New products and updated standards aid engineers
by Kyle DelPiano
The average death rate for Legionnaire’s disease is one in 10, according to statistics from the Centers for Disease Control and Prevention (CDC), making it the deadliest waterborne disease in the United States today. For those with weakened immune systems who con- tract the disease in a hospital or health- care facility, the death rate is one in four.
These alarming statistics, coupled with increasing numbers of reported cases — up 62 percent between 2014 and 2018, from 5,166 to 8,453 — are spurring action on multiple levels to educate those on the front lines as well as the general public in order to lower risks and save lives.
Legionnaire’s disease is a severe type of pneumonia caused by the Legionella bacteria. The bacteria exist naturally in low numbers in rivers, lakes and streams, but in the complex plumbing and HVAC systems of commercial buildings, Legionella bacteria can proliferate and sicken inhabitants.
According to the CDC, transmission is generally spread through aerosolized water droplets or through aspiration, although most healthy people do not get Legionnaire’s disease after being ex- posed to Legionella. Those over 50 years of age or having certain risk factors, such as a history of smoking or a compromised immune system due to illness or
medication, are susceptible to infection. Efforts within the commercial building sector to reduce the risk of Legionella bacteria growing and spreading are focused on promoting the effective design and maintenance of water systems in buildings. Designers of water systems must rely on guidelines and codes produced by industry and government organizations as the scientific community continues research and manufacturers invest in product development to eradicate the deadly disease.
ASHRAE Standard 188 was revised in 2018 to provide building designers and building owners more comprehensive guidance in the design, operation, maintenance, repair, replacement and expansion of a building’s potable and nonpotable water systems and components. Also in 2018, OSHA, the Occupa- tional Safety and Health Administration, updated information on prevention, identification and management of Legionella exposure.
A number of previously accepted common practices in water system design are no longer recommended, so it’s vitally important designers, contractors and facility maintenance personnel are up to date on best practices.
In plumbing systems, a hot water recirculation system is commonly specified because it helps conserve water by supplying hot water to a fixture more quickly. General practice in hot water recirculation systems was to shut down the system during periods of no demand, but that has been condemned by OSHA and ASHRAE because stagnant water in pipes creates a favorable environment for bacteria to grow.
Note some of the basic design proce- dures for a hot water recirculation system:
• Determine required recirculation flow rate based on the heat loss of the sup- ply pipe to the farthest faucet or riser at a given delta temperature
• Size recirculation line — determine flow-friction head loss in recirculation line, heater supply pipe, etc.
• Select pump based on flow requirements and head loss for the system
• Avoid unbalanced riser flows to maintain distribution temperature
• Never recirculate across a pressure- reducing valve
• Recirculation pumps must be constructed of non-corrosive materials such as brass, bronze and stainless steel
Chapter 7 of OSHA’s technical manual recommends waterborne bacterial con- trol measures be included in the design whenever hot water is supplied to high- risk occupancies, such as hospitals or nursing homes and also hotels. Legio- nella control should include:
• A constantly running recirculating pump
• Minimal dead legs to reduce stagnancy in system
• Minimum water heater temperature — water heaters maintained below 140 degrees Fahrenheit create ideal conditions for Legionella bacteria to thrive and contain scale and sediment that can harbor bacteria
• Point of usage thermostatic mixing valves at every fixture if supply hot water is over 105 degrees and a report of balanced circuits
• Sizing of recirculating pump on total flow required to balance the system not just on the heat loss of the supply mains; select larger pump if needed to ensure a balanced system
Conditions that favor bacteria growth in building water systems include biofilm, scale and temperature. When Legionella forms biofilms in the pipes leading to faucets and showerheads, water can still contain Legionella.
Temperatures between 120 and 140 degrees are optimal for propagation of Legionella; best practice is to maintain hot water temperature at the high- est temperature allowable code or local regulation. Legionella bacteria exposed to 140-degree water will die within 30 minutes. However, higher temperatures pose a scalding risk to people — a 3-second exposure to 140-degree water
can result in a first-degree burn. If Legionella is already in pipes and faucets, 140-degree water will not be sufficient because of the temperature drop that occurs when it leaves the tank on its way through the distribution system.
A thermostatically controlled balance valve located at the end of each hot water supply branch automatically adjusts or throttles to maintain the temperature set point. As the return line cools down, which can occur during periods of low demand, the valve opens. If the return line temperature heats up, the valve will never close completely, ensuring a minimal flow rate to prevent stagnant water in the recirculation lines.
Thoughtful system design to reducing the growth and spread of Legionella bacteria includes the latest product advances such as thermostatically controlled balance valves and ECM recirculating pumps, as well as adherence to industry best practices, codes, guidelines and a Legionella mitigation plan. As the scientific community continues its search for solutions, design engineers on the front lines will be challenged to keep Legionnaire’s disease at bay.
Kyle DelPiano is the Business Development Manager – CBS market for Xylem AWS. He is a graduate of Auburn University with a bachelor of science degree in polymer and fiber engineering, concentration in mechanics.