The exact operating temperatures for a commercial system should be determined based on applicable codes, standards, engineering requirements, and the building’s water management plan. Of course, system specifiers and contractors also must account for scalding risk when higher water temperatures are used.

The key point: Setting the water heater to a particular temperature does not guarantee that the same temperature will be maintained throughout the building. Contractors should look for areas where water temperatures may drop, particularly portions of the system that do not maintain at least 120°F. Proper pipe insulation can help prevent significant temperature losses as hot water travels through the distribution system.

2. Design and Balance Recirculation Properly

A properly designed and balanced recirculation system plays an important role in maintaining consistent temperatures throughout a commercial hot water system.

The recirculation line should maintain the temperature for which the system was designed. Contractors should verify that balancing valves are functioning properly and that water is being distributed throughout the system as intended.

An improperly balanced system can result in some branches receiving adequate flow at the right water temperature while others experience insufficient circulation.

In applications such as restaurants, a recirculation system should maintain above 120°F, while mixing devices at individual fixtures temper water to an appropriate delivery temperature and provide scald protection.

This approach illustrates why Legionella risk management cannot be viewed solely as a water heater issue. The water heater may produce the required temperature, but the DHW distribution system must maintain it.

3. Minimize Stagnant Water

Stagnation is another critical consideration. Oversized piping increases the volume of water contained within the system, which can increase the amount of time water remains in the piping before being used or circulated.

Capped or abandoned branches can create similar conditions. Although a branch may no longer serve an active fixture, it can remain connected to the system and contain stagnant water.

Low-use plumbing fixtures also deserve attention, particularly if located at the end of a branch. During installation and commissioning, contractors should ask building owners which fixtures are rarely used and assess whether water in those locations could become stagnant.

This is especially important in commercial facilities where building occupancy and water use can vary significantly. A plumbing system that performs properly under normal operating conditions may have different characteristics in areas that see little or only intermittent use.

Storage Tank vs. Tankless Systems

Storage-tank and tankless water heaters differ in how they handle hot water, of course. Storage systems keep heated water available in a tank, while tankless systems, such as the Noritz NCC199CDV Commercial Condensing Tankless Water Heater, heat water on demand; that is, as the water passes through the unit, rather than maintaining a large volume of hot water.

Because water remains in a storage tank between uses, periods of low demand can increase the time it is held within the system. Depending on storage temperatures, the tank can create conditions that support Legionella growth. Sediment and scale can also contribute to environments conducive to biofilm development. Tankless systems, by definition, limit the amount of water held within the heating equipment. That, in turn, should reduce opportunities for Legionella growth within the unit.

However, using a tankless system does not eliminate the risk throughout the building. As noted, stagnation and favorable temperatures can still occur in the piping, fixtures, and the recirculation system.

For contractors, the type of water heater is only one consideration when addressing Legionella risk. The hot-water distribution system must also be designed and maintained to support proper temperatures and water movement.

Evaluate and Test the Entire System

Replacing an existing commercial water heating plant presents an excellent opportunity to evaluate the condition and performance of the entire DHW system. Because no particular type of water-heating equipment, tankless or storage, prevents Legionella, the CDC identifies routine legionella testing over time as the best method for validating a water management program. We recommend following these steps.

1.     Check the recirculation line to confirm that it maintains the intended temperature. Verify that the balancing valves are working properly.

2.     Walk the system, checking the fixtures throughout the building. Pay particular attention to those farthest from the water heater. These fixtures can help identify whether an adequate temperature is being maintained throughout the distribution system.

3.     Identify rarely used fixtures and ask building owners or facility maintenance personnel which fixtures are rarely used. A fixture that appears normal during commissioning may still represent a potential stagnation point if it receives little use after the building is occupied.

4.     Look for cross-connections that could allow cold water to mix with hot water. Identifying and correcting these conditions can help the system maintain its intended temperatures.

5.     Recommend routine maintenance appropriate for the water-heating equipment. For tankless water heaters, flushing frequency depends in part on local water hardness. Maintenance may be performed annually under typical conditions, or as frequently as every six months in areas with very hard water.

Legionella Risk Management is a System-Wide Job

The DHW distribution system, recirculation loop, pipe sizing, system layout, fixtures, and operating temperatures all influence how water moves and whether portions of the system remain at the proper temperature.

For plumbing and mechanical contractors, that means Legionella risk should be considered during design, installation, commissioning, and maintenance. The objective is not simply to select equipment capable of producing hot water. It is to install a complete system that maintains appropriate temperatures, promotes water movement, and minimizes opportunities for stagnation throughout the building.

Juan Rivera is a Sales Application Engineer at Noritz America with a bachelor’s degree in Mechanical Engineering. He has experience in domestic hot water systems, as well as a background in mechanical & plumbing design. His work focuses on supporting engineer’s contractors & reps with system design, equipment selection, and application guidance for tankless and storage water heating systems. He collaborates with sales teams and customers to address project requirements and ensure effective system performance.