Cask breathers: what one does to your yield, your waste bin and your stocktake

A tapped cask is on a clock. Cask Marque's guidance is to empty it within three days of going on sale, and from day four the drain starts taking what your customers didn't. A cask breather is a £47 valve that slows that clock by keeping air out of the cask. CAMRA has been neutral on it since 2018, most drinkers can't taste the difference in blind trials, and for a pub whose second handpull sells slowly it can be the difference between a firkin yielding 60-odd sellable pints and yielding 50. Here's what it actually does, the arithmetic, and the one way it makes things worse.

Every pint I've ever poured down the drain at the end of a cask had already paid its duty. That's the bit that stings. The breather question comes round every autumn, when pubs put an extra ale or two on for festival season and then watch them slow to a crawl midweek. Cask Ale Week runs to 27 September this year with a record number of pubs taking part, so plenty of cellars are carrying more cask right now than they will sell fast. This is the licensee's version of the breather argument, with sums, not the beer-forum version.

The three-day problem, in pints

The timetable from Cask Marque's cellar guidance: once a cask is on sale, aim to empty it within three days. A firkin holds 72 pints nominal. After sediment, line fills and sampling you're realistically selling somewhere in the mid 60s from a well-kept one, and I use 65 as a working figure.

So to clear a firkin inside the quality window you need to be pulling around 22 pints of that one ale every day it's on. Plenty of pubs manage that on the house bitter over a weekend. Very few manage it on the third handpull on a Tuesday.

Author's arithmetic on a firkin with 65 sellable pints. Your yields will differ; the shape won't.

Sales rate	Days to empty the firkin	Pints still in it at the end of day three
22 pints a day	3 days	None
15 pints a day	4.3 days	About 20
10 pints a day	6.5 days	About 35
6 pints a day	10.8 days	About 47

That right-hand column is the choice you face on the slow lines. Sell those pints past their best, bin them, or slow down what the air is doing to them.

What a cask breather actually is

CAMRA's own technical explainer describes it as "a relatively simple automatic device which applies carbon dioxide (or another neutral gas) at exactly atmospheric pressure". As beer is drawn out through the pump, the valve tops up the space above the beer with the same volume of gas, so the cask never pulls in air. No pressure is applied to the beer. It is not the same as pushing beer with gas; the beer still comes up the line because someone pulls the handpump.

What that does, per the same explainer, is slow oxidation, which is where the stale cardboard and sherry flavours come from, and stop airborne bacteria getting into the cask. What it does not do is fix anything else. A dirty line will spoil beer on a breather just as surely as off one. So will a warm cellar, and so will a beer that was tired when it arrived.

The CAMRA thing, since someone at the bar will raise it

CAMRA's position since 2018 is neutral: "neither encouraging or objecting to their use". Before that the line was that breathers had a place in limited circumstances, mainly low-turnover outlets, with a worry that they'd encourage pubs to overstock. Worth knowing: in CAMRA's own taste trials, only a very small minority of drinkers could tell breather-served cask from traditionally vented cask.

The old worry was fair, mind. A breather that lets you keep a beer on sale past its prime is not doing you a favour. More on that below.

What it costs and what the sellers claim

Cellar equipment supplier RLBS sells the breather valve itself at £47.10 and pre-made aspirator set-ups from £150.09 to £363.96 depending on configuration, plus a few quid for spiles and disconnects. It runs off your existing CO2 supply, and because it only tops up at atmospheric pressure the gas use is small change next to what your keg dispense already burns.

The claim on their page is that it extends the quality life of a cask "up to 3 times" against the roughly three days you get unassisted. That's the vendor's claim, not mine, and I'd treat it the way you treat any vendor claim: the honest read is a few extra days, with the beer still declining, just slower. Your palate stays the referee. Pull a taster every session on any cask past day three, breather or not.

The money side

Say one slow cask a week leaves you binning 12 pints. At the average cask pint price of £4.54 (CGA's on-trade measure, reported in May 2026), that's £54 a week in lost sales, call it £2,800 a year, from one handpull. My arithmetic, and deliberately conservative; the table above shows a 10-pint-a-day line leaving three times that in the cask at day three.

Against that, a one-off £47 to £364 on kit. Even if a breather only rescues half those pints, it pays for itself inside a couple of months. And the drain gives you nothing back: duty on drain-poured beer from an opened cask is not something you can claim, and the supplier credit routes for beer that arrived faulty or spoiled unopened have their own rules, which I've covered in the ullage and wastage claims guide. The short version is that the end of a slow cask is almost always your loss, at full cost, duty included.

Where it earns its keep, and where it's a plaster

It earns its keep on the second and third handpull, on Monday to Wednesday trade, on a stronger ale that sells steadily but slowly, and on the week after a festival when you're carrying more cask than usual. If you've just run a beer festival off my cask festival planning guide, the leftover casks are exactly the use case.

It's a plaster if every cask in the cellar needs one to survive. That's not an air problem, that's an ordering problem. If the maths in that table says a line can't clear a firkin in under a week, the answer is a pin instead of a firkin, fewer handpulls in winter, or dropping the line. A breather used to dodge that decision just moves the loss from the drain to the customer's glass, and they only forgive that once. The old CAMRA worry about overstocking was right; the fix is to buy to your real pull-through, which is what your par levels should be telling you anyway.

What it does to your stocktake

Cask is already the flakiest line in most stock results, and yield assumptions are why. If your stock system assumes 66 sellable pints a firkin and your slow lines are actually yielding 55 because the last inch goes down the drain unrecorded, your variance shows a phantom loss and you end up suspecting people for what is actually chemistry. Two habits fix it.

First, log every drain pour in the wastage log with "end of cask" as the reason, at the time, not from memory on count day. Second, watch yield per cask by line, not just overall draught variance. Do that and a breather shows up in your numbers as the slow lines' yield climbing towards the fast lines'. If it doesn't, either it's fitted wrong or air was never your problem, and both of those are worth knowing before you buy a second one. The cellar management basics guide has the rest of the routine, and the calibration guide covers getting the container sizes right in your system, which matters more than any gadget.

Common questions

Does a cask breather spoil real ale?

In CAMRA's taste trials only a very small minority of drinkers could tell the difference. What definitely spoils real ale is serving your slowest line on day five with no protection at all. Judge it against that, not against a perfect cask that empties in two days.

Is beer served on a breather still real ale?

CAMRA has been neutral since 2018, neither encouraging nor objecting. The beer is still live, still unpressurised, still pulled by hand. Whether your regulars care is a conversation for your bar, but the taste trials say most of them wouldn't know.

How long does a tapped cask last without one?

Cask Marque's working rule is three days on sale. Sellers claim a breather stretches quality life up to three times that; treat that as a claim and taste the beer every day either way.

Can I claim the duty back on a cask that's gone off?

Not by pouring it away. Full unopened containers can go back for supplier credit, and destruction on site only ever recovers the duty element with the brewer's sign-off. The end of an opened, mostly sold cask is your loss. The ullage claims guide walks through what's claimable and what isn't.