Abstract A series of experiments was performed to characterize the time response of a large-scale open calorimeter to square-wave pulses in terms of peak heat release rate, width of the peak, and conservation of energy. Quantitative heat release rate measurements of full-scale fires up to 2.7 MW were conducted using the principle of oxygen-consumption calorimetry. A remotely-operated natural gas burner provided a reproducible heat source and near-square-wave inputs to the system. The calorimeter was capable of resolving the actual peak heat release rate value for fire transient events having a full width at half height of 15 s or greater. However, if the full width at half height measured by the calorimeter was less than 11 s, the measured peak value underestimated the actual peak heat release rate by 15% or more. Even if the peak heat release rate could not be fully resolved, the calorimeter was able to provide an estimate of the total heat released to within about 5%, demonstrating conservation of energy by the system.