I have been brewing in 5gals for years and have had no problems making this ESB. I have been using 5gal glass carboy's up until this point, and the Grainfather system.

I have never had an infected batch, however I just tried to make my first 10gal batch using a 14gal stainless steel Chapman fermenter and the batch is infected. We did add some extra non-boiled water at the end, but other than that, everything was properly sanitized as normal and the recipe was followed as normal (except we doubled the recipe for 10gal).

Did it get infected because of the 4gals of headspace in the new fermenter? Or did we do something else wrong?

Note, there was some extra lag time to start seeing action on the airlock (about 12hrs), which was longer than I normally see action. It also appeared to last about 4 days hard instead of the normal 2. Also, I think I pitched the yeast a little hot (like 88deg).