20% Evaporation Rate?

Does a 20+% evaporation rate seem like a lot? I downloaded Beersmith and am feeding default info into it. I brewed today and started with 8 gallons in the kettle and ended up with 5.5 gallons. I have a 10 gal SS pot and a SQ14 burner (first time I used this today). I have always had to add water back to the fermenter to hit my target OG and volumes. I'm working on trying to avoid that. But I don't know if I'll be able to start with that much volume in my kettle.

I've never found evaporation rates to be useful as a percentage so much as a fixed number. I boil off a gallon per hour, whether my pot is completely full or nearly empty. With a biggish burner like that, yours might be higher.

How long was your boil? 2.5 gal per hour is a lot, in two hours not so much. Regardless, you definitely don't need to boil off that much. Throttle that burner back and save some propane.

Temperature and humidity levels make a big difference to. My last brew I did a allgrain outdoors when there was a high of -19C, it got down to -24C by the time I did my boil and it was dry. I lost 2.5 gallons in an hour. When it was boiling it looked lime a steam engine

I had the same issue. I usually account for about 1.25 gallons of boil off per hour. However, I typically don't brew all grain in winter because I'm a wussy when it comes to cold.....

However, I had the itch for a brew day a couple of weeks ago, so I decided to brave the icy chill of 40 degree weather. (told you I'm, a wuss). I lost over 2 gallons and my gravity was a little high. I topped off to where I wanted my gravity to be and dumped the yeast. We'll see how it goes.

I have an SQ14, and start my boils with 7g. I boil for 75 minutes, and end up with 5.25g (+- 1 pint) in the fermenter. However, that isn't all boil off. I have some trapped in the CFC, and a whole bunch absorbed by the whole hops. If I'm making something very hoppy, I add more volume pre-boil.

-a.

I boiled off 2 gallons in my brew yesterday. Once the wort reached a boil, I forgot to turn the burner down, so it was going pretty vigorously for the first 30 minutes

I did a 90 minute boil. So about 1.7 gals/hour.

I use a 12.5 gallon pot with a 16" diameter. I start with 7.5 gallons and end up just over 5 after an hour.

I have to put fans in the kitchen and open doors and windows otherwise eveyrthing ends up soaked.

If you want to know accurately, you can do a trial boil with water. As Malfet said, it really doesn't matter how much water you have in the pot providing you don't boil it dry. If you weigh the pot before and after the boil, you can get a very good idea of the boil off rate.

-a.

With my stupid wide pot a 2.5 - 3 gallon boil off is my average with the sp-10 burner.

I use a 210,000 BTU banjo style burner and get ~1.25 gal per hour boil-off. I would collect about 6.5 gal for a 5.0 gal batch (1.25 to boil, rest to shrinkage and trub/break). I had a couple [more] experienced brewers tell me that my boil was really too strong and that I could throttle back a bit. I did... I still maintain a strong rolling boil but not so strong that I boil off an excessive volume of water.