Author 
Message 
John McGrann (141.158.180.121)
 Posted on Monday, December 29, 2003  04:51 pm: 

I should know this one. Is volume lost during boiling a percentage of the total starting volume, or a constant amount? I've been starting with 6.5 gal, and the first 23 times wound up very close to 5 gal, but my batch this weekend left me with about 5.5 gal. The one before that was a total fup where I wound up with 6 gal, but I think that one was the result of a measuring error on my part in the beginning. Anyhoo, I'm going to watch it the next time to see how it comes out before making any big changes, but I would like to know whether a 1 hour boil generates the same boiling loss whether you start with 6 gallons, 6.5 gallons, or whatever. 
Bill Pierce (24.141.63.119)
 Posted on Monday, December 29, 2003  05:07 pm: 

You can express boiling losses either way, as a percentage of the total wort volume or as a specific amount. They are expressed as the volume per hour or the percentage per hour. ProMash, for example, will allow you to use either one (in my system I use 1.3 gallons per hour). It varies for each system and depends on the burner, the kettle geometry, the air temperature and the vigor the of boil, among other factors. There would be a slightly different loss for a different volume but it's unlikely to be enough to affect your calculations greatly. After a few batches on your system you should be able to estimate your boiling losses with a fair degree of accuracy. 
Greg Brewer (12.107.171.4)
 Posted on Monday, December 29, 2003  07:12 pm: 

Water vapor is just another way for your system to release the heat you are putting into it. Once the whole thing is boiling, the rate of evaporation is just about constant (gal/hr) assuming the same equipment, ambient conditions and heat input. Total volume has a much bigger impact on how long it takes to achieve the boil than on the rate of evaporation. That said, if you use a percentage basis, it will not change much in the range of volumes you cited anyway. Don't forget hops absorption rates when calculating how much will end up in your primary. More hops will soak up more wort, leaving you with less to ferment. 
John McGrann (141.158.176.43)
 Posted on Tuesday, December 30, 2003  02:02 pm: 

"More hops will soak up more wort, leaving you with less to ferment" Now that's something I never thought of. My most recent batches have been pretty lightly hopped, so that could be a contributing factor. Thanks to both of you for the information, that should help. 
Tom Gardner (198.81.26.39)
 Posted on Tuesday, December 30, 2003  03:43 pm: 

I use the same gallons/hour figure for each batch regardless of its volume like Greg recommends. 
Denny Conn (140.211.82.4)
 Posted on Tuesday, December 30, 2003  08:49 pm: 

I use gal./hr. also. Percent makes no sense to me...I boil off 1.5 gal./hr. whether I start with 7 gal. or 10 gal. If I tried to do it as %, it would be off one way or the other. 
Jim Layton (209.246.130.16)
 Posted on Wednesday, December 31, 2003  04:39 am: 

My experience matches Denny's. My boiloff rate is pretty linear over 90 minutes. 
