BTU range for direct fire 7bbl CDC kettle?

[BeerBoy]

Wondering if there's anyone out there using (or has used) an old CDC (or similar) direct fire kettle and could share what burner they're using and if you know what it's putting out, BTU-wise?
I've got an Adams Speed Flame rated to 400,000 BTU and the flue temp was measured at 825-850F about four feet above the flue exit - seems like a lot of $$$ going out the stack.
Any idea of what the "sweet spot" of BTUs would be to get that lovely rolling boil and yet not waste natural gas?

Thanks,

FarmerD
Dave's BrewFarm - A Farmhouse Brewery
Wilson, WI

[gitchegumee]

I've got a CDC of similar size that uses a Midco with 400,000 BTU/hr burner. A bit big, but you can regulate the heat down. Never measured the stack gas temperature, but no matter what it will be wasteful. A burner technician will be a big help to find your sweet spot. We use about 5 gallons of "propane" per 90 minute brew. It's a pretty low heat rate fuel mix, though. Good luck!