Over the past several years, I often have spoken about second-order thinking. Farnam Street Media addressed first- and second-order thinking in a recent blog post (www.fs.blog/second-order-thinking); the first paragraph is quoted from The Most Important Thing by Howard Marks:

“‘First-level thinking is simplistic and superficial, and just about everyone can do it (a bad sign for anything involving an attempt at superiority). All the first-level thinker needs is an opinion about the future, as in, ‘The outlook for the company is favorable, meaning the stock will go up.’ Second-level thinking is deep, complex and convoluted.’

“Second-order thinking is more deliberate. It is thinking in terms of interactions and time, understanding that despite our intentions, our interventions often cause harm. Second-order thinkers ask themselves the question, ‘And then what?’ This means thinking about the consequences of repeatedly eating a chocolate bar when you are hungry and using that to inform your decision. If you do this, you’re more likely to eat something healthy.”

As we evaluate plumbing systems today, we can discover many second-order issues. One of the largest and most misunderstood second-order issues is hot water recirculation systems. 

These systems suffer from several issues as a result of first-order thinking. To clarify, hot water recirculation systems entail both a hot water supply and a hot water return. Complex hot water return systems have two or more parallel branches; the pattern was to provide a solution for a problem. Problem, solution. Problem, solution. Problem, solution. 

However, each solution had consequences that were not originally considered. And where issues really begin to manifest themselves is in these complex hot water return systems due to a misapplication of plumbing science. This includes unbalanced or imbalanced hot water systems, gross oversizing of hot water return systems, or shutting off the recirculation pump, among others. And the consequences manifest themselves in the form of reduced water safety and sustainability. 

This article will review the root causes of these issues, what the industry is doing to help fix them, and what the future might hold. 

Past State 

Over the past several decades, our industry has witnessed several hot water return systems problems. The biggest issues among hot water return systems are:

• Lack of hot water balancing. Many hot water return systems are not balanced correctly (or at all) post-construction. As a result, numerous parallel branches in complex hot water return systems have little to no flow. Those branches with stagnant-velocity conditions (or even full stagnancy) amplify the proliferation of Legionella pneumophila and lead to increased water use and energy waste as plumbing fixtures have to remain open longer in order to get hot water delivered.

• Oversized hot water systems. Hot water return system design often uses unnecessarily large safety factors, making systems grossly oversized. These engineering practices led to higher energy costs than required to maintain temperature. These higher energy costs became a target for energy engineers who, in turn, looked to limit the operation of the recirculation pump to minimize energy usage. 

However, this decision increased the likelihood of stagnant-velocity conditions in the most hydraulically remote portions of the hot water return system. By oversizing the hot water return system, the basic purpose of the hot water return system, which is to maintain a set temperature, is diluted. 

On the other hand, other plumbing professional designers select hot water recirculation pumps that can’t meet the system’s flow and pressure requirements. These systems become health and safety concerns, as design flow rates are never met and temperatures stagnate. 

• Hot water system technology limitations. Until the last decade, hot water system equipment had several limitations. An example is hot water master mixing valves were not designed for recirculated systems, meaning temperature “creep” occurred, as noted in Working Pressure magazine (https://bit.ly/3SsdkfO). This, along with energy conservation measures, means shutting off the hot water return pipe was mandated in energy codes. 

A potential problem with starting/stopping the recirculation pump is the short-circuiting of parallel branches of hot water return systems. A hydraulically remote branch doesn’t hit the design temperature, but because a hydraulically proximate (i.e., closed) loop does meet the temperature and, due to imbalanced systems, the temperature at the pump increases to the high-limit temperature and shuts off. 

The hydraulically remote branch possibly never meets the design temperature and remains in a temperature range leading to waterborne pathogen amplification (e.g., Legionella). See Figures 1a, 1b, 1c and 1d, in which blue piping indicates cold water, red piping indicates hot water, and purple piping indicates hot water, but on the lower end temperature-wise. Notice that in Figures 1a through 1d, pipe C never reaches the design temperature (an imbalanced system may be a contributing factor).

Present State

Over the past 10 years, the plumbing industry evolved to address hot water system control. These solutions ranged from new technologies to design techniques to codes and standards development. These advances in plumbing science have been observed by the industry in the following ways:

• Digital mixing valves. Over the past decade, these valves have made larger roads into the hot water system market. More engineers are specifying digital master mixing valves over mechanical master mixing valves, with the improved performance to control hot water systems and the ability for data logging to comply with ASHRAE 188. From a systems-level impact, by electronically controlling a diabatic mixing, more control can be exerted on the water temperature delivered throughout the building.

• Automatic balancing valves. Another trend to keep an eye on is more engineers are moving away from manual balancing valves and toward two automatic balancing valves. To define, manual balancing valves require a test and balance contractor to come in after they are installed to ensure each parallel loop is receiving its designed flow rate. Automatic balancing valves are self-regulating, which, in this case, means no balancing is needed as the valves determine the needed flow rate to maintain the performance of the system. 

