In this paper, a virtual Air handling unit (AHU) level water flow meter is developed by using a control valve as a measurement device. The flow through the valve is indirectly calculated using two measurable inputs (differential pressure measurements and valve opening positions) and three constants (inherent valve characteristics, valve coefficient and the valve authority). Thus, the non-intrusive virtual flow meter introduced in this paper provides a solution to one of the measurement barriers and challenges: a low cost, reliable energy metering system at the AHU level. Due to the fact that the virtual flow measurement is achieved by an indirect calculation based on multiple inputs, possibility of measurement errors is significantly increased comparing with a direct flow measurement. In this paper, we first introduce mathematical models of the virtual flow meter. Sensitivity and error propagation analyses are carried out afterwards to investigate the flow accuracy. As results, the valve stem position is a dominant variable in determining the accuracy of the virtual flow meter and thus the error impacts from other inputs except the valve stem position are minor. When the valve stem position errorranges from 0.5% to 2.0%, the accuracy of the virtual flow meter ranges from 1.96% to 7.83% under the valve authority of J and the accuracy ranges from 0.92% to 3.60% under the valve authority of 0.3. This simulation study has proven that it is feasible to use the control valve as the measurement device at the AHU level to provide satisfactory results. More studies will be carried out on experimental tests in next phase study.