The other day I made beer with my new 25 gallon pot and I confirmed what I suspected:  A 55,000 BTU burner cannot boil 15 gallons of wort in any reasonable amount of time.  My brewing day was cold and windy, so I never got the wort above 200°F (93°C).  The question I needed to answer is how many BTUs do I need to boil my wort?

What is a BTU?

A BTU (British Thermal Unit) describes the amount of energy to raise 1 lb of water 1 degree Fahrenheit in one hour.

lbs of water x temperature rise = BTUs required for one hour
(1 lb of water x 1 degree Fahrenheit) = 1 BTU for 1 hour

Water equals 8.3 lbs per gallon.  To boil one gallon of water starting 70°F in 1 hour you will need 8.3 x (212-70) = 1,178.6 BTUs.  On my brewing day, I needed 15 x 1,178.6 = 17,679 BTUs to boil my wort in an hour.  With my 55,000 BTU burner, I should have no problem boiling my wort right?  The total boil time should take about 20 minutes.

What’s wrong with the calculation?

This calculation assumes the heat transferred from your burner to your water is 100% efficient.  This will never happen no matter how much you beg the beer gods.  The BTUs listed on your burner are like mileage listed in a car.  Your mileage will vary.  On a nice hot day (unlike my brewing day), 50% is a good number to hope for.  Under ideal weather conditions, my boil time should be around 40 minutes with a 55,000 BTU burner.  Less than ideal brewing days can drastically increase your boil time.

Another factor to consider is evaporation.  Evaporation removes energy from your wort.  This means you need additional energy just to keep your wort at a boil due to heat loss and evaporation.  Boiling your wort down one gallon will require another 8,000 BTUs.

Since I started 17 gallons (boiling down to 15 gallons) of wort, I needed 17 x 1,178.6 = 20,036 BTUs.  I was boiling down 2 gallons, so this adds another 16,000 BTUs for a total of 36,036 BTUs for one hour.  Assuming 50% efficiency, I needed 72,072 BTUs to boil the wort in one hour.  On my cold brewing day, I would have been lucky to get 25% efficiency.  I needed a bigger burner, and if I had calculated it this in advance I would have known (instead of suspected) it.

My 55,000 BTU burner would have been ok for a 5 gallon brew, but scaling to a much larger brew I needed a bigger burner.  The next day I purchased a 210,000 BTU burner (I tell people it has enough power to burn a hole in the sun).  Taking the above equations into account, hopefully this will help you make an educated decision on which burner to use.