Evaporation

My first AG batch in my new 20 gallon kettle had a very low amount of wort that went into the primary. I shook it off to inexperience.

2 more batches later and the same problem was occurring so I decided to take some accurate measurements.

Nut Brown Ale
8.5 lb grain bill
Mashed with 3 gallons of water at 152
Added 1 gallon at 175 to mash out
Sparged with 4 gallons of 165

I sparged into my wine bucket and measured volume at over 6 gallons. Don't have a measurement for 7 gallons

Then I did a 60 minute boil, cooled, and put into primary where I had 3.5 gallons of wort!!!!!!!!

So I lost about 3 gallons of water in the boil. The gravity was crazy high at 1.1+ so I added 3 gallons of bottled water and the gravity was 1.050.

I live in NW Arkansas near the Ozark Mountains at 1400 feet.

Is there too much surface area in my 20 gallon monster kettle? Why am I losing so much to evaporation?

Did you leave the heat on high the whole time?
Before your next batch put water in to kettle, boil for an hour, what you have left is your boil off amount. You add that much extra next time.

Your altitude, potentially cold weather (dry air) and large surface area probably did it. You can also boil off more by having your burner at 100% full throttle all the time. The boil should be hard, but it doesn't need to be crazy.

Also- do you transfer trub into your fermentors? I do but get crazy amounts of cold break (around a gallon) so that affects my final volume. If you're leaving that behind, you also need to adjust your preboil volume. You probably just didn't start out with as much as you needed.

Beersmith 2 always helps me hit my water amounts just right.

I had the same issue man. I had 5.75 gallons in my brew kettle and after 60 minutes was left with about 3.25 gallons. I left the heat on high the whole time and didn't realize it. In the end it ended up working out but just small annoyance.

You have a high boil off rate. It doesn't mean anything.
Like aubiecat said, you just need to do some experiments and figure out what that volume is.

Thanks for the responses. It wasn't a big deal, it just never seemed to happen with my old kettle. I'll just plan for it next time and watch my heat level during the boil.