Avoiding a Boil Over When Home Brewing

by Brad Smith on May 3, 2014 · 36 comments

FoamingA boil over when home brewing can be a real mess, particularly if it happens on the stove top. Even if you are brewing outside, you risk losing precious wort and hops over the side if your pot boils over. So this week I provide some quick tips on avoiding a boil over.

Why the Pot Boils Over

Wort and water contain a lot of dissolved gasses that are forced out of solution, which is why they are prone to boil over. Wort made with malt contains a lot of proteins, which also tend to foam as they start to boil, and often the foam is what goes over the side.

The risk is largest as the wort is just starting to boil. Wort is also at risk of foaming and boiling over when you add hops or other additives – as these tend to act as nucleation sites. Wort that is starting to boil can really foam up and boil over if you add hops to it.

Wort that boils over on the stove can create a real mess. The brewer is faced with either having to turn off the heat and clean up the mess, or continue with the boil which effectively bakes the wort on the stove – making it really hard to clean up.

Methods for Stopping a Boil Over

  • Get a Big Enough Pot – Using a pot with sufficient headspace is important. You don’t want to try to boil a pot that is full – an extra gallon or two (4-8 liters) of space is required in a typical 5 gallon (19 liter) batch. For 5 gallons/19 liters of extract, this means getting a 5 gallon (19 liter) pot and limiting your boil to 3.5 gallons (14 liters). For all grain brewers, you need a much larger pot – typically 8-10 gallons (30-40 liters) to make a typical 5 gallon (19 liter) batch.
  • Don’t (ever) Cover Your Pot – Covering your pot while heating can result in foaming and boil over when you remove the cover. In addition, covering your wort during the boil traps many volatiles such as DMS which you want released during the boil. So don’t cover your pot when boiling.
  • Stir the Pot – Stirring as you start to boil will help to break up the large bubbles that can cause boil over as well as reduce the foaming. Stirring also keeps you focused on the pot, so you can turn down the heat or remove heat if needed.
  • Use Glass Marbles – Some brewers add glass marbles to their pot. The glass marbles (don’t use plastic!) are inert and can be safely boiled, and having them in the bottom of your pot will break up the large bubbles that are the main cause of boil over. Some brewers also use pennies, but I try to avoid adding metals such as copper which can impact the stability of the finished beer.
  • Use Fermcap (foam control) – A few drops of fermcap at the start of the boil will suppress the foaming and help to prevent a boil over. This is a very effective solution, and relatively inexpensive.
  • Use a Spray Bottle with Clean Water – Spraying cool water over the top of the pot will suppress the foam and help to keep it at bay.
  • Skimming – Though I’ve not used this method myself, some brewers report success with skimming the foam and proteins from the top of the boil pot as they form. If you can prevent the foam from building up, it won’t be able to boil over.

Do you have your own ideas for reducing boil over? Leave a comment below!

Thanks for joining me on the BeerSmith Home Brewing Blog. Be sure to sign up for my newsletter or my podcast (also on itunes…and youtube…and streaming radio station) for more great tips on homebrewing.


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{ 34 comments… read them below or add one }

Butch Dorris May 3, 2014 at 1:11 pm

I keep a spray bottle with with water and hit the foam once it starts to form a head. This seems to work well and reduces the risk of a boil over.

Ken Schauer May 3, 2014 at 1:59 pm

Holding off on the hop additions until after the pot boils helps tons. Turning the heat down once it starts boiling as a preemptive strike and bringing it up to a hard boil slowly.

Removing the hot break seems to help. That’s the oily super fine bubbles that ride the big boil over bubbles.

Sheppy May 3, 2014 at 5:29 pm

Personally, I am a big fan of the Fermcap. Have not worried about boil offers since I started using it.

Sheppy May 3, 2014 at 5:30 pm

Boil OVERs. Not boil offers.

Scott Ickes May 4, 2014 at 3:47 am

The most common way to prevent a boil over is to have a spray bottle of cold water handy and just spritz the foam with it. It knocks it down immediately! My water bottle is always right next to my boil pot.

Scott May 4, 2014 at 3:51 pm

Does the addition of wheat in the mash contribute to the amount of foam that gets created at the start of boiling? It sure seems like it.

Calvin Nin May 4, 2014 at 3:53 pm

I pre-rinse my hops and other additions with the wort after the mash to reduce nucleation sites. I notice that I get crazy boils right after adding things to the pot and this hleps. I put rinsed things in a clean bowl and throw them into the boil when it is time.

dan May 5, 2014 at 10:03 am

A watched pot never boils. So the moment you turn away it boils! Just kidding, but I use an electronic remote thermometer that alarms at about 195 f and alerts me to watch the batch. Otherwise you can stand there a long time and Still walk away at a most inopportune time.

Denny Conn May 8, 2014 at 1:09 pm

I used to be a big believer in Fermcap, then I learned that commercial breweries that use it are supposed to filter the beer to remove it. Supposedly, it can block your kidneys. Now, I realize we don’t use much and that’s it’s a long shot, but why invite health problems if you don’t have to? I’ve gone back to using a spray bottle and burner control. Just as effective and no health worries.

MJC May 8, 2014 at 1:20 pm
happy hillbilly May 10, 2014 at 9:52 am

STIR STIR STIR!!! I find stirring the top two inches of the boil keeps down boilovers . have to watch though especially right at start of boil

beerbrewer May 13, 2014 at 1:52 pm

I use an electric fan that blows right on top of the kettle. It keeps the foam from forming but allows the vigorous boil we all desire.

