One of the benefits of the Hottop roasters is the filtering system. The top filter ensures clean air is drawn into the roasting chamber, and the rear filter removes smoke particles from the atmosphere. This ensures a much lower smoke output than from non-filtered roasting systems.
The top filter minimises the smoke that escapes from the top of the unit. Once the heating element shrinks you’ll notice a lot more smoke escaping, meaning it’s time to change the filter. This normally happens after about 100 or so roasts. Roasting beyond this will affect the air flow and cause inconsistent behaviour, particularly compared to your earlier roasts.