How to purge a beer keg and the dispense line during service
Purging a beer line correctly during service prevents foam surges, gas pockets, irregular flow, and waste when changing a keg. But purging does not mean opening the tap uncontrollably until you waste beer: it means removing gas or foam from the right area, restoring continuous flow, and leaving the system stable for service. The key is knowing when to purge, how to do it, and when the problem is not purging but pressure, temperature, cleaning, coupler, or system design.
Quick summary
Purging a beer line is used to remove gas, foam or trapped air after an empty keg, a keg change, a service stoppage or an incident in the line. If there is a cellarbuoy or FOB, purging is done at the detector to refill the chamber with beer and avoid filling the whole line with foam. If there is no cellarbuoy, you purge from the tap until the flow is continuous, clear and without bursts of foam. Afterwards you must check pressure, temperature, taste and flow rate.
Purging, cleaning and pressure adjustment are not the same thing
Purging removes gas or foam from the circuit at a specific moment. Cleaning removes residues, yeast, biofilm, beer stone and line contamination. Pressure adjustment balances the system so the beer comes out with the correct flow and foam. Confusing these three concepts is one of the most common causes of product loss and incorrect diagnoses.
Guide contents
- What purging a beer line means
- When you need to purge
- When you should not purge without a diagnosis
- Purge with cellarbuoy or FOB
- Purging without a cellarbuoy
- Step-by-step guide to changing the keg and purging
- Pressure, temperature, and foam
- How to reduce waste
- Common mistakes
- Cleaning and maintenance
- What to check or buy
- Frequently asked questions
What purging a beer line means
Purging a beer line consists of expelling gas, foam or trapped air from the circuit so that the beer once again flows continuously from the keg to the tap. It is a common operation when changing an empty keg, especially when the line has been partially filled with foam or gas.
In a short system, purging can be done from the tap, collecting the initial beer and foam in a container. In a long installation, it’s advisable to use a cellarbuoy or FOB, also known as a foam detector, to prevent the entire line from filling with foam when the keg empties.
Removes trapped gas or occasional foam to restore a steady flow.
Removes dirt, residues, microorganisms and deposits from the line.
Balance pressure, flow, temperature and flow control.
When to purge a beer line
Purging makes sense when there is a product interruption, gas entering the circuit or foam in the line. The most common case is a keg change: the keg empties, the coupler starts taking gas, the flow stops or foam reaches the tap, and you need to recover stable liquid beer from the new keg.
| Situation | What happens | What to do |
|---|---|---|
| Empty keg | The system starts sending gas or foam instead of beer. | Change the keg and purge until you recover a continuous beer flow. |
| FOB/cellarbuoy tripped | The detector has shut off the flow to avoid filling the line with foam. | Tap a new keg, purge the FOB and reset it correctly. |
| Air or gas entering the line | Bubbles, bursts of foam or intermittent flow appear. | Check the connection, coupler, seals and purge after correcting. |
| Product change on one line | Previous product or foam may remain in the line. | Clean if the product changes, or purge if you are only replacing a keg of the same product. |
| Long service break | The first beer may come out warm, foamy, or stalled. | Purge only as much as necessary and check chilling, line and cleanliness. |
| Long-draw system without detector | When the keg empties, a lot of line can fill with foam. | Consider installing a cellarbuoy/FOB to reduce waste. |
When you should not purge without a diagnosis
Not all foam problems are solved by purging. If the beer comes out foamy all the time, if the pressure is poorly regulated, if the keg is warm, if the line is dirty, or if the tap is obstructed, opening the tap to purge will only increase waste.
Don’t purge liters and liters without knowing the cause
If after a small purge the beer still comes out foamy, with off-flavors, irregular flow or high temperature, stop service and check the system. Purging does not fix incorrect pressure, a warm keg, a dirty line, a misconnected coupler or a blocked tap.
How to purge with a cellarbuoy or FOB
The cellarbuoy or FOB detects when the keg runs out of beer and prevents the line from filling completely with foam. This is especially useful on long lines, cold rooms far from the bar, multi-line setups or high-volume venues.
When the keg empties, the detector cuts or restricts the flow. After connecting a new keg, you open the cellarbuoy purge to expel the gas or foam from its chamber until it fills with beer again. Then you reset it or leave it in service position, depending on the model.
1. Detect an empty keg
The flow drops, foam appears, or the cellarbuoy/FOB runs empty or gets blocked.
2. Close service
Avoid continuing to pour from the tap if the system has already detected a lack of product.
3. Change the keg
Disconnect the empty keg and connect the coupler correctly to the new keg.
4. Open the detector purge
Vent gas or foam until the cellarbuoy chamber fills with beer.
5. Reset the device
Leave the float or mechanism in service position according to the model’s instructions.
6. Check the tap
Pour a small sample and confirm a stable flow, without bursts of foam or off-flavors.
The FOB is also cleaned.
The cellarbuoy or FOB must not be left out of maintenance. If it gets dirty, it can block, contaminate the beer flow, cause false shutoffs, or make line cleaning more difficult.
