[Pi]

Seems I am getting variable results. On Tuesday I did a 2 hour boil. Started with 8.5 gallons, and after 1hr, i was down to 7.5. But after the second hour was down to 5.5.Same everything. what changed?

[morticaixavier]

Was the 5.5 measured during the boil or after things had settled down? Is it possible it wasn't really 7.5 after one hour?

the geometry of your kettle, the relative humidity and temperature where you are boiling and the vigor of the boil are the main parameters here.

[PORTERHAUS]

I wonder as the volume decreases if the vigor of the boil increases as there is less liquid to keep boiling meaning the same heat energy is applied to lesser volume that may require less heat. Never thought about it.

[PORTERHAUS]

Like mentioned above, at what was the condition of the water when you measured the 7.5 gal? The volume of liquid increases when hot. If it was taken at boiling...it might trully been less volume.

[decibeldanny]

Have you ever brewed from a recipe where the author has described the boil as a rolling boil?  A true boil is found at 212 degrees, however, you will see boil-like traits below that.  There are recipes out there (for food, not beer) that call for a "quiver" "bubble" or "simmer."  Though a simmer or bubble is obviously not a boil, with this school of thought a "quiver" could be mistaken as a true boil.  I can see that this would be even easier to mistake when your wort is darker.  Once you reach 212 degrees the temperature of the water will not rise any further.  It is possible that your boil for the first hour was actually a sub-boil (even if not by much) and then your second half was full-fledged boil.

[Pi]

I took my first and last measurements ~175deg, but the 1 hour measurement was during the boil. I think morticaixavier nailed it:

Was the 5.5 measured during the boil or after things had settled down? Is it possible it wasn't really 7.5 after one hour?

[brewsumore]

Also, I find that a breeze, by driving off steam contributes to a slightly higher evaporation rate than during no breeze.  So there's the potential for a changing rate mid-boil depending on if a breeze develops.  I'm not saying that happened to you.

[GS]

This thread seems to have died, but I wonder... was there any wind blowing? I brew out doors, and I have found that the only times I have been able to boil off 6 quarts in 60 minutes is when there is no breeze whatsoever. When the breeze picks up, it literally blows the beat out from under the kettle. On the average, my boil lasts 90 minutes and I adjust the hopping schedule accordingly.

[GS]

Oh yeah I almost forgot. The windier it is, the more propane I burn.

[Whiskers]

The more wind, the less likely you are to be delivering what you could be in terms of heat from the burner to heat into the wort.  The power delivered to the wort is the overwhelming contributor to boil-off rate. 

[hopfenundmalz]

Wind sreens around the burner help in windy conditions. I have one made from Aluminum roofing flashing. wraps around the burner. It has enough gap so that the burner air inlet is not obstructed.

[Jimc]

Hey all I brew 10 gal batches and collect 14 gal of wort to get to
11 gals in the fermenter that's after evaporation equipt losses. To what
Advantages if you boil longer is this supposed to give u a better original
Gravity as it is boiling off water I generally evaporate 1-1/2 gal per
1 hr boil

[morticaixavier]

Hey all I brew 10 gal batches and collect 14 gal of wort to get to
11 gals in the fermenter that's after evaporation equipt losses. To what
Advantages if you boil longer is this supposed to give u a better original
Gravity as it is boiling off water I generally evaporate 1-1/2 gal per
1 hr boil

if your efficiency is off and you don't mind losing out on volume you can boil longer to increase your gravity. The other effects are that it will increase hop utilization, thus increasing bitterness, and it will increase Color. Very long boils can create some Maillard reactions that will change the flavor profile as well.

[mabrungard]

But also recognize that longer boils will concentrate the ionic content of your brewing water. If you are boiling off more than about 10 to 15 % of the original volume, you may need to reduce any mineral additions to compensate for that concentration.