There are three types of automatic balancing valves: thermostatic, pressure-independent and electronically actuated. Many engineers gravitate toward this solution as it gives them more confidence that their design will be implemented, especially after decades of not receiving test and balancing reports during construction. 

• Hot water recirculation pumps with electronically commutated motors. Variable-frequency drive pumps have been on the market for years; however, their adaptation to recirculation pumps is especially noteworthy in the past five years. Electronically commutated motor recirculation pumps are often designed with automatic balancing valves to maximize energy efficiency. 

By modulating flow or pressure to meet the performance needs of the pump, the horsepower also can modulate up or down, allowing for potential energy savings without sacrificing performance.

• ASPE Legionella Design Guide. Officially titled “Engineering Methodologies to Reduce the Risk of Legionella in Premise Plumbing Systems,” this design guide from the American Society of Plumbing Engineers (ASPE) has been in progress since 2019; it was released in mid-August for a 45-day public review. This guidance document includes many engineering principles that should be followed to minimize Legionella’s proliferation in plumbing premise systems. 

Two sections especially of note are the hot water return design and pressure zone design appendices. The hot water return design section specifically covers the Hardy-Cross methodology for determining flow, head and temperature values in parallel branches of a complex hot water return system. 

The pressure zone design section offers helpful insights to further reduce the risk of waterborne pathogen proliferation by walking design professionals through good practices to prevent pressure imbalances in domestic water systems, especially in multilevel buildings.

• IAPMO Water Demand Calculator (WDC). Worth noting is the International Association of Plumbing and Mechanical Officials (IAPMO) WDC and its potential to reduce potable water system sizes by more accurately estimating peak flow rates. The calculator only applies to residential occupancies, but IAPMO’s WDC task group is gathering the data necessary to apply this tool to commercial buildings. 

By reducing the size of the pipe, hot water return systems can operate more efficiently, especially when paired with the Hardy-Cross sizing methodology for hot water return systems.

• IAPMO 2024 UPC Hot Water Return Provision. The 2024 Universal Plumbing Code (UPC) includes a new provision giving design professionals the needed language to prioritize public health and safety over energy efficiency when necessary. Since 2015, the International Energy Conservation Code (IECC) has included provisions to turn off hot water return pumps when the design temperature is met. Unfortunately, this can lead to hydraulically remote sections of piping not reaching design temperature (especially if the system is imbalanced). 

The 2024 UPC includes language exempting health-care and hospitality facilities from shutting off the return pump. During the technical committee meetings, these two occupancy types were targeted because they had the highest number of Legionnaires Disease cases. While the technical committee commented on this potential correlation between codes, ultimately, the provision was added to show leadership in the realm of public health and safety. 

Future State

While there have been many innovations within the past decade, there is a high likelihood that further innovations are yet to come, especially in the realm of standards and codes. 

• ASPE Commissioning Standard. This standard will detail the methods for verifying that plumbing systems in new and renovated commercial, industrial and multifamily residential buildings are installed and perform as designed by the engineer and intended by the owner. 

• ASPE Thermal Disinfection Standard. This proposed standard will provide guidelines to the health-care and mechanical/electrical/plumbing engineering communities on approaches to optimally implement and manage the thermal disinfection process. Thermal disinfection is one of the primary Legionella mitigation methods used in health care today.

Several plumbing manufacturers developed products to facilitate implementing a thermal disinfection process, including balancing valves, master and point-of-use mixing valves, water heaters, fixtures, etc. This standard will provide direction on how to effectively implement and manage the thermal disinfection process using these components.

• Inspection of hot water recirculation systems. One of the subjects that came up continuously in the uniform codes technical committee meetings was the inclusion of an American Society of Sanitary Engineering (ASSE) 12080 Legionella risk mitigation specialist qualification. Many commenters indicated displeasure with any mandated credential being included in the code. The most common reason cited for this was that codes do not require credentialing. 

However, this is not true. National Fire Protection Association 99, Health-Care Facilities Code, in Chapter 5, Medical Gas Piping Systems, includes requirements for the contractor, third-party inspector and third-party verifier all to be credentialed toward the ASSE 6000 series. Thus, the precedent for credentialed professionals in code already exists. 

Therefore, as was argued during the technical committee meetings, it is not too far of a stretch to require plumbing installers working in health-care facilities to gain ASSE 12080 credentialing. 

Additionally, with several cases of waterborne pathogens in health-care and hospitality facilities due to hot water systems issues, it could be argued that there is a need for credentialed third-party inspectors to verify that the plumbing engineer’s design was implemented during construction. These types of hot water system inspections could similarly save lives just as they have done in medical gas.

Hot water systems have been suffering from a lack of second-order thinking for decades. In the past decade, however, we have seen massive strides in improving second-order thinking. As a result, hot water system control has improved drastically. From new technologies to new techniques, and innovative standards to leading codes, the entire plumbing industry is quickly adapting to address this concern. What an exciting time to be part of this plumbing renaissance and revolution!

By Christoph Lohr, P.E. CPD, LEED AP BD+C, ASSE 12080, IAPMO Vice President of Technical Services and Research