How to purge without a cellarbuoy
If the system doesn’t have a cellarbuoy or foam detector, purging is usually done from the tap. This option works for short lines, portable units, or simple setups, but it creates more waste in long lines because the foam can travel through the entire circuit to the point of service.
1. Place a container
Collect the initial foam and beer. Don’t purge directly onto the drip tray if it doesn’t have enough capacity.
2. Open the tap in a controlled way
Let gas, foam, or irregular beer run until a stable flow is restored.
3. Watch for the change
The line must go from foam or gas bursts to clear, continuous liquid beer.
4. Close the tap
When the flow is stable, close it and let it rest for a few seconds if there has been a lot of foam.
5. Pour a test sample
Check foam, flavor, temperature and flow before returning to normal service.
6. Record your losses
If each keg change wastes too much product, consider installing a FOB/cellarbuoy.
Step by step: changing a keg and purging during service
Changing the keg should be done calmly, even during peak hours. A quick but poorly done change can cause foam, loss of pressure, gas leaks, product waste, and longer delays.
| Pitch | Action | What to check | Common mistake |
|---|---|---|---|
| 1 | Confirm that the keg is empty. | Flow, foam, keg weight or FOB detector. | Change the pressure before confirming the keg. |
| 2 | Close or isolate the line if appropriate. | Tap closed, drip tray ready and gas under control. | Leaving the tap open while handling the keg. |
| 3 | Disconnect the empty keg. | Coupler released without stressing seals or connections. | Forcing the coupler or turning it incorrectly. |
| 4 | Tap the new keg. | Correct type of coupler, firm connection, and no leaks. | Using an incompatible or poorly seated coupler. |
| 5 | Purge at the cellarbuoy or tap. | Gas/foam discharge until continuous beer is restored. | Purging too much or not enough. |
| 6 | Check pressure. | Stable pressure gauge within the range defined for that system. | Randomly raising or lowering pressure. |
| 7 | Serve a sample. | Foam, flow rate, temperature, aroma, and flavor. | Returning to service without checking the first glass. |
Do your keg changes generate too much foam?
We can check whether the issue lies in purging, pressure, temperature, line length, coupler, tap, compensator, cleaning, or the absence of a cellarbuoy/FOB.
Request technical inspection View cleaning and maintenancePressure, temperature and foam after purging
After purging, the beer should pour with a stable flow. If it still comes out foamy, you must check the three main variables: temperature, pressure and hygiene. A warm keg releases CO₂ easily; incorrect pressure throws the system off balance; a dirty line creates turbulence, blockages and off‑flavors.
| Symptom | Possible cause | What to check |
|---|---|---|
| Continuous foam after purging | Warm keg, high pressure, warm line, or dirty tap. | Keg temperature, cooler, pressure and cleaning. |
| First burst of foam and then fine | Warm final section, warm tap or line with bubbles. | Column, final stretch, insulation, and cold recirculation. |
| Very slow flow | Low pressure, blocked line, flow restrictor closed or FOB not properly reset. | Regulator, tap, detector, line and head. |
| Flat beer | Low pressure or loss of carbonation. | Gas, regulator, leaks, temperature, and time connected. |
| Off flavor | Dirty line, old product, contamination, or insufficient cleaning. | Thorough cleaning of line, tap, coupler and cellarbuoy. |
| Visible bubbles in the line | Gas inlet, leak, damaged gasket, or poor connection. | Coupler, seals, fittings, check valves, and pressure. |
How to reduce waste when changing kegs
Shrinkage during keg changes is not completely eliminated, but it can be greatly reduced with a well-designed installation and proper routine. On long lines, the biggest savings usually come from preventing the line from filling with foam when the keg empties.
- Train staff in keg changing and purging.
- Use a cellarbuoy/FOB on long lines.
- Keep kegs cold and stabilized.
- Check pressure before service.
- Identify each line and each coupler.
- Log recurring incidents.
- Continuing to pour when the keg is empty.
- Purging from the tap on long lines without a detector.
- Increase pressure to “push faster.”
- Not resetting the FOB after the change.
- Working with warm kegs.
- Not cleaning lines, taps, and sensors.
Common mistakes when purging a beer line
| Error | Consequence | Solution |
|---|---|---|
| Purging for too long | Unnecessary waste and economic loss. | Purge only until continuous flow is restored. |
| Not purging after the keg is empty | Bursts of foam, irregular flow, and a poor first pour. | Purge at the FOB or tap before going back into service. |
| Not resetting the FOB | No beer comes out or the flow is insufficient. | Reset following the installed model. |
| Adjusting pressure without a clear rationale | More foam, flat beer, or overcarbonation. | Adjust pressure according to temperature, style and line. |
| Confusing purging with cleaning | Persistent off-flavor and contaminated line. | Carry out a full cleaning if there is an odd smell or taste. |
| Ignoring leaks at the coupler or fittings | Gas ingress, loss of pressure, and bubbles in the line. | Check seals, fittings and non‑return valves. |
Cleaning and maintenance: what purging can’t fix
Purging is a service operation. Cleaning is a hygiene and maintenance operation. A line with residues, biofilm, beer stone, yeast, or contamination is not fixed by purging. The first beer might come out clear, but the flavor, aroma, foam, and system safety will still be compromised.
