How to change a keg in a pub cellar: step by step, the British way, without losing a pint every time

How to change a keg: check the new keg has had 48 hours in the cellar, lift the coupler handle up to shut it off, twist the coupler off the empty keg, check it's the same fitting as the new one, twist it on and push the handle down. Then reset the fob detector and pull the line through into a jug until the beer runs clear. Two minutes, and the only beer you should lose is what's in that jug.

Every guide on this that comes up first is American, written for a coupler most British pubs don't use and with gas pressures in PSI. This is the British version, with the bits the American guides leave out: the fob detector, which gas cylinder is which, when a keg should come off even if it isn't empty, and how to stop a keg change turning into a pint of waste every time.

Before you start

• Is it the right keg? Same product, in date, and the oldest one in the cellar. Tennent's and the Enterprise technical guide both say a keg needs 48 hours in a cooled cellar before you connect it, otherwise you'll get dispense problems and fob.
• Is it the same fitting? Inn Express's fitment guide lists six coupler types in UK cellars: A-type, Grundy, KeyKeg, M-type, Sankey and U-type. The coupler on your line has to match the spear on the keg. If a brewery changes its keg, or a guest beer arrives, check before you're stood there with a full bar.
• Is the cellar at 11 to 13°C? If the cellar is warm the new keg will fob whatever you do.

How to change a keg, step by step

1. Stand to the side, not over the keg. Tennent's puts this as a heading of its own: don't stand with your head above the keg while you're changing it. It's under pressure.
2. Shut the coupler off. On most UK couplers you pull the handle out and lift it up. That closes the gas and beer valves.
3. Take the coupler off the empty. Twist it to release it from the spear. A drop of beer comes out, that's normal.
4. Wipe the coupler and the new spear. Clean water and a clean cloth. A sticky coupler is where fruit flies and yeast start.
5. Fit the coupler to the new keg. Line it up, twist it on until it seats, then push the handle down until it locks. If it won't go on, you've probably got the wrong fitting, don't force it.
6. Reset the fob detector. When a keg runs out the float in the fob detector drops and stops gas and foam going up the line. The Enterprise guide says if the float sticks, release it with the button at the base and leave it in the down position. Follow the instructions for your own detector to bleed it until the bowl is full of beer with no air at the top.
7. Pull it through at the bar. Pour into a jug, not a glass, until the beer runs clear and stops fobbing. Then pour a proper pint and look at it.
8. Write it down. Date on the keg, the keg change on the sheet, and what went in the jug on the wastage log.

Check the gas while you're there

You're in the cellar anyway, so look at the cylinder on the board. Inn Express's guide gives the colour code. A CO2 cylinder has a grey body and a purple valve guard and no ID tag. A mixed gas cylinder has a grey body with a green shoulder and a pink valve guard, and must have an ID tag showing the mix. Their instruction is blunt: if a mixed gas cylinder has no tag, don't use it.

Their guide lists 30/70 mixed gas for bitters and stouts, and CO2 or 40/60 mixed for lagers, ciders and pale ales, and says to check the exact mix with the brand owner. Don't change the pressure on the regulator. Tennent's and the Enterprise guide both say that's a technician's job, and turning it up is how a lot of fobbing starts. If you smell or hear a leak, the Enterprise guide says turn off the gas if you can, ventilate the cellar and call your lead brewer. The cellar gas guide has more, including what each cylinder costs per pint.

When a keg should come off even if it isn't empty

When to change, from the brewer guidance below

Container	Change it when	Source
Keg	Empty, on sale longer than 5 days, or past its best before date	Enterprise technical guide, Tennent's
Cask	Empty, or on dispense longer than 3 days	Enterprise technical guide

Cask Marque says a keg on sale longer than about five days absorbs too much gas and fobs. If a line can't sell a keg in five days, that line is costing you money, and the answer is usually fewer lines or a smaller keg rather than more patience. I worked the sums in how many draught lines a pub needs, and out of date beer covers the best before side.

What a keg change costs, and how to keep it small

The jug at step 7 is waste you can't avoid, but you can stop it growing. Say you change ten kegs a week. If each one costs you half a pint pulling through, that's 260 pints a year, about £520 at roughly £2 a pint cost. If a stuck fob detector or a warm keg makes it two pints a time, it's 1,040 pints and about £2,080. Same number of kegs, four times the waste. Those are my illustrative sums, so measure your own: tip the jug into a measuring jug for a week and you'll know.

The other loss at a keg change is the beer you leave behind. The fob detector cuts off when it sees gas, and some couplers and spears leave beer in the bottom of a keg that's gone back as an empty. You'll only know if you check.

One idea: weigh your empties once a month

Get a set of bathroom scales in the cellar. Weigh a keg you know is bone dry, write the weight on the wall, and once a month weigh a few of the empties waiting to go back. Anything over that weight is beer you paid for and didn't sell. A kilo over is about 1.7 pints, because a pint of beer weighs a little over half a kilo. If it's happening on one line, look at that coupler and that fob detector. It's the same weighing trick that makes stocktakes accurate, and the keg calibration guide and how many pints in a keg go through it properly. If empties are going missing altogether, that's a different problem, covered in empty keg returns and missing container charges.

If it still fobs after the change

Run through the list in why is my beer fobbing. If it's only the new keg, it probably hasn't settled. If it's every line, it isn't the keg change at all, it's the cellar, the cooler or the gas.

Common questions

How do you change a keg in a pub?

Lift the coupler handle up to shut it off, twist the coupler off the empty, wipe it and the new spear, twist it onto the new keg and push the handle down. Reset the fob detector, then pull the line through into a jug at the bar until the beer runs clear.

Do you turn the gas off to change a keg?

Lifting the coupler handle shuts the gas and beer valves to that keg, so on most systems you don't need to turn the cylinder off to change a keg. Do turn it off if you're changing the gas cylinder itself, and follow your own equipment's instructions.

How long should a keg settle before connecting?

48 hours in a cooled cellar, according to both Tennent's and the Enterprise technical guide. Connecting a keg straight off the lorry is a common cause of fobbing.

How long can a keg stay on?

The Enterprise guide says change a keg that's been on sale longer than five days, or past its best before date. Cask Marque says a keg left on longer than about five days absorbs too much gas and fobs. Casks should be sold within three days of going on.

Why won't the coupler go on the keg?

Usually it's the wrong fitting. UK cellars use several types, including A-type, Grundy, KeyKeg, M-type, Sankey and U-type. The coupler has to match the keg's spear, so check before you force it.

What is a fob detector?

A bowl with a float on the beer line that cuts off when a keg runs empty, so gas and foam don't go up the line. After a keg change it needs resetting, and if the float sticks there's usually a release button at the base.