I am exploring the idea of switching to natural gas. In doing so I am exploring potential cost savings of natural gas versus propane.

The Blichmann Floor Burner can use propane or natural gas. With propane the Blichmann is rated at 72,000 BTU/hr and with natural gas it is rated at 60,000 BTU/hr.

First propane: one 15 lb. propane refill costs me $20. One lb. propane yields approximately 22,000 BTU. To burn the Blichmann for one hour it will take 72,000/22,000 = 3.27 lbs. of propane. Propane costs $1.33 per lb. meaning running the Blichmann for 1 hour costs $1.33*3.27 = $4.35. 1179 BTU are required to bring one gallon of water to 212F from 70 F. I boil 7 gallons, so 1179*7 = 8253 BTU are required to raise the temperature. The Blichmann in propane mode would require approximately 8253/72000*0.8 = 0.14 hours or 8.6 minutes to boil water.

Next natural gas: Based on my gas bill one CCF costs $0.744. One CCF of natural gas has approximately 100,000 BTU. Running the Blichmann for one hour will require 60,000/100,000 = 0.6 CCF which in turn costs $0.744*0.6 = $0.45. The Blichmann in propane mode would require approximately 8253/60000*0.8 = 0.17 hours or 10.3 minutes to boil water. The difference in boil time will add a few cents to the natural gas bill.

My boil times seem too low. I used 80% efficiency. Maybe I should use a lower efficiency. Not sure.

Is the above math correct?

Bottom line it looks very cheap to use natural gas. However, since I would have to hire a plumber to run a gas line in my back yard it would likely take quite a while (I only brew once per month) to recover the upfront costs. I am assuming the plumbing costs would be $200 or more.