During 2020, Peter Skinner, P.E. and Gary Klein wrote a series of seven articles for Plumbing Engineer providing an in-depth look into the engineering decisions made in the design and construction of three 35-unit apartment buildings in upstate New York. Those articles focused on the effort to right-size the plumbing and the water heaters for customer satisfaction and energy efficiency.  

In this article, we detail how right-sizing also directly impacts Legionella growth in building water systems 

When Peter and Gary began the engineering review of the premise plumbing and water heating design, the foundations for the three buildings in Phase II were already completed. This meant all the big architectural decisions had already been made and were literally and figuratively cast in concrete.  

As with many things related to dwellings, it is all about location, location, location. Three decisions that should have been considered in the schematic design of the building are:  

1. Location of the wet rooms in each apartment relative to each other and to the source of hot water within the apartment. 

2. Location of the wet rooms in each apartment relative to those in the adjacent apartment. 

3. Location of the mechanical room relative to all apartments. 

Time-to-Tap 

The first principle is determining time to tap — how long the tenants wait for hot water. This issue applies to a 35-unit apartment as much as a 350-bed hospital or a 35-story office building. 

This decision rests with the architect. The plumbing engineer and the plumber cannot overcome the impact of an inherently inefficient architectural building layout design.  

In the simplest of terms, the closer the fixtures are to each other and the source of hot water, the less time it takes and the less water and energy that runs down the drain.  

This is the reason why ASHRAE Guideline 12 states: “Planning to control the conditions that increase the potential for Legionella growth should begin with the architecture, design, and engineering of buildings. Building layout and location of water fixtures will significantly affect the number, length, and complexity of pipe runs; the occurrence of dead legs; and the use of water and energy.” 

ASPE says that a good time-to-tap is no longer than 10 seconds. Research conducted by Carl Hiller almost 20 years ago documented that the source of hot water cannot be more than 5 seconds away from each hot water fixture to deliver 105 F or hotter within 10 seconds.  

This is very close. How close? It depends on the volume (length and diameter) in the pipe between the source of hot water and each fixture. The pipe length has already been determined by the architect. Most plumbing codes stipulate a minimum nominal diameter of 0.5 inches on branches and twigs. Some codes allow as small as 0.375 inches for some fixtures.  

Using the IAPMO Water Demand Calculator enables the right-sizing of the trunks and branches. Additional information, which is available, is needed on the pressure drop in 0.375-inch pipe and fittings to enable a plumbing engineer to be able to make a case for its use on the twigs. 

Why is time to tap so important? 

• Customer satisfaction increased;
• Water and energy loss reduced while waiting for hot water to arrive;
• Energy loss further reduced when the piping in the 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 risers 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 templated 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 distribution 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 multifamily 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 return piping that would have normally been installed from the distant mechanical room to the risers.  

The underlying theme of all these design decisions is to reduce the volume in the premise plumbing. Less stored water volume in the piping equals less Legionella risk, water waste and energy consumption. The end result is healthier buildings and increased customer satisfaction.  

Right-Sizing the Premise Plumbing 

Once the architectural decisions have been made, it is now time to use IAPMO’s Water Demand Calculator (WDC) to right-size the premise plumbing. It is currently able to be used for single-family and multifamily occupancies. The WDC brings Roy Hunter’s work into the modern era for these two occupancies because there is data to support updating the probabilities of simultaneous use.  

It allows plumbing designers to use modern flow rates and these updated probabilities to more accurately size plumbing systems while still leaving a good margin of safety in the design. 

What is the calculated savings on this project from right-sizing?  

Construction costs: 

• $5,000 in first costs of the piping;
• $5,000 in the fittings, valves, pipe insulation and installing the smaller diameter pipe; 
• $20,000 by reducing the water heating system by a factor of four;
• The service entrance was reduced from 3 to 1 1/2 inches, which meant a smaller water meter, lowering the first costs still further; 
• $850 less per apartment to build this building.  

Operational costs: 

• $2,500 per year projected operational savings in water, sewer and energy included.

Every item discussed previously directly impacted Legionella risk. The volume of the water in the premise plumbing system was reduced by about half from the design used in Phase I. Once again, less volume = less water aging = less Legionella risk = healthier buildings, all while saving water and energy and increasing customer satisfaction. 

Gary and I have worked together on these issues for many years because we understand that Legionella risk management and water and energy conservation are objectives integral to each other.  

After more than 20 years of work in the Legionella field leading outbreak investigations and resolutions as well as performing building audits, I’ve found the two single most critical variables for controlling Legionella risk are temperature and water age (time-to-tap). 

If the temperature at the fixture is more than 120 F and the time to tap is less than 1 minute, then the likelihood of any Legionella issues is very low. And if the temperature at the fixture is less than 130 F, then the likelihood of scalding is very low. 

The next article in the series will go into more detail on temperature, temperature control and temperature impacts related to Legionella risk in plumbing design.