Knowing the evaporation rate of your equipment is important when
trying to brew beer to a target original gravity. There a few things
that you can consider to solve the problem you are having. Beginning
with more water is one way that you can compensate for your high rate
of evaporation. If you choose to do this I would collect wort from the
mash bed until the gravity falls to about 2 °Plato or a specific
gravity of 1.008. Collecting low gravity wort leads to grainy,
astringent flavors in the finished beer. So if you want more volume
before the onset of boiling I would simply add water to the kettle
instead of running extra sparge water through the grain bed.
Another option is to add water to the hot wort following boiling. At
Springfield Brewing Company we actually use this method as our standard
method of brewing so that we can adjust our wort gravity to
specification for our brews. This also allows us to produce more wort
since we can brew slightly higher gravity wort and dilute after
boiling. Since kettles can only boil so much wort, adding water after
boiling is one way to boost wort volume.
Adding water after the boil is a way of brewing more beer and may make
sense depending on the size of your kettle. Adding extra water to the
wort before the boil, however, may not be required if you can regulate
the heat that you are applying to the wort during boiling. In fact, if
this problem occurs in a commercial brewery it is really a problem of
energy waste. Most beers these days have a total evaporation during
boiling between 4-8%. The trend is moving more toward 4% or less in new
kettles, but for some beers this evaporation rate is too low unless the
kettle is really designed to strip DMS during or after boiling using
some type of stripper.
From what you describe you are removing more than 20% of the volume
during boiling. This is really excessive and for most beers you are not
gaining anything with this high rate of evaporation. In fact, if you
like brewing some of the lighter styles this amount of evaporation is
probably leading to higher colors and is likely lending some flavors
that would be good to not have. I would turn down the heat during
boiling to bring your total evaporation down closer to 10-12%. If you
have flavors like DMS in the finished beer you could boil a bit harder,
but I do not think this will be the case.
All the methods described above can be used to hit your target gravity.
I would choose the method that makes the most sense for your needs.
“Most sense” can also be interpreted as best tasting beer. Good luck!