A plumber is designing a domestic hot water recirculation system for a multi-story residential building. The system requires a minimum flow rate of 0.5 L/s to maintain adequate hot water at the farthest fixture. If the total equivalent length of the recirculation loop is 150 metres, and the maximum allowable pressure drop is 40 kPa, which pipe material and nominal diameter would be most suitable to achieve the required flow while keeping velocity within acceptable limits (typically < 2.5 m/s)?

Question: A plumber is designing a domestic hot water recirculation system for a multi-story residential building. The system requires a minimum flow rate of 0.5 L/s to maintain adequate hot water at the farthest fixture. If the total equivalent length of the recirculation loop is 150 metres, and the maximum allowable pressure drop is 40 kPa, which pipe material and nominal diameter would be most suitable to achieve the required flow while keeping velocity within acceptable limits (typically < 2.5 m/s)?

Answer options:

• 25 mm Type L Copper
• 32 mm PEX-b ✅ 38 mm Type M Copper
• 50 mm schedule 40 Steel

Correct answer: 38 mm Type M Copper

Explanation: To meet the flow rate and pressure drop requirements over a long run, a larger pipe diameter and smoother material are generally preferred. 38 mm Type M Copper offers a good balance of flow capacity, friction loss, and cost-effectiveness for this application, ensuring the velocity remains acceptable. Smaller pipes would likely result in excessive pressure drop or velocity, while significantly larger pipes might be overkill and add unnecessary cost.