I am considering getting a either a 7 gallon or a 14 gallon conical fermenter and am wondering if it would be okay to ferment a standard 5 gallon batch in the 14 gallon or if that would be a bad idea. Thoughts?
You can, but the beer profile will be different. With more head space above the fermenting beer, the CO2 will not be pushed out as efficiently. This will create a higher pressure of CO2 pushing on the beer, which will decrease yeast growth, and increase ester formation.
If you want consistency then I would not recommend brewing in the wrong size vessel. If you don't care, go for it.
For further reading http://www.scientificsocieties.org/JIB/papers/1992/1992_98_6_509.pdf
Actually I agree with brewchez. I did some further reading and it does appear that you can ferment a 5 gallon beer in a 14 gallon conical. Where what I said really applies is to fermenters that are 15' or higher. At this point the hydrostatic pressure at the bottom of the tank begins to inhibit yeast growth. Additionally on Blichmann's website, it says at the bottom that you can brew 5 gallons of beer in a 14 gallon conical.
I am now interested to trying to brew a 5 gallon batch in my 15 gallon cylindrical-conical and comparing it to a beer fermented in a glass carboy.