In a multi-story building, how does gravity affect cold water pressure, and what design strategies help maintain pressure on upper floors?

• A. Pressure increases with height; install larger diameter main and reduce valve usage.
• B. Pressure remains constant with height; no special strategies needed.
• C. Pressure drops with height; use booster pumps, pressure zones, and pressure-reducing valves on separate floors or zones to balance pressure.
• D. Pressure drops with height but only due to leaks; seal all joints to maintain pressure.

Answer: (C)

The main idea is that gravity creates a hydrostatic head, so pressure in a vertical piping system changes with elevation. In a multi-story building, the water column above fixtures on upper floors is taller, which means the usable pressure there is lower than at the base. That’s why pressure tends to drop as you go higher.

To keep upper floors adequately fed, designers use booster pumps to raise the pressure where it’s lacking and divide the system into pressure zones. Each zone or floor can have its own pressure-reducing valve or regulating device so the overall system delivers comfortable, safe pressure on every level without over-pressurizing the lower floors. These strategies balance the pressure throughout the building, compensating for the gravity-driven drop. Leaks or friction can affect pressure, but the primary issue here is the height of the water column, not just leaks.