Experimental data recorded for a heating, ventilating, and air conditioning system showed that the daily heated water loop energy calculated by summing the hourly-averaged loop heat transfer for the heated water loop is almost 35% higher than the sum of the daily energy consumed by variable air volume hydronic reheat coils in each zone. The main reason for this difference was attributed to the time delay of the loop return water temperature caused by the water traveling along the loop. This study examines methods to evaluate the heated water loop energy when there are time varying conditions. Data for a heated water loop are used to demonstrate the effects of the time delay on the evaluation of the loop energy. A pipe heat transfer model characterizing the transient response of a fluid flowing through a single pipe is developed to examine the effects of time varying conditions on the pipe thermal response. Based on the pipe heat transfer model, a loop model is constructed to simulate the actual loop with multiple hydronic reheat coils. The comparisons of the simulation results with the data indicate that the loop model is capable of predicting the loop return water temperatures and loop heat transfer based on data. (C) 2004 Elsevier B.V. All rights reserved.