Two Foamy Taps After a Line Change
A taproom pours 12 lines from a walk-in cooler held at 38 °F. Taps sit about 4 feet above the keg couplers. CO2 is set at 14 psi, which holds the house beers at roughly 2.5 volumes. Ten lines use 3/16-inch ID vinyl choker coils about 6 feet long and pour clean, well-carbonated glasses. During a remodel, two lines were re-plumbed with 12 feet of 5/16-inch ID barrier tubing; both now pour fast, with a glass that is more than half foam and beer that tastes noticeably flat. Kegs on those two taps also seem to empty faster than the others. The bar manager wants to drop the regulator to 8 psi to slow the pour. Meanwhile, one of the good taps foams only on the first pour of the day and is fine afterward. Cellar records show the two problem taps are a 4.8% lager and a 5.2% pale ale, both carbonated to 2.5 volumes.
1. Explain, with numbers, why exactly two taps foam while the other ten pour correctly.
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The balancing equation. A draught line must dissipate the applied pressure through restriction (line friction) plus lift, plus about 1 psi at the tap:
Applied pressure ≈ line restriction + lift + ~1 psi
- Lift: 4 ft × 0.43 psi/ft ≈ 1.7 psi.
- The ten good lines: 3/16-inch ID vinyl is roughly 2.2 psi/ft, so 6 ft ≈ 13.2 psi, plus 1.7 psi lift ≈ 14.9 psi. The 14 psi setting is balanced, which is why they pour cleanly.
- The two new lines: 5/16-inch ID barrier tubing is roughly 0.4 psi/ft, so 12 ft ≈ 4.8 psi, plus 1.7 psi lift ≈ 6.5 psi. Against 14 psi applied, that is about 7.5 psi of excess pressure.
What that excess does. The beer leaves the keg far faster than it should, so the pressure is dumped across the tap rather than along the line. CO2 comes out of solution as a burst of foam in the line and at the spout. That explains all three symptoms at once:
- Half a glass of foam - CO2 breaking out at the tap.
- Flat-tasting beer - the CO2 left the beer as foam and went up the drain; the keg itself stays at 2.5 volumes.
- Faster keg turnover - high flow rate plus pouring off foam means more beer per glass sold.
The style and carbonation level are irrelevant here. Both problem beers are at 2.5 volumes, the same as the beers that pour well, so this is purely a line-restriction problem, not a carbonation or recipe issue.
2. Evaluate the bar manager's proposal to drop the regulator to 8 psi, and give the correct fix with verification.
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The 8 psi proposal is wrong. At 38 °F, 8 psi only supports about 1.9 volumes of CO2. The beer would slowly go flat in the keg over days, so you would trade a fast, foamy pour for a slow, under-carbonated one and still not have a properly balanced line. Lowering pressure masks under-restriction; it does not correct it.
Correct fix: restore restriction, keep 14 psi.
- Replace the 5/16-inch tubing with 3/16-inch vinyl: about 6 ft gives roughly 13.2 psi, which balances 14 psi applied with the 1.7 psi lift. This also makes the two lines consistent with the other ten, which simplifies future troubleshooting.
- Alternatively keep 1/4-inch barrier tubing and lengthen it to about 14 ft (roughly 0.85 psi/ft), or fit an epoxy restrictor or flow-control tap at the shank.
- Never compensate for a badly balanced line by changing the regulator setting that sets carbonation.
Verify after the change.
- Pour and time a pint; a properly balanced line fills a 16 oz glass in roughly 8–10 seconds with a clean head of about 1 to 1.5 inches and no breakout in the line.
- Check the beer tastes properly carbonated, not just foamy on top.
- Measure the line temperature at the tap and confirm it stays at 38 °F during pour.
- Record line length, internal diameter, applied pressure, and temperature for all 12 lines so the next remodel does not repeat this.
3. A different tap foams only on the first pour each morning and pours perfectly for the rest of the day. Diagnose it and give the fix, and explain why it is not a line-balance problem.
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Diagnosis: warm beer in the line and tower. Overnight, the beer sitting in the line between the cold box and the tap warms toward room temperature. Warm beer cannot hold 2.5 volumes at 14 psi, so CO2 breaks out, a gas pocket forms at the high point, and the first pour pushes foam. Once cold beer from the cooler reaches the tap, the rest of the day pours normally. The periodic nature is the tell.
Why it is not balancing. A line that is under- or over-restricted pours badly all day, every pour, and would show up on the flow-rate check. A fault that appears once and then self-corrects is a temperature fault, not a restriction fault. Do not touch the regulator - dropping pressure here would just make the later pours slow and eventually flat.
Fix. Keep the trunk line and tower cold: run a cold-air fan or glycol loop into the tower, insulate the trunk and the shanks, and confirm the tap shank is not warmed by room air or by a nearby heat source. Check that the cooler's air circulation reaches the point where the line exits the box, and that the tower's fan is actually running. Then re-measure tap temperature at the spout in the morning before the first pour.
Second-order checks. If the temperature is confirmed cold and the first-pour foam persists, look for a small gas leak or a failing check valve at that tap, or a shank that is partly blocked with debris. Both are less common than a warm tower.