Cleaning must include the line, tap, coupler, fittings, cellarbuoy/FOB, drip tray and all elements in contact with the beer. In high-volume installations, with many lines or long runs, it is advisable to document keg changes, incidents, cleanings and inspections.
If it smells bad or tastes odd, it’s not a purge: it’s maintenance
If after changing the keg and purging correctly the beer still tastes sour, metallic, rancid, dirty or different from the original product, the line needs to be cleaned or checked. Do not keep purging litres hoping it will fix itself.
What to check or buy to purge better
A good purge depends on a well-designed installation: correct coupler, stable gas, suitable line, leak-free fittings, foam detector if appropriate, clean tap and regular maintenance.
| Need | Recommended product or service | Internal link |
|---|---|---|
| Change the keg correctly | Extractor head compatible with the keg type. | Keg couplers |
| Control pressure | Regulator, pressure gauge, CO₂ cylinder or suitable gas blend. | Gas and regulators |
| Avoid leaks and bubbles | Fittings, gaskets, quick-connects, non-return valves, and clamps. | Connectors and fittings |
| Renew the beer line | Food‑grade tubing with the correct diameter for the installation. | Dispensing tubes |
| Improve flow and head | Tap with flow control, clean tap and well-chilled tower. | Beer taps |
| Reduce losses in long-draw systems | Compatible Cellarbuoy, FOB or foam detector. | Dispensing accessories |
| Eliminate off-flavors | Cleaning canister, detergent, adapters, and professional cleaning. | Cleaning and maintenance |
| Resolve recurring issues | Technical check of pressure, cooling, line, tap and connections. | Installation and technical service |
Purge checklist during service
- Confirm whether the keg is empty before touching the pressure.
- Close the tap and prepare a collection container.
- Untap the empty keg without forcing the coupler.
- Tap the new keg with the correct coupler.
- Check that there are no gas or beer leaks.
- Purge at the cellarbuoy/FOB if present.
- If there is no detector, purge from the tap only as much as necessary.
- Wait until the beer runs continuously, clear and without bursts of foam.
- Check pressure and temperature if the foam persists.
- Taste the product before returning to normal service.
- Log an incident if it recurs with every keg change.
- Schedule cleaning if there is off-flavor, bad smell, irregular flow, or persistent foam.
Reduces foam, waste, and service interruptions
At Install Beer we can help you check the entire system: couplers, gas, pressure, cooling, cellarbuoy/FOB, lines, taps, cleaning and maintenance so that every keg change is fast, clean and with less product loss.
Consult with Install Beer View cleaning and maintenanceFrequently asked questions about purging beer kegs and lines
What does it mean to purge a beer line?
It means expelling gas, foam, or air that has built up in the line to restore a continuous flow of beer from the keg to the tap.
When do you need to purge a keg of beer?
Usually when changing an empty keg, when the line has taken in gas or foam, when the cellarbuoy or FOB has cut the flow, or when the pour is irregular after an incident.
How do I know the line is already purged?
When the beer flows continuously, clear, without gas surges, without excessive foam and with a steady flow. Afterwards it’s a good idea to check flavor and temperature.
What is a cellarbuoy or FOB?
It is a foam detector that cuts or limits the flow when the keg is empty, preventing the line from filling completely with foam and reducing waste during keg changes.
Does purging fix a beer that always pours foamy?
Not necessarily. If the foam is continuous, you need to check temperature, pressure, tap, flow restrictor, cleaning, coupler, line, and keg condition.
Should I increase the pressure to purge faster?
No. Increasing pressure without a clear rationale can increase foam, alter carbonation or create more problems. Pressure must be adjusted according to temperature, style, container, distance and line.
Does purging replace line cleaning?
No. Purging removes localized gas or foam. Cleaning removes residue, contamination, biofilm, beer stone, and line, tap, coupler, and fitting dirt.
Why does beer taste strange after purging?
There may be dirt in the line, spoiled product, contamination, lack of cleaning, oxidation, or problems in the tap or coupler. In that case, the system must be cleaned and checked.
Is it worth installing a FOB on long lines?
Yes, it is usually advisable on long lines or high-volume setups because it prevents the entire line from filling with foam when the keg empties and reduces product loss.
Can Install Beer check a line with purging or foaming issues?
Yes. Install Beer can check pressure, cooling, couplers, lines, taps, cellarbuoy, cleaning and maintenance to improve service and reduce waste.
Technical note: purging procedures may vary depending on keg type, coupler, cellarbuoy/FOB, line length, cooling system, pressure, gas and the beer being served. Before changing the pressure or dismantling components, it is advisable to follow the procedure for the installed equipment or request technical support.