For a 5 gallon batch what should be my wort volume before a 90 minute boil?
Not a straight forward answer. It will depend on your environmental conditions and pot geometry. On dry windy days I boil off about 1.5 gallons an hour, on humid days it’s closer to 1.2.
What he said. It depends on what your boil off rate is. But a simple answer is you should have at least 6.5 to 7.5 gallons pre-boil volume. Depends on what your target pre-boil volume should be for that recipe and your target per-boil gravity. You’ll lose at least 1 gallon in a 60 min boil and if its a hoppy beer you can lose up to another 1/2 gallon or more to hop absorption and trube. Hope this all helps.
+1 to the above. Add a measured amount of water to your kettle, boil for 60 minutes, and measure what’s left. Then when you brew, add the amount that boils off (your pot’s evaporation rate) to your target volume and you’ll know how much wort to collect - ie., say you boil off 1.2 gallons in that hour and want to be left with 5 gallons of wort post boil, then plan to collect 6.2 gallons of wort to be boiled for 60 minutes. Obviously if you’re doing a 90 minute boil, you’d need to collect more wort to account for a longer boil.
EDIT - Steve’s right that environmental conditions can change your boil off rate, and also there is ~ 4% expansion at boiling temps as opposed to cooled volume but this gets you in the ballpark.
Meaning cool you’re test boil before measuring the end amount.
It mainly depends on the heat added to your wort. So it depends on your burner and the efficiency of the heat transfer to your wort.
In my case the 5.5 kW burner on natural gas results in about 3 liter per hour.