Residual pressure is defined as

• A. The pressure remaining in a water supply system while water is flowing
• B. The pressure before pumping begins
• C. The maximum pressure during a cycle
• D. The pressure loss due to friction

Answer: (A)

Residual pressure is the pressure remaining in the water supply system while water is flowing. This matters because as water moves through pipes, fittings, and elevations, friction and hydraulic losses reduce the pressure from the static value. Measuring residual pressure shows how much pressure is actually available at the system when there is demand, which helps determine if there’s enough supply to sustain the required flow during a fire.

Static pressure, by contrast, is the pressure when no water is moving (no pumping or flow). The maximum pressure during a cycle isn’t the concept here, and while friction causes pressure loss, the residual isn’t simply the loss itself—it’s the remaining pressure after those losses under active flow.