Fermentation Temperature

Both enter the heat calculation as direct multipliers on total sugar fermenting per hour: Must Volume sets how much total sugar is present, and Fermentation Rate sets what fraction of that sugar is converting per day. Doubling either one roughly doubles Heat Generated, and at this calculator's default values the two effects are nearly identical in size -- neither one dominates the heat load the way a single input often does in simpler calculators.

Not directly -- Starting Brix sets the total available sugar, but Heat Generated depends on how much of that sugar is actively converting per hour, which is set by Fermentation Rate (the Brix drop per day), not by the total Brix present. A high-Brix must fermenting slowly can generate less heat per hour than a lower-Brix must fermenting aggressively; Starting Brix mainly affects Estimated Duration, since more total sugar at the same rate simply takes longer to ferment through.

Temp Rise divides total heat generated by the thermal mass available to absorb it, and this calculator treats a tank's thermal mass as proportional to its own volume (must behaves close to water for this purpose). Since both heat generated and thermal mass scale together with Must Volume, the two effects cancel out -- a 200-gallon and a 2,000-gallon tank fermenting at the same Brix and rate see roughly the same temperature rise per day if neither is cooled, even though the larger tank is releasing far more total heat.

Treat it as a starting point for sizing, not a final spec -- it assumes you want to remove exactly the heat this calculator estimates is being generated, using a simplified constant heat-of-fermentation figure and a fixed duty assumption. Real fermentation heat output varies across the fermentation curve (lower during lag phase and near completion, higher mid-ferment), so a properly sized chiller typically needs headroom above this steady-state estimate to handle the peak, not just the average.