60/Plumbing Engineer August 2021
apartment cools down after being used; Legionella risk is reduced because less volume in the piping means less water aging; Legionella risk reduced because less water aging increases disinfectant residual. Key Design Criteria So, what design criteria are critical to minimize the risks of Legionella growth in plumbing systems? The same exact criteria as needed to minimize water and energy loss. Figure 1 shows the layout of the corridor apartments and identifies the time-to-tap for each fixture. Figure 2 presents the same information for the corner apartments. Notice the differences, all due to location. What could be done to improve the design? 1. Locate all wet rooms close to each other and to the source of hot water within the apartment. This reduces the horizontal distance and, therefore, the volume of water in the cold and hot water piping. Why would this have worked? Each apartment has a mechanical closet adjacent to it with access from the hallway. The cold and hot water ris- ers come into these closets and each branch has a shut-off valve. The volume of water in each apartment's piping depends on the distance from these valves to the fixtures and on the diameter of the piping. The closer these valves are to the fixtures also makes a difference, particularly for the hot water piping. In the corner apartments, switching the location of the kitchen and the bedroom would make a significant improvement. Additional volume must be considered for the time- to-tap. This additional volume depends on the length and diameter of the hot water branch line coming from the circulation loop to the hot water shut-off valve. The closer the loop can come to each shut-off valve, the smaller the extra volume. 2. Locate the wet rooms in adjacent apartments, so they share a common plumbing wall and the same drain and vent stack. This is common practice in tem- plated hotel design, but too often not the case in office buildings, hospitals or multifamily dwellings. The plumbing codes are clear: fixtures must be within a short horizontal distance to share the same vent stack. If you locate the hot and cold risers close to the vent stack, then the distance and volume to each fixture will also be small. Another advantage of this back-to-back configuration is that there can be fewer risers, which should result in less pipe volume. 3. Locate the mechanical room so it is closer to all the fixtures in the building. It is very common to see a mechanical room off in one corner of the basement or first floor. This results in a long-distance, high-volume header to the furthest risers. A central location would result in a two-zone distribu- tion system. Each zone serves half of the apartments, which means each zone can be sized to match a smaller load. Bring the service entrance pipe to the mechanical room and distribute it from there. This works for both cold and hot water. Each hot water zone would have its own return pump and controls. Tim McDonald, founding principal of multifam- ily building Onion Flats, took this idea even further. He installed a water heating system below each stack, one system for each group of seven apartments. So yes, he had four small mechanical spaces instead of one larger one. The big win: This decision eliminated the long run of large diameter supply piping and correspondingly long
Figure 1: Corridor Apartments - Time-to-Tap. Figure: John MacArthur, Beardsley Architects + Engineers; Table: Gary Klein
Legionella
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