Part II: How to Manage Risks in Plumbing System Design

Part II of this two-part series explores how to manage risk in system design

Part one of this article introduced current trends affecting sanitation safety and the continued increase in Legionnaires’ disease cases in the US. According to the US Centers for Disease Control and Prevention (CDC), nine out of 10 legionella outbreaks can be prevented with more effective water management. This article will attempt to outline how plumbing engineers can implement fundamental design principles and execute comprehensive strategies that support and facilitate effective water management.


To manage legionella Risks start with conceptual planning

Managing risk should be a priority for the building owner and should be discussed at the beginning of project planning. Engineers should include recommended control measures in early conceptual and schematic design documents. This ensures that control measures are included in the initial cost estimates and provides an opportunity to discuss the owner’s priorities with regard to management legionella.

As the project moves through design development, engineers should focus on supporting the key principles of water management:

  • Maintain the water temperature outside legionella growth range;
  • Prevent water stagnation;
  • Provide adequate disinfection; and
  • Prevent limescale, corrosion and biofilm growth.

During construction, it is critical for engineers to review construction submissions through the lenses of: legionella checking and critically assessing all cost-saving proposals. The continued rise in construction costs exposes effective design strategies and innovative products to removal from the project to save money. This is when an owner’s commitment to manage risk is vital to the successful implementation of proven controls.

Engineers must balance public health and safety, good engineering practice, sustainability, operational efficiency and owner requirements, while being cost-conscious.

Focus on the basics

Good engineering practices should start by focusing on fundamental legionella control measures. These control measures often have little or no impact on costs. Two of the most common control measures are maintaining the water temperature (hot and cold) outside legionella growing range and preventing water stagnation. Unfavorable temperatures and water stagnation increase the chance of bacteria and biofilm growth.


Balanced hot water recirculation is critical

One of the biggest challenges to maintaining water temperature is the domestic hot water recirculation system. Without proper balancing of the recirculation system, hot water circuits further away from the heat source (ie boiler) will struggle to reach or maintain the design temperature, while the hot water circuits closest to the heat source will maintain the design temperature constant.

The reason is that water chooses the path of least resistance. To balance a system, valves are installed on each hot water return circuit to create resistance. This resistance encourages water to choose a different path, while still maintaining enough flow through the circuit to overcome heat loss. A well-balanced system ensures that hot water is maintained and delivered quickly and consistently throughout the building.

Multiple issues can affect the proper balance of a system, including system design, valve selection, commissioning, scaling, renovations, repairs, or neglected maintenance. The bottom line is that most recirculation systems do not work correctly. As such, there are multiple balancing valve technologies on the market designed to improve system balancing. Automated balancing valves are the best option for ensuring a building maintains consistent hot water temperature throughout the system. They provide a perfectly balanced system based on custom parameters, automate weekly maintenance to prevent limescale build-up, monitor temperatures in real time on each return circuit and continuously log data for verification, risk assessment and compliance.

Address stagnation throughout system design

Eliminating the possibility of water stagnation with system design alone is not possible. However, engineers can reduce the chance of stagnation and support regular water changes by using good engineering practices and manufactured solutions:

  • Consider conduit routing, fixtures and fittings to minimize dead spots and eliminate dead ends.
  • Provide means to facilitate corrective or routine flushes (Routine flush is a preventative action performed during periods of low water usage or on infrequently used bypasses, mains, branches, and outlets.).

An overlooked potential for stagnation and bacterial growth is the period between substantial completion and occupation of the building. Engineers can address stagnation during this time by incorporating an automated system designed to periodically flush mains and branches based on time, temperature, or usage. This ensures regular water changes while the building is unoccupied, minimizing the growth of bacteria and biofilms while preserving residual levels of disinfectant.

Minimize aerosol formation, corrosion, scale and biofilm

legionella is transferred via aerosolization. Contaminated water droplets enter the lungs where the bacteria can grow, which can lead to severe pneumonia. When building water systems, misting can occur at showers, faucets, decorative fountains, cooling towers, humidifiers, etc. Engineers should consider choosing plumbing fixtures that minimize the potential of producing aerosols.

Sediment, scale and corrosion provide habitat and nutrients for legionella. It is important to address these points through a thoughtful selection of equipment and piping materials:

  • Water filtration removes sediment and ensures nutrient control.
  • Water treatment reduces limescale in hard water facilities.
  • Supplemental disinfection allows facilities to monitor remaining disinfectant levels.
  • Choose piping material that is corrosion-free and impervious to degradation by chlorine, monochloramine, or chlorine dioxide disinfectants.

Provide control, monitoring and data recording capabilities

Plumbing systems often lack the control and monitoring capabilities necessary for effective water management, even in the newest and most critical facilities. When controls and monitoring are installed, they require almost exclusively manual supervision and operation. Manual verification is highly dependent on operator time, dedication and expertise.

Engineers should consider designing smarter, more intelligent systems that take advantage of market innovations to simplify water management. These smarter systems automate fundamental controls and enable operators to quickly respond to potentially dangerous system changes, conduct risk assessments and verify compliance with an established water management program.

What does a comprehensive plumbing design look like?

Two long-term care facilities in the US Southwest wanted to reduce the risk of: legionella without the use of continuous additional disinfectant treatment. Engineers provided a comprehensive plumbing design by implementing a multi-barrier approach. She:

  • Control measures applied at source, such as water treatment to reduce limescale; ultrafiltration for nutrient control; and UV disinfection to destroy harmful microorganisms.
  • Selected equipment to reduce potential risks, such as booster pump without accumulator tank and flow-through expansion tank at the boiler.
  • Designed outdoor hot water storage and distribution temperatures legionella growth range.
  • Selected CPVC pipes for hot and cold water distribution.
  • Used “flow splitters” in the cold water system to minimize dead legs.
  • Recirculated hot water near fixtures (minimizing the chance of bacteria growth/survival).
  • Features an intelligent system that can automatically flush valves to ensure regular water changes, control limit monitoring and continuous data recording.
  • Equipped with ports for temporary additional disinfection (cure or routine).

Plumbers play an essential role in creating safe and healthy buildings. By making a conscious decision to incorporate fundamental principles and comprehensive strategies into the design process, engineers can minimize the impact of emerging trends and give building managers the systems they need to successfully manage risk.

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