St. Joe Blues May 13, 2014 at 2:01 pm

I do a lot of canning (tomatoes, salsa, applesauce) as well as beer making, and have the same issues with boil over. What I’ve found works real well is a breeze blowing over the top of the pot during those critical moments when boil over can occur. It reduces the surface temps enough to allow for a controlled release of the foam. In a pinch, you can even blow across the surface of the wort.

Grayson Saunders May 13, 2014 at 3:39 pm

We use a huge inline extraction fan that causes lots of to kill the foam in our 500liter pot.put in hops slowly,turn heat back up for a good hot break after a slow start

Dennis T. Stevens May 14, 2014 at 3:13 pm

I agree with Denny Conn above. I use to use Fermcap when I brewed in a 5 gallon kettle to avoid boil overs. I did some research and didn’t like the implications that if you don’t filter some of the silicon could remain in your kidneys. I was able to obtain a 17 gallon SS pot from a restaurant that was closing for about $50. This much surface area has prevented any boil overs since I started using it.

Mark Galli May 16, 2014 at 7:50 am

Can you use a spray bottle with Star Sans to control boil over? Any reason not to? That’s a spray bottle I already have handy when I’m brewing.

Matthew Sipos May 16, 2014 at 10:16 am

I purchased better burners. I was using a typical turkey fryer burner where the heat was concentrated in the center of my 15 Gal brew pot. By spreading the heat out over a larger surface, better heat control, a little bit of stirring is all I needed to control my wort. My last boil, I had pulled 13.5 gals for an hour boil and never got close to a boil over. I am also a firm believer in a good rolling boil and not a hard boil.

Julien Paquette May 27, 2014 at 8:56 pm

I use a large pot, slowly bring the wort to a boil, add my hops as soon as the temp hits 100 Celsius and start smoothing the top of the wort with a spoon. The rolling starts a couple of seconds after, then I adjust the heat to get a perfect rolling boil. It never boils over and the hops stay in the wort.

mark gooch May 30, 2014 at 1:42 am

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TripleHopped June 7, 2014 at 3:13 pm

I’m a firm believer in the Skimming technique. I use an 8 1/4 gallon brew pot and start with 8 gallons for all my beers. No boil overs here.

Cameron June 24, 2014 at 5:35 pm

I gotta echo the “breeze” or fan comments.

But I use the low tech method. I blow on it. When excessive foam starts to build up, I make sure my temperature is low enough and I just blow on the foam. The passing air breaks the bubbles and allows the gasses to escape. Eventually at hot break the foam starts to break down anyway.

Easy and completely free.

Scott June 24, 2014 at 5:47 pm

If you brew outside, chances are you have a garden hose handy. Get an adjustable spray head that has a mist setting. No pumping and it does double duty spraying down hops stuck to the pot wall!

Digglr June 25, 2014 at 12:09 am

Mark Galli… I use Star San to stop my boil overs all the time… works very well, and as you said.. always on hand. Never had any ill effects on my beer.

Paulsner July 3, 2014 at 12:11 am

I keep a few ice cubes handy. When the boil starts, I turn down the heat and drop a few ice cubes in the wort. The foam subsides immediately but the boil continues.

Ron Thiessen July 26, 2014 at 1:57 pm

Consider going to an electric element in your boil kettle the next time you upgrade. It is instant on and instant off with no flame and no other heat sink to cause boil overs. When you cut power to the element the boil stops immediately and settles within seconds. Foamier worts may require you to do this two or three times but the whole process only takes a few minutes. An added advantage is that electric elements won’t cause caramelization.

Captjpr3 August 15, 2014 at 11:10 pm

I just got a bigger pot. Of course then the temptation is to brew a bigger batch. Self control is MY issue.

ABIMBOLA D October 14, 2015 at 11:05 am

what percentage should be the standard for wort kettle headspace

Brewbuzzard December 21, 2015 at 5:37 pm

I leave enough head space, skim the top of the boil(this also is good for the finished beer), add hops after 30 minutes (I boil 90 minutes), and always have a spray bottle with filtered water near by. As long as I do these things I never have a boil-over.

Matt February 25, 2016 at 7:41 pm

I read somewhere that someone who had an SS mash paddle used that to control boil overs. If it started to boil over he just stuck it in the kettle to drop the heat by allowing the paddle to absorb some thermal energy!

Mark February 27, 2016 at 12:11 pm

I have had good luck with adding a few hop pellets as the wort comes to a boil, the oils help reduce the foam. A tip that Sam Calagione lists in his book “Extreme Homebrewing”. It has worked for me everytime.

MJ Rodney September 30, 2016 at 8:42 am

I second the vote for a fan blowing across the top of the pot. The breeze is not enough to affect the rolling boil, and it keeps the hot break/boil over at bay.

I only watch my pot for the first 3 or 4 minutes, just to sure, but from then on, it maintains itself.

It always seems to work.

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James Mccartan March 9, 2019 at 3:20 pm

I use Temp control and never let my boil go over 99c or below 97c and never more than 28 litres in a 31 litre pot also no lid never get a boil over.

Mark Bilodeau March 27, 2021 at 5:26 pm

Chalk up another vote for using a fan to blow air across the surface of the boil. I use an 8 gallon pot on the stove top to do 5 gallon batches, so it can get pretty dicey at times. But I found that at small office fan set to low blows enough air across the top (along with stirring just until the hot break subsides) does the trick.

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