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So Im brewing my Belgian blond (half batch) and I start my boil with 13.2 L and end up with 7L way above my gravity target. I watered it down and im going with it. but that seems like a ton of boil off.. could it have anything to do with the extreme cold temps outside? go easy on me this is only my second all grain but I figured beer smith would make it easy, man was i wrong.

If you can get a good rolling boil going when it's really cold outside, you'll notice a lot more boil off than you would when it's above 0º.

Would it be that if you normally do a 23l batch and you boil off 6 liters during the boil, it would be the same amount of boil off for a 11.5 liter batch..so it just seems like a lot when you're doing a half batch?

I don't know if the above statement is correct, but I'd think the boil off volume would be the same regardless of the batch size (provided you're using the same pot).

yeah 6 litres an hour sounds like around what i get.

Im only doing 11.5L allgrain .. the other night when i did my first i started the boil with 14L and finished with 11.7L and it was just as cold outside..I just find it crazy to lose that much in only one hour. I suppose there is a larger learning curve ahead of me then I realized.

and after watering it down to hit my target gravity I only have 10L

2.3l off a one hour boil is pretty low. Something wrong there.

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well in beer smith it said to start with 13.85 and I would end up with 11,5 thats useing the values that beer smith is preset with ...

Have you setup your equipment profiles in beersmith yet?

yes I did i have everything set up ... the evaporation rate is set at the default 13.7 an hour I also used the boil off calculator to confirm everything.

Jimmy wrote:Would it be that if you normally do a 23l batch and you boil off 6 liters during the boil, it would be the same amount of boil off for a 11.5 liter batch..so it just seems like a lot when you're doing a half batch?

I don't know if the above statement is correct, but I'd think the boil off volume would be the same regardless of the batch size (provided you're using the same pot).

I agree with you Jimmy. I'm doing allgrain BIAB 1/2 batches (11L), and I've thought about this quite a bit. Here are my two cents.

From what I understand there are two things that affect boil off rate. The area of the pot and your heat input. I don't think the volume of wort should have anything to do with it. So if you are using beersmith defaults that are set to %boiloff per hour you can't directly apply these to smaller volume boils. For example, if you are using a 10% boiloff per hour with a 23L batch, then beersmith will predict a 2.3L boil off after 1hr. However, if you use this boil rate with a 13L batch, then beersmith will predict a 1.3L boil off after 1 hr. So if the boil rate is only affected by heat input and area of the pot, you should still get the same total volume of boil off in a 13L pot as a 23L pot. For example, if you lost 2.3L after a hour with a 23L batch I would expect you to loose about 2.3L after an hour for a 13L batch as well. The only difference is that your new boil rate is 18% instead of 10%.

To sum it up there are two metrics you can use to think about your boil rate:
1) Boil rate as a percent (volume boiled per total wort volume per hour)
- This measure might be good if you want to talk about melanoidin development from the boil??
2) Absolute boil rate (volume boiled per hour)
- This measure is good if you want to scale down (or scale up) your brewing and want to predict how much boil off you will get.

Should I have my burner on full tilt, I thought the temperature of boiling was contant? But you refered to heat input? Should I just boil 10l of water a d see what I'm left with in an hour, or does the density of sugar have anything to do with boil off? Again this is extreamly confusing being very new as well as when I did my first batch I only lost 2.3l and now with the second I lost around 7 such a wide gap is leaving me dazed like I'm caught in headlights.

At any given time, the temperature of boiling wort should be constant whether or not your burner is set full tilt, or half way.

Here is an example to hopefully better explain what I'm talking about:
Lets say you put the burner on full tilt to get the wort up to a boil with the burner dial set to 10 (or 11 depending on how awesome your burner is), and after you've achieved a good rolling boil, you back off the heat (burner dial set to 5). The wort still boils but it's not as vigorous of a boil. As long as there are ripples on the surface you are still boiling. In this example, you can keep your boil going with your burner dial set to 10 or set to 5. If you set the dial to 10, you are providing more heat input to the pot and you will get more of a rigorous boil and more evaporation, while a burner dial set to 5 you will get less heat input, less of a vigorous boil, and less evaporation.

I brew inside, so I have lots of control over my boil, but if you are outside you have to take into account for wind, relative humidity, temperature, etc...

I expect the density of sugar to change the boiling temperature of wort. So at the end of the boil once you have more concentrated wort, the boil temp might be a bit warmer, but its effect on the homebrew scale is probably minor.

To answer some of your questions:

Q) Should I have my burner on full tilt?
A) Yes until your wort boils, then ease of the burner dial enough just so you maintain the boil

Q) But you refered to heat input?
A) What I meant is the amount of BTU's going into your pot.

Q) Should I just boil 10l of water a d see what I'm left with in an hour?
A) I think this would get you in the right ballpark to find an evaporation rate.

And to try to explain your change in evaporation, did you have your burner dial on the same setting both days?

My heat the first time would have been about 1/2 to 3/4 so it wasn't as high beer smith automatically selected a rate of 1.89 l an hour is this to small? I have a 42l kettle.. Should I select "use boil of as an hourly rate?

1.89L/h seems low for a 42L pot. The 50L pot in BS equipment profiles shows 5.8L for a 60 minute boil. If you do other than 60 minute boils, you will need to check off "use boil off as hourly rate" and the boil off will adjust for time. I have a 58L keggle and consistently boil off 6L/hour. Hope that helps.

All advice was great. I did a one hour with 10l of water and lost 6.5 L I will factor this in next time and then tweak it over time.