Many studies demonstrate the suitability of the heat production rate for analysing and controlling aerobic growth and production processes ranging from the μL to the m³ range. Recently anaerobic bioprocesses leading to biofuels (e.g. ethanol, n-butanol), biogas, solvents (e.g. acetone, 1,2 butandiol) and small organic acids (e.g. acetate, butyrate) have attracted industrial attention. However, calorimetric monitoring of such processes challenges particularly the sensitivity of the applied sensors. A simple and cost-efficient solution for large scale reaction calorimeters is needed. We show that the sensitivity of a standard heat flux reaction calorimeter (here tested for RC1e) can be improved by the factor of 10 to (1.886 ± 0.012) K W⁻¹ by simply adding an additional internal thermal shield. Signals of about 5 mW L⁻¹ can be detected this way and the steady-state response time is about 10 min. Both criteria of relevance (sensitivity and steady-state response times) are sufficient for anaerobic growth processes. This was successfully demonstrated at the example of the acetone–butanol–ethanol (ABE) fermentation using Clostridium acetobutylicum. Here the calorimetric signal matches perfectly other simultaneously measured on-line signals and parallel measurements with a high-sensitive microcalorimeter.