high evaporation loss

Howdy all.

So, I'm trying to do some small, 3 gallon all grain batches (something I can do inside with my electric stove, since it's all of -10 F. outside these days), and was trying to find out my evaporation loss rate, just so I get get my all my mash tun run offs and sparging to push out the right amount of wort to boil.

I tried to run an evaporation test on my stove and pot I have.

Basically, it's a glasstop electric stove (it can boil about 4 1/2 gallons, but not much more than that
I am boiling in a 32 quart pot.

For a quick test, I decided to boil 2 gallons of water for an hour (lid off of course) to see what my evaporation loss rate was.

I am also at around 4500 feet elevation.
A 2 gallon boil for 1 hour left me with 1.3 gallons:
About 35% loss rate!

I had a good vigorous boil going, but nothing TOO nuts.
This seems waaaay to high.
Any ideas? Is it just my elevation? Is it that it's bone dry? Is my burner up too high and I should tone it down?
Is my boil pot too big for what I'm trying to boil?

Of course, when doing a 5 gallon batch and when boiling around 4 gallons I don't get anywhere near this loss rate. I suppose it's a function of a weaker boil, that I don't loose as much then.

Anyhow, what should I do? turn down the heat some? Or live with that number?

If you started with .7 gallons, after an hour you'd have a 100% loss rate!

Seriously, the % is completely meaningless. You're loosing .7 g / hr (sounds normal for a stovetop boil) with your setup. So if you want 3 gallons post-boil, start with 3.7 g.

I'm a small batch indoor AG brewer and also do 3 gallon batches. I normally do 90 minute boils and my evaporation rate is a little higher than yours so I sparge to get 4.5 g pre-boil.

In winter, on propane, doing a 12 gallon boil. I loose 2 gallons an hour.

20%

In summer, same boil, same rig I can loose 1 gallon or less.

It all has to do with humidity at the time of boil.

The trick is to plan for more volume at the end but still hit target gravity. If you are lucky, once in a while you'll hit spot on.

Most times I have a quart more beer, post trub, than I have keg volume.