Losing 8 liters evaporation during boil?

[Awfers]

I made a Dunkles Weissbier on Monday, 5.25kg of malt, ran out 26 liters of liqour out of the mash tun, boiled it for 1.5 hours (rigourous for the frist 5 minutes, then turned down to a gentle simmering boil) and ended up with just shy of 18 liters in the fermenter, with not much left in the kettle (maybe a liter, max).

I was going more for a loss of 4 liters (+/- 10%) and ended up with losing twice as much.

Is this rate of evaporation normal? To lose 8 liters (2 gallons) over 1.5 hours?

Note: It was raining yesterday, so 90% humidity. The system is meant for 40 liter batches, so I was half batching in the system, but I've experience the same thing in my smaller 30 liter Vollrath kettle.

Thanks for your input.

Cheers,
Awfers

 

[FuzzeWuzze]

Seems in line with what i see.

I lose 1.5 gallons over a 60 minute boil, and like you I turn it down to a small rolling boil.

 

[RoadKing]

It sounds like a lot. I lose 1.9 gallons per hour on a rolling boil. It's best to measure your boil off for future reference.
If you don't have a measuring stick, now would be a good time to make one. Take a measuring cup and start filling your boil kettle and make marks on a stick
of some sort in half gallon (litre) increments. (I used an oak stick and a sharpie. The numbers are still good after years of brewing.)
You'll only need a couple gallons of water for your test, boil for 30 minutes and remeasure. multiply by 2 to get the gph / lph rate.
Keep in mind that the water will expand by 4% at boil, so when I did it, I put the water in cool boiled then let it cool again to take final measurement.
The boil off rate is pretty static, as long as you keep the boil intensity about the same.

 

[kh54s10]

My boil off is 1.75 gallons in an hour. It seems very consistent.