Flat draft beer is almost always a system problem, not a beer problem. The keg left the brewery carbonated correctly. Something between the keg and the glass knocked it out. Here's how to find it.
Check Your CO₂ Pressure First
Low serving pressure is the most common reason draft beer comes out flat. According to the Brewers Association Draught Beer Quality Manual, most American lagers and ales should be served between 10 and 14 PSI at a keg temperature of 38°F. Some styles — nitro stouts, high-carbonation Belgians — fall outside that range, but that's the baseline for most of what's on your lines.
If your regulator is set below that range, the beer will lose CO₂ as it travels from keg to faucet. Check the gauge. If pressure has drifted, reset it and give the keg a few hours to stabilize before pulling pints.
Also check your CO₂ tank. A near-empty tank can hold pressure at the regulator but still underperform under load. If you're pouring heavy and the tank is low, swap it out before troubleshooting anything else.
Temperature Problems Cause Flat Beer Too
CO₂ stays dissolved in cold beer. When beer warms up — even briefly — CO₂ escapes. If your walk-in is running warmer than 38°F, or your lines run through a warm section before reaching the faucet, you'll lose carbonation before the pour.
Long draws are particularly vulnerable. If your lines run more than 25 feet from keg to faucet, you need either a glycol system or a well-insulated trunk line to keep the beer cold the entire way. A glycol chiller circulates cold liquid around the beer lines inside a foam-insulated bundle. Without it, a long draw in a warm bar will go flat even with perfect pressure settings.
Check the temperature at the faucet, not just at the keg. Stick a thermometer in the first pour of the day. If it's above 40°F, the line is warming the beer.
Why Is My Draft Beer Flat Even with Good Pressure and Temperature?
If pressure and temperature check out, look at the lines themselves. Dirty lines can cause flat beer in a couple of ways. Biofilm — the microbial buildup that develops inside unclean lines — can absorb CO₂ and strip carbonation from the beer as it passes through. A 2021 study published in PubMed documented how bacterial biofilm in draft lines actively affects beer chemistry, including carbonation levels.
Old or deteriorating tubing is another culprit. Vinyl beer lines have a service life. As the material ages and the inner wall degrades, it becomes porous enough to allow CO₂ to diffuse out of the beer during transit. A 2024 study in ScienceDirect confirmed that tubing age and inner wall condition have a measurable effect on CO₂ retention in beer lines. If your lines are more than two years old and you can't find another cause, replacing them is worth considering. [INTERNAL LINK: when to replace draft beer lines and jumper lines]
Check for CO₂ Leaks in the System
A slow leak anywhere in the gas side of your system will bleed pressure without triggering an obvious alarm. The regulator, coupler connections, gas hose fittings, and manifold joints are all potential leak points.
The easiest way to check: turn off the CO₂ at the tank, note the regulator reading, and come back in an hour. If the reading has dropped, you have a leak. Use a spray bottle of soapy water on every connection while the gas is running — bubbles will show you exactly where it's escaping.
Mixed Gas Setups Add Another Variable
If you're running nitrogen blends — typically 75/25 nitrogen-to-CO₂ for stouts, or a custom blend for certain styles — the blend ratio matters. Using a straight nitrogen mix on a beer that requires CO₂ will pull carbonation out of the beer over time. Make sure the blend on your mixed gas regulator matches what the brewery recommends for that style.
When to Call Someone
If you've checked pressure, temperature, leaks, and the lines are reasonably new, the system needs a professional eye. Foamy and flat problems often share root causes, and a trained technician can diagnose the whole system rather than chase one variable at a time.
Flat draft beer is costing you money on every pour. Call Philly Draft Cleaners at (267) 282-1002. We serve bars and restaurants across Philadelphia and the surrounding area.