Bought a grainfather a few months ago, and I have been really happy with it, on the whole. However I seem to have a problem with the chiller unit. I have never been able to chill a brew down to 25ðc in under an hour. Used my old copper coil last time at the same flow rate, and the brew to 25 minutes! Has anyone else had this problem? Is a slow or fast rate of cold flow best?