A plumber is investigating a complaint of low water pressure at the second-floor bathroom shower in a residential home. The street main pressure is 350 kPa, and the ground-floor fixtures have adequate pressure. What is the most likely cause of the low pressure at the second-floor shower?

Question: A plumber is investigating a complaint of low water pressure at the second-floor bathroom shower in a residential home. The street main pressure is 350 kPa, and the ground-floor fixtures have adequate pressure. What is the most likely cause of the low pressure at the second-floor shower?

Answer options: ✅ An undersized branch line serving the second-floor bathroom.

• A malfunctioning pressure reducing valve on the main service.
• An excessive pressure within the street main.
• A leak in the main water service line before the house.

Correct answer: An undersized branch line serving the second-floor bathroom.

Explanation: If ground-floor fixtures have adequate pressure and the street main pressure is good, the problem is localized. An undersized branch line to the second-floor bathroom, particularly if it serves a fixture with high flow demand like a shower, would restrict flow and cause low pressure specifically at that location. A malfunctioning PRV would affect all fixtures, and street main issues or service line leaks would typically impact the whole house.