Insane evaporation rate!

A few days ago I brewed a 5.5 gallon batch of IPA and ended up with only 3.75 to 4 gallons of wort in the carboy??? I am fairly new to brewing still (only 8 all-grain batches) but haven’t missed gravity this badly. I also can’t seem to sink my brewing with the beersmith software yet. I collected 8.2 gallons of 1047 wort and ended up with about 4 gallons of 1077 wort on a 90 minute boil (shooting for 5.5 @ 1066). How much does ambient temp affect evaporation rate? It was 32f in my garage but I am using a 20 gallon megapot (really wide opening). I am willing to take a bit of a hit on efficiency in order to stick with this pot and get 5 to 5.5 gallons in the end. When I mess around with the numbers, it claims I would be at about 25 -26% evaporating rate in order to hit my numbers. Anyone else dealing with this problem? I should have added more water during the boil to compensate…does anyone do this regularly? Thanks for any advice, I just want to be able to start with the right amount of liquid so that the hopping doesn’t have to be adjusted on the fly to compensate for adding water all the time.

I realize this is a question that take time to answer and may be a pain so I greatly appreciate your responses!

Evaporation rate is directly proportional to the surface area of the boiling wort. If a normal guy has a boil kettle maybe 14 inches in diameter but your kettle is much larger at more like 22 inches in diameter, your boiloff rate will be higher by a ratio of the square of the radiuses. In this example, your radius is 11 inches versus 7 inches, which could increase your boiloff by a ratio of 121/49 = 2.5 times. So if a normal guy boils off one gallon per hour with his 14-inch kettle, you will easily boil off 2.5 gallons for the same recipe with your huge kettle. I don’t know the actual radius of your kettle but this is why your boiloff rate is so enormous.

To compensate, you need to either sparge more, or add more clean water before the boil to account for the boiloff rate.

Or you could experiment with boiling less, maybe only boil a typical batch for 45 minutes or 30 minutes and see how you like that.

Assuming you choose to sparge more than the average schmuck, I don’t see why you couldn’t experience 90%+ brewhouse efficiency since you are collecting more sugars in your sparge than the average schmuck. If you don’t care that much about efficiency (which personally I do NOT), then just add clean water which is probably easier and might experience less odds for tannin extraction. In no case do you want your pH to exceed about 5.6 or so, or your sparge gravity to decline below 1.010, or you could pull tannins out of the grain.

Not too complex of a response. Yeah, I’m an engineer… But it really just has to do with the size of your kettle. Bigger kettle surface area = bigger boiloff rate. Plain and simple.

The low humidity in winter can also increase your boiloff rate slightly, in theory, although personally I doubt this has much effect.

I’ve noticed a big difference in boil off when comparing a warm, humid, summer day to a cold, dry, winter day. In the summer, I boil off around 1 gallon per hour. In the winter it’s closer to 1.5 gallons per hour. I live in New Jersey, so summers are humid while winters are quite dry.

Thank you so much for your response…that makes sense and it really helps to know that it would be best to add water before boil to avoid that massive sparge and tannin extraction. What do you use to measure your mash ph with? So when I see a recipe in say Zymurgy for 5 gallons…that simply means 5g at the end of boil? If I wanted 5.5g in carboy and figure .5 gallons loss to trub and deadspace I would just bump the percentages of grain and hops to 6g? Thanks again!!!

I have a pot that is probably about the same size as yours and I do not experience nearly this much boil off. My “average” is to collect 7.5 gallons which leaves me with almost exactly 5.3 gallons in the fermenter after a 70-75 minute boil. But I have very little dead space in my mash tun and lose very little to cooling.

I have come up way short on one or two occasions and have come to the conclusion that I mis calculated how much I started with - pre-boil. This probably happened when my first runnings were something like 2.75 gallons and I miscalculated the amount of sparge water by being dumb.

Relative humidity and altitude all come into play as well as the afore mentioned surface area.

Also, how vigorous of a boil are you maintaining. I keep my heat up pretty high up to the point it starts to boil, but then turn it down to maintain a low, rolling boil