Temporary fixes—adjusting the regulator, bleeding a keg, apologising to customers—mask underlying instability. Cylinder-based gas systems, in particular, create a cycle of inconsistency: pressure drops, you compensate, the new cylinder arrives at a different pressure, and you start again.

The venues that serve flawless pints day after day have one thing in common: they took the variables out of the equation.

The Long-Term Fix

On-site nitrogen generation removes the guesswork. Instead of chasing cylinder pressure, swapping bottles, and hoping your blend is right, you get:

• Stable pressure from a continuous supply

• Precise blending for every beer style

• Consistent quality whether it’s Friday night or Tuesday afternoon

• Lower long-term costs with no delivery fees, cylinder rental, or emergency orders

At Draught Doctor, we help pubs move from firefighting to consistency. Because great "cellarmanship" shouldn’t require a chemistry degree and a prayer.