Generally speaking, for a drum roaster using direct heat source the air temp needs to be at least 20 Deg C above the bean temp in order to maintain thermal momentum. In the earlier stages of the roast, that differential needs to be greater. This concept applies for most of the smaller drum roasters from Probat, Geisen, Has Garanti, Proaster, etc. where the heat is applied in close proximity to the drum - either a controlled distance or via the deflector shield. Drum roasters using indirect heat source, e.g. oven or hot-box, etc. are in effect air roasting and this type of roasting requires a much higher air temperature, e.g. 400 - 500 Deg C. Of course, all this depends upon thermocouple placements, etc. and the individual design of the roasting platform. Without knowing anything about your HotTop, it would be interesting to target a 13 - 15 minute roast and see what difference you discover. This may require a reduction of batch size in order to reach those targets. It may also be impossible due to inadequate power. If your roaster principally uses convection rather than conduction, the general rule of thumb is shorter roast times for convection and longer roast times for conduction. The reasons these rules (and I use the word rule very loosely) seem to make sense is due to the way in which heat is transferred into the core of the bean. If you try shorter roast times with a higher level of conduction, you will risk scorching and tipping. If you try longer roast times using convection you risk baking. Finding the ideal range takes a lot of trial and error. Every roasting platform is different.
