A numerical study has been conducted to characterize the transient pressure in a building water supply system with an air chamber by utilizing a commercial code that employs the method of characteristics. Some results produced for the purpose of verification in the study agree quite well with the previously reported. Several parameters are then varied. Among them are the valve closure time, the wave speed, the static pressure, the polytropic exponent, the air chamber volume, the inner diameter and the shape of orifice in the air chamber, etc, while the water temperature and velocity are kept constant at <TEX>$20^P{circ}C $,/TEX> and 0.8 m/s, respectively, Results reported in this parametric study may be useful to understand the unsteady behavior of the system.