What residual pressure should I design for in a potable water system (at the top of the structure)?
Most North American plumbing codes require at least 30 psi at the top of a building. This is to ensure any existing or new building has enough pressure to operate water-conserving flush valves. The maximum pressure allowed under most codes is 70 psi. Since there is a large design window, temptation is to design for a residual pressure somewhere in the middle (50 psi). Practically, it is undesirable to supply more than the minimum at the top of the building. Though designing for a higher pressure does not change water usage for flush valves and gravity tanks; sinks, showers, and other fixtures increase their water usage with pressure. Though in the design phase, it is not recommended to underestimate losses, once the building is occupied, it is recommended to reduce the pressure down using a PRV or a VFD on the pump system to the minimum pressure required. This ensures that the building owner does not pay for wasted water over the life of the plumbing system. In the case of a VFD system
