A plumber needs to select a domestic water pump for a cottage located 20 metres above a lake. The total equivalent length of piping from the lake to the highest fixture in the cottage is 75 metres, and the desired minimum pressure at the highest fixture is 200 kPa. The design flow rate is 0.8 L/s. What calculation is most critical in determining the required discharge pressure of the pump?

Question: A plumber needs to select a domestic water pump for a cottage located 20 metres above a lake. The total equivalent length of piping from the lake to the highest fixture in the cottage is 75 metres, and the desired minimum pressure at the highest fixture is 200 kPa. The design flow rate is 0.8 L/s. What calculation is most critical in determining the required discharge pressure of the pump?

Answer options: ✅ The sum of the static lift, friction loss, and desired residual pressure.

• The total volume of water to be stored in the pressure tank.
• The diameter of the suction line from the lake.
• The number of fixtures connected to the system.

Correct answer: The sum of the static lift, friction loss, and desired residual pressure.

Explanation: The pump's discharge pressure must overcome the static lift (elevation gain), the friction loss in the piping system, and still provide the desired residual pressure at the furthest fixture. The other options are related to system design but are not the primary calculation for pump discharge pressure.