The Plumbing Prescription: Proper Flow, Better Water

A groundbreaking study conducted by researchers at Drexel University offers crucial insights into a surprising yet vital factor for building safety: the power of flow velocity, or how fast the water travels, in plumbing.  

The study, “Premise Plumbing Flow Rates, Linear Velocity, and Nitrification,” suggests that higher flow rates and smaller-diameter pipes are key to maintaining water quality and reducing the presence of harmful pathogens reaching the user.  

The research clearly shows that rightsizing and prioritizing higher flow velocities are vital for maintaining water quality. This aspirational research study serves as a critical first step toward developing evidence-based guidelines for plumbing system design, providing a clear call to action for the industry to ensure safer, cleaner water for all. 

A Real-World System Approach 

For years, research has been conducted using recirculation systems, synthetic water chemistries and brand-new pipe material, but this research highlights that what’s inside the pipes — and how it moves in real plumbing systems — is important.  

By using an aged-in-place plumbing system supplied with real municipal water at Drexel University, the researchers moved beyond theoretical lab conditions. They tested pipes made of PEX, CPVC and copper, observing how flow velocity, pipe diameter and usage frequency affected water quality. 

Researchers also tested various pipe sizes and materials with both high-frequency (five times a week) and low-frequency (weekly) flushing to simulate real-world usage patterns. 

The Key to Safer Water Is Rightsizing 

The findings are compelling. Higher flow velocity leads to better water quality, with higher residual disinfectant levels, lower turbidity (cloudiness) and less measured microbial activity. This suggests that a strong, consistent flow helps to scour pipes, preventing the slimy buildup of microorganisms that can harbor pathogens called biofilm.  

The study found significant benefits for biofilm control in smaller-diameter pipes (such as 3/8″), which naturally encourage higher flow velocities. 

Flushing Out the Risks of Stagnation 

The study also underscores the critical role of frequent flushing and regular use. Pipes that were flushed only once a week showed significant water quality degradation, regardless of pipe material.  

This demonstrates that stagnation is a major risk in building water systems, as it allows disinfectants to decay and microbial activity to increase. This finding reinforces the need for building designs that promote regular water use and limiting stagnant conditions like low-use fixtures. 

Redefining Plumbing Codes for Modern Safety 

Historically, plumbing design has relied on decades-old methodologies that assumed much higher water flow rates than we see today. This has created a critical knowledge gap, where current plumbing systems with low-flow fixtures and infrequent use lead to stagnant water, reduced disinfectant effectiveness, and a higher risk of microbial growth and discharge. The study’s findings directly address this issue, providing a clear call to action for the industry to move toward evidence-based design. 

The study’s use of a real-world, aged-in-place plumbing system sets it apart from previous research and makes its findings highly relevant for practical application. While further research is needed, this work provides a strong foundation for future studies and a clear direction for improving the safety of our built environment from the inside out. 

As the industry moves forward, this and similar data should be used to develop future plumbing code requirements, ensuring that they are built on a solid, scientific foundation. By incorporating requirements that prioritize higher flow velocities and strategies to prevent stagnation, we can design safer, more efficient water distribution systems and ultimately improve public health.