To figure out your efficiency, you need to know a few things;
LP puts out 21,591 BTUs per pound
It takes one BTU to raise the temp of one pound of water one degree F
Water weighs 8.3 lbs per gallon at 68F
Therefore, if you have 5.5 gallons or wort, it weighs about 46 lbs.
If you raise the temp from 68 to 212, that's 144 degrees.
46 times 144 equals 6624. So, at 100% efficiency, it only takes 1/3 pound of LP to bring the batch to a boil.
I use about 3 lbs of LP to boil 5.5 gallons for an hour. I figured out it's about 40%~50% efficiency, which is fairly normal.
Burning 10 lbs per 5.5 gallon batch has to be less than 15% efficiency. The main reason you'd get so low an efficiency is that the flame is mostly exhausting past the bottom of the kettle... in other words turn the burner down and you'll actually get a boil faster with a lot less fuel. Also, check to see how far the flame is from the bottom of the kettle. If the flame is climbing the sides, that's your problem. If the flame is too far away, that's another problem. The tips of the flame should dance across the bottom of the kettle. Lastly, look to see of you have blue flames or yellow flames. If you see yellow, adjust the air mixture just until you get pure blue flames.
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