This article was written by Mike Cudahy, codes and standards specialist for the Plastic Pipe and Fittings Association. The article was published on NAHBNow as a benefit of membership in the NAHB Leading Suppliers Council (LSC).

The way water moves through our buildings has a direct impact on public health, according to a recent study conducted by Drexel University and commissioned by the Plastic Pipe and Fittings Association, an LSC member.

This research, “Premise Plumbing Flow Rates, Linear Velocity, and Nitrification,” moved beyond theory to use real-world, aged-in-place plumbing systems and municipal water. By shifting the focus from oversimplified studies to the critical dynamics of design flow and velocity, and measuring activity inside the pipes and also exiting the pipes, the study offers crucial evidence to inform the next generation of plumbing codes and design practices.

The Study’s Results

The study measured how factors such as flow velocities and pipe use frequency influence water quality indicators, specifically looking at microbial activity inside the pipes, nitrification and the potential emission of pathogens at the tap.

The findings suggest a clear path forward for system design: Higher-flow velocities demonstrated the lowest microbial activity, less nitrification and better overall water quality.

Conversely, maintaining water quality during periods of prolonged stagnation proved challenging across all common pipe materials tested.

One of the most significant takeaways challenges conventional wisdom: Pipe material (PEX-b, copper or CPVC) did not significantly impact the prevention of opportunistic pathogen growth. The research found that design — specifically ensuring proper velocity and minimizing stagnation, which maintains disinfectant — is the paramount factor.

This research suggests future plumbing codes should pivot toward establishing updated hydraulic performance requirements, which haven’t been touched in decades.

What’s Next

A goal of the Plastic Pipe and Fittings Association is for this work to directly inform safe right-sizing of North American water distribution systems and assist in the improved design of premise plumbing systems.

Choosing the proper pipe sizes in the development of the plumbing codes is a complex decision, requiring careful consideration of the trade-offs between specific surface area and linear flow velocity. We encourage industry professionals to reference this evidence-based approach in local and state code change discussions to support necessary changes to plumbing codes to maintain water quality

However, while the study offers a powerful foundation, the authors note limitations, including focusing on a single domestic water supply and not addressing hot water systems — a critical area for pathogen risk.

Therefore, this research is intended not as a final answer, but as a robust aspirational model platform to be referenced and emulated for further experimentation. The ultimate hope is that this evidence-based approach will support necessary changes to plumbing codes, leading to healthier, more sustainable and better-designed buildings for everyone.