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!