Why do I have a big difference in boil-off rate between 2 (6.6 and 3.5 gls) batches?

I have just finished my second BIAB brewing.

My pot is a 13.2 gallon steel pot, 15.74"-15,74" in dimensions.

My 1st batch (3.5 gallon planed, achieved too) started off with 6.6 gallons of water and I ended up with 3.4 gallons in the fermenter of which 1.5 gallon of water boiled off in 60 mins.

However...my second batch (6.6 gallon planed, achieved too) was a lot bigger.
I started off with 9.2 gallons of water and used 1.3 gallon of water to sparge (I didnt do it with my 1st batch..but the sparge water was planned when I calculated the amount of water needed), then I ended up with 6.8 gallons in the fermenter. Meaning only 0.8 gallon of water evaporated. The boil wasnt actually as fierce as during the 1st brewing....I guess it is due to the bigger batch size. So, this might explain why water didnt evaporate. I really dont know.

Is this possible that my boil off rate dropped this much just because of the size of the batches? Will this affect the taste of the final brew?

P.S. My calculation doesnt account for any water losses durning the brew...just the actual amount of water that is boiled off.

You didn't evaporate as much water due to the boil's intensity.

Your burner was not providing enough heat to the large batch as it did to the 3 gallon batch, since the larger batch requires more heat to reach the same boil-off rate.

There will be a few differences. You will start with a lower gravity beer. Color development and hop utilization will occur less in a less aggressive boil. Also, you may have less hot break development as a result of lower boil intensity.

You're still making beer though.

novahokie09 said:

You didn't evaporate as much water due to the boil's intensity.

Your burner was not providing enough heat to the large batch as it did to the 3 gallon batch, since the larger batch requires more heat to reach the same boil-off rate.

There will be a few differences. You will start with a lower gravity beer. Color development and hop utilization will occur less in a less aggressive boil. Also, you may have less hot break development as a result of lower boil intensity.

You're still making beer though.

Click to expand...

Thanks for the reply. Do you think the beer will be a lot different due to all the factors you mentioned (such as the hop utilization, less hot break..etc).
I understand that I am still making beer...but I dont wanna make low-quality beer. I'd rather lower the amount of water and make smaller, but better batches...probably that's the best option.

Did you make the same beer in the larger batch? If so, just save a bottle from the smaller batch and see which you prefer. Some people might not even notice a difference. Better palates will be able to differentiate.

You can still make the bigger batches, but you need to realize your final volume will be larger due to less boil off. You can account for this a few ways. First, you can use the same volume of water, and increase your starting grains and increase your hopping amounts to arrive at the same color, IBU and OG. The other option would be to just start with less water in order to hit your target volume into the fermentor. This would require you to know your boil off rate and calculate how much pre-boil volume you will need. You could also just boil longer to get the OG you want.

This is all part of dialing in your system. Just stick with it and take ample notes. This will allow you to learn your system. If I were you'd, I'd just keep making larger batches but account for how your system works.

Elysium said:

Thanks for the reply. Do you think the beer will be a lot different due to all the factors you mentioned (such as the hop utilization, less hot break..etc).
I understand that I am still making beer...but I dont wanna make low-quality beer. I'd rather lower the amount of water and make smaller, but better batches...probably that's the best option.

Click to expand...

Your larger batches will be of no less quality. You just need to account for the different amounts of boil off with different size batches.

wilserbrewer said:

Your larger batches will be of no less quality. You just need to account for the different amounts of boil off with different size batches.

Click to expand...

But surely the less intense boil affects the chemistry (I mean the things that change, boil off) inside the wort and the way things boil off. Or am I wrong?

Elysium said:

..my second batch (6.6 gallon planed, achieved too) was a lot bigger.
I started off with 9.2 gallons of water and used 1.3 gallon of water to sparge , then I ended up with 6.8 gallons in the fermenter. Meaning only 0.8 gallon of water evaporated. .

Click to expand...

So on your second batch, you used 10.5 gallons of water total, yet ended with 6.8, and only evaporated .8 gallons? Where did the other 2.9 gallons go?

Are you boilng on a stove top?

You could try boiling the larger batches longer?

wilserbrewer said:

So on your second batch, you used 10.5 gallons of water total, yet ended with 6.8, and only evaporated .8 gallons? Where did the other 2.9 gallons go?

Are you boilng on a stove top?

You could try boiling the larger batches longer?

Click to expand...

grain absorption: 1.5 gallon, trub was a bit big: 1.3 gallon, and I think I had a bit of a shrinkage too. That's where the 2.9 gallon went.

I dont use a stove top.
I use 2 big gas rings that are attached to a gas cylinder. It's a thing they use here in Spain.