I did a brew on Saturday and cooled, got in the fermenting bucket and added yeast, added starsan to the chamber in the double bubble airlock and got it under the stairs. Ever since then, the starsan gets sucked into the fermenter, fairly quickly as well (after I replace it). I've heard this is due to temperature change creating a vacuum. I've also heard that that shouldn't happen with this type of airlock. Its not happened before to this extent (i.e. I've never had to replace the starsan).

Any ideas why this is happening? And is there a way I can prevent it? I'm not too concerned right now as the yeast is still working and producing carbon dioxide but when it finishes I still plan to leave it (three weeks in total) so I don't want the possibility of oxygen getting in.


4 Answers 4


This is caused by a drop in temp before co2 is being produced. Just cap the fermenter in sanitized foil until you're past the lag phase, or cooled to fermentaion temp. Though a little bit of starsan won't hurt much, foil is better than an open airlock IMO.

I don't put air locks on until the wort is at fermentaion temp. I also remove the airlock then foil cap when cold crashing to prevent the same issue.

  • ok thanks, useful info. Regarding the foil, at the moment the airlock has that little red cap on it. Is that as good as putting the foil on?
    – Rich Prag
    Commented Oct 23, 2017 at 18:32
  • @RichPrag I would use foil. Rubber caps will pop off when fermentation starts. Rubber corks too. I've even seen rubber corks get sucked into carboys from temp drops. Commented Oct 24, 2017 at 12:26

You're putting too much liquid in the bubbles of the airlock. If each side is only filled about one-third, none of the liquid will either be sucked out or blown out.

The fluid in a properly-filled airlock will not get sucked out. As you can see in the picture of the properly-filled airlock below, there is not enough fluid to overflow the bubble chambers in either direction.

enter image description here

Also, as barking pete has answered, you might as well just fill the airlock with water.


In the comments below Wyrmwood says:

The pressure in a fermentation chamber with a good seal after moving from ale temps to a fridge to cold crash is plenty to empty a standard airlock or to suck a half-filled quart jar dry with a blow off tube.

My reply is that sure, it could suck fluid out of a blow-off container through a blow-off tube, but no, it won't suck the fluid out of properly-filled airlock such as the one pictured here. It will suck air through the fluid, but the fluid will be left behind.

  • This is wrong. Cold crash with an airlock (or any change in temp/pressure) and all of the sanitizer will get sucked in. It just needs negative pressure.
    – Wyrmwood
    Commented Nov 6, 2017 at 18:16
  • 1
    @Wyrmwood: Try a thought experiment. If there was 1 drop of fluid in the airlock, would it get sucked in? How about 2 drops? 3 drops? Keep going up until each side of the airlock is 1/3 full. Now stop. You have a properly-functioning airlock whose fluid will not get sucked back or blown out.
    – Jeff Roe
    Commented Nov 6, 2017 at 18:31
  • Try a real experiment. Fill an airlock any way you like, then blow gently. Notice only air flows. Now blow hard - notice, whatever you filled it with is ejected. Now try again with Star san and suck as hard as you can.
    – Wyrmwood
    Commented Nov 6, 2017 at 20:36
  • 2
    I agree that Wyrmwoods proposals might work as described but I doubt the contents of a 5gal/25litre fermentor could match the maximum output rate of my lungs. So the fact that blowing/sucking hard enough can empty the air-lock is not so relevant. Standard airlocks work reasonably well on 100L fermentors and they really blow the CO2 out. But I can still blow harder! Generally I fill and airlock so that just a little more than the narrow bottom bend is filled. Mr Pasteur might have argued no liquid is necessary anyway. Commented Nov 8, 2017 at 5:11
  • The pressure in a fermentation chamber with a good seal after moving from ale temps to a fridge to cold crash is plenty to empty a standard airlock or to suck a half-filled quart jar dry with a blow off tube. This is empirical, not theoretical. I recommend plain water for your blow off or air lock and highly recommend switching to a water filled airlock if using a blow off tube before cold crashing. YMMV.
    – Wyrmwood
    Commented Nov 8, 2017 at 16:12

One sensible suggestion is don't put starsan in the bubble trap - just put water in there. There is no need to use an antiseptic in the air lock.

  • Actually you use sanitizer for this reason. If there's an unexpected temp drop (control failure) and air lock fluid sucks in, you don't want a fluid that can't neutralize the contaminates its trapped. It would basically be worse than leaving a fermentor open to air, if the contaminated water sucks into your brew, it gets all the bad at once. Commented Nov 8, 2017 at 3:33
  • Would it really be worse than leaving a fermentor open to the air? The top of the airlock is smaller than the neck of the fermentor so less would be collected. There is also a slight positive pressure - more so than one would get from the open fermentor. That would tend to deter entry to airborne particles. So what was collected would be less than what might fall into the open fermentor. Commented Nov 8, 2017 at 4:59
  • yes, I should have phrased it dry air lock, not open air. Commented Nov 8, 2017 at 5:05
  • 1
    for example if you get a fruit fly trapped in the airlock, you would want this to be sanitised before any chance of it being suck back into your brew.
    – Mr_road
    Commented Nov 15, 2017 at 11:49
  • I take your point but I don;t think starsan would sanitise it. I would rather recommend a gauze filter to prevent the flies, etc. I do like your vodka idea though... Commented Nov 16, 2017 at 0:39

I use vodka in my airlocks, you know, just in case it gets sucked in.

  • how much effect does this have on the brew if it does get sucked in?
    – Rich Prag
    Commented Nov 15, 2017 at 12:07
  • very little as it is such a small quantity of vodka, but even at such a small volume if it were full of bacteria and wild yeast from the environment it could easily spoil the brew.
    – Mr_road
    Commented Nov 20, 2017 at 16:24

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