What matters - Boil-off volume, percentage or time?

Deej · Feb 2, 2025

Hi everyone, first proper post since joining the forum

.

I have a 30 litre kettle (BIAB) but normally brew half size batches as I like to have more variety/more brewing. One downside of this is you lose quite a large percentage of the wort to boil-off (3l/hr), kettle (2l below tap) and fermenter (2l trub).

I can't do much about the kettle and fermenter losses, but I was wondering whether it's really necessary to boil the wort for an hour as per most recipes. If I brewed full size batches I'd still lose 3 litres to the boil, but this would proportionally be half the amount.

I appreciate there are numerous reasons for the boil stage, but to achieve these should you be targeting:

Boil-off volume (litres)?
Boil-off percentage?
Boil-off time?

Hopefully that makes sense!

Cheers, Dave

Stu's Brews · Feb 2, 2025

Welcome to the forum - you've accidentally picked a quite contentious topic for your first post!

I think the easiest answer to your question is that the important bit is boiling for the length of time required to achieve all the necessary boil actions. The boil-off volume and percentage are a consequence of that.

The contentious part is what the time should be; a 60 minute boil is fairly standard but some will go shorter and a few others will go longer.

Ultimately, the only way to know what is right for your setup and/or tastes will be experimentation.

Caramel Ox · Feb 2, 2025

Well you aren’t losing wort during the boil it’s only water…you could choose to start with more water or add more water at any stage.

Rwilts · Feb 2, 2025

Highly recommend some brewing software. You can set a profile of your equipment and tell it how much wort you want at end of process and it will tell you starting volumes etc...

This may also answer some of your questions

Agentgonzo · Feb 2, 2025

Boil off should normally target a percentage of the volume and a target time.

The percentage is normally 4-12% for a 1 hour bill going by a large selection of professional breweries across all sectors across the world. For homebrew, this normally translates to 2-3L in a 30L kettle.

If you do a half batch, you'd want to reduce the boil power (many units allow you to do this). Boiling too hard has negative effects on your wort (sorry, it's late so I can't remember the details at the moment), ends up driving off more water than is necessary and is just a waste of energy. If you can't reduce the boil power, you can either boil for a shorter amount of time (can of worms!) or top up with extra water or mash/sparge with more water to compensate

As for boil length, yes it's controversial for some reason

. Traditionally 60 mins but people are also happy with 30 or 90 or 120 minute boils.

MashBag · Feb 2, 2025

I like 45 mins. Top up to final volume in the fv. I don't use the taps, because of the losses. Syphon. If you catch a little trub it will settle out later.

No Maths or software required

foxy · Feb 3, 2025

Rwilts said:
Highly recommend some brewing software. You can set a profile of your equipment and tell it how much wort you want at end of process and it will tell you starting volumes etc...

This may also answer some of your questions

Hardly qualifying for a post when David Heath in 2015 was asking how to make kit beer.
There is plenty of information about wort boiling from qualified experts as Stu's Brews mentioned it is a can of worms. At the end of the day no one gives a monkeys how long you want to boil the wort, it is up to you to read all the relevant information and do your own thing, and there is plenty of empirical information out there about the why's and wherefores of the length of boils.

Agentgonzo · Feb 3, 2025

MashBag said:
I don't use the taps, because of the losses. Syphon. If you catch a little trub it will settle out later.

If you've got a tap on your kettle, you can always tilt the kettle a bit to get more wort out and (like Mashbag) stop when you get a bit of trub. It doesnt' matter if some gets into the fermerter. You can probably get an extra litre out like this.

Deej · Feb 4, 2025

Some really useful replies there, much appreciated.

I've been using brewfather recently and it's great. My equipment profile is pretty dialled in so the brews turn out as expected. I generally end up with about 8 litres bottled from about 15 litres of strike water.

I think I'll continue doing one hour boils for now but will keep them very gentle. Will save a little bit of gas at least!

Cheers

Davegase · Feb 5, 2025

Rwilts said:
Highly recommend some brewing software. You can set a profile of your equipment and tell it how much wort you want at end of process and it will tell you starting volumes etc...

This may also answer some of your questions

I've not tasted any of David Heaths beers. But I will say he has worked very hard to learn beer making over the years and to share what he has learned.
He now has over 62,000 subscribers to his YouTube channel, so he must be doing something right.
He now has close connections to kegland so obviously some people will hate him for that

Agentgonzo · Feb 5, 2025

Davegase said:
He now has close connections to kegland so obviously some people will hate him for that

Haters gonna hate...

What matters - Boil-off volume, percentage or time?

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