Adjusting boil off rate

[jmancuso]

I have been brewing 5.5 gallon batches in my 20 gallon kettle and my boiloff rate has been pretty high. I'm planning my first 10 gallon batch how much will my boil off rate change? Or will it stay the same?

 

[arturo7]

In the same vessel the actual amount boiled off should not change. The percentage boiled off will.

 

[Bobby_M]

Well, caveat... if your burner is strong enough to maintain the same vigor of boil, it will not change. Sometimes more wort means a less aggressive boil and therefore slightly less boil off. With 6.75 gallons in the kettle, I lose 1.5g/hr. With 13, I lose about 1.25 gallon per hour. Splitting hairs of course.

 

[jmancuso]

I have been boiling off a little over 1.75 g/hr. I have been doing 90 min boils and starting with between 8-8.25 gallons and ending up with 5.5 gallons in the fermenter. It seems to me there should be less boil off if the kettle is more full.

 

[ajwillys]

The two posters are right of course, but to clarify, wort volume does not affect boil off. These factors do:

1) Boil Vigor (as Bobby said)
2) Surface Area of liquid (kettle diameter)
3) Ambient weather conditions (temperature, humidity, wind)

That last is obviously only a problem if you boil outside.

 

[ajwillys]

I have been boiling off a little over 1.75 g/hr. I have been doing 90 min boils and starting with between 8-8.25 gallons and ending up with 5.5 gallons in the fermenter. It seems to me there should be less boil off if the kettle is more full.

The percentage should be lower. The amount should be the same (assuming all other factors remain constant). Since the percentage is lower, if you use brewing software that relies on percentage of total volume, you will need to adjust it.

 

[jmancuso]

So if boil vigor is the same and ambient temperature is the same, I should start with 16 gallons in the kettle to end up with 11 in the fermenter?

 

[Bobby_M]

No, if you boil off 1.75g an hour and want to boil for 90 minutes, you would start with 13.625 gallons to end up with 11.

If you're using brewsmith, the boil off rate should be something like 19%. Having to adjust the boil off rate because it is incorrectly factored as a percentage is probably the biggest goof with that software design.

 

[Scimmia]

No. If boil vigor is the same and atmospheric conditions are the same, you will still boil off 1.75 gallons/hr. For a 90 minute boil, that means 2.625 gallons. For 11 gallons in the pot, you'd start with 13.625 gallons.

 

[arturo7]

And damnit, don't forget that last .025 of a gallon!