Recently, I was contacted by a colleague regarding my thoughts on IPC (International Plumbing Code) Section 608: Protection of Potable Water Supply. A designer within their office had provided RPZ (reduced pressure zone) backflow preventers on the incoming potable water to the softened domestic hot water system for a critical-care facility, but the facility staff was pushing back, saying there was no code-based rationale for including these devices.
The designer believed the RPZ was required based on IPC Section 608.3: Devices, Appurtenances, Appliances, and Apparatus. This section states: “Devices, appurtenances, appliances, and apparatus intended to serve some special function, such as sterilization, distillation, processing, cooling, or storage of ice or foods, and that connect to the water supply system, shall be provided with protection against backflow and contamination of the water supply system. Water pumps, water-powered sump pumps, filters, softeners, tanks, and other appliances and devices that handle or treat potable water shall be protected against contamination.”
After Some Thought, I Provided This Response
The two categories of backflow prevention are containment and isolation. Containment is where a device is required to separate the public and private systems as required by the Clean Water Act and enforced by the U.S. EPA and local purveyors. Isolation, on the other hand, is the internal protection of the potable water system within a structure or facility that separates potable water from potentially contaminated sources.
So why would one want to place a backflow preventer on a potable water softener? Does the designer in this case think the domestic hot water is nonpotable? Hot water can be considered potable, in that some people may drink from that system or use it for washing or cooking, but every connection that has hot and cold water could be a cross-contamination point if the hot water was not potable. One would only provide a water softener with backflow protection if it was not part of a potable system, but instead served some special function or process. So, to directly answer the question, the backflow preventer on the water softener system is overkill and would have an adverse impact on system pressure.
One must closely read the code language and understand the underlying reasoning. In this case, in my judgment, the last part of the code section’s statement is relevant: “…shall be protected against contamination.” Nowhere within the code specifically provides for backflow protection to be provided on a domestic hot water system served by a water softener. The code simply requires one to protect the potable water system from contamination.
Thus, a designer needs to ensure that any potential contamination sources are “isolated” from the potable system. The potable system provides safe drinking water to the occupants of the facility regardless of its temperature, so anything within that potable water system must be protected and remain potable. However, things within the potable system do not need to be protected from each other (i.e., hot and cold systems). The protection needed is between potable and nonpotable systems.
This is why, as design professionals, we need to fully know and understand our systems. You must be continually aware of what feeds your systems and what could potentially contaminate those systems.
It is also the responsibility of the owner and their facility staff to understand their installed systems. They must always be on the lookout for potential contamination sources as these systems get modified over the years. They must also be aware of the code-mandated requirements for annual backflow preventer inspection, testing, and, when necessary, rebuilding.
Here’s How Things Can Go Wrong by Not Paying Attention to the Overall System
Let me tell you about a project that inadvertently had the potential to contaminate the potable water system for an entire campus. This campus, which I will not identify, is a critical care facility that gets its potable water from the local municipal water purveyor. The supply is fed from a street main that has a “separation” valve between the two taps that serve the campus. This arrangement allows the owner and municipality to determine from which direction the facility receives its water supply.
In the case of this campus, a 500,000-gallon underground reservoir acts as a reserve should they totally lose the municipal supply. The two water services are served by RPZ-style backflow preventers located below grade in separate meter pits. Important note: RPZ backflow devices should never be located below a potential flood level; however, the municipality allowed this arrangement since the meter pits had a gravity waste system to the existing storm system on top of a hill.
Before continuing to trace the system, let’s take a moment to consider a potential contamination source: the reservoir. At the inflow and outflow points, the water velocity can be significant. However, within the reservoir, water velocity is almost nonexistent. This condition allows for stratification of the water by temperature and density, with almost no mixing occurring. Since the velocity within the reservoir is so low, scouring and/or cleaning of the surfaces does not occur. Additionally, while the incoming water may contain residual disinfectant, this residual may be consumed within the reservoir. Hence, little to no remaining disinfectant may be in the water delivered to the facility’s potable system. (Stay tuned: In a future article, I will go into more detail about reservoirs.)
Back to the potable system: Transfer pumps move the water from the reservoir to the central plant. Additionally, the two water services are arranged to bypass the reservoir and transfer pumps should conditions warrant. Once in the building, booster pump packages (cold and hot) are used to maintain pressure within the campus. The cold system simply distributes potable water to all fixtures including the domestic hot water system along with “isolated” HVAC equipment. Hence, the water remains protected and potable.
For some reason the plumbing engineer decided to provide a duplex RPZ backflow arrangement to feed the water softener and hot water generators. While this “separates” the potable cold water from the hot water system, why? The hot water system within a potable system should be potable, so this backflow seems unnecessary. If contaminates were within the hot water system, they could not flow back into the potable cold system as the RPZ would prevent the backflow. However, this hot water system, if contaminated, would also contaminate the delivered water at every mixing fixture.
Over the years the hot water system had been modified many times. One such modification, in the interest of energy conservation, allowed for pre-heat to be provided by interconnection to one of the large chillers utilized in the central plant. The HVAC engineer did not specify that the heat exchange on the chiller be double walled; hence, a potential contamination source was interjected into the potable domestic hot water system. When this was discovered as part of the water management team’s effort to address Legionella, the interconnection to the chiller was removed. Theoretically, the system was now again potable.
However, as we continued to trace the miles of piping, another issue was found. As part of the water softening system, the original brine makeup system had been replaced. The new bulk brine system consisted of a large fiberglass tank with makeup water and a transfer pump to move brine to the softeners. As the piping was traced back to what should have been a potable cold water source, it was found to have been connected into the boiler system’s feed water makeup system. This in itself would not have been a problem if the connection was on the potable side of the “isolation” RPZ that served that system. However, the correct point of connection for the brine makeup water was not selected; it was connected on the downstream side of the RPZ that protected the potable system from the boiler system. This created a potential high-hazard condition in that boiler system water and associated chemicals could be introduced into the brine storage tank, and if the brine were contaminated, it would contaminate the mineral within the water softeners and then contaminate the domestic hot water system. The RPZ arrangement that served the softener’s domestic hot water system would do little if anything to prevent such contamination of the potable water system.
So, in closing, you must fully know and understand the water system that you design and ensure that it is installed as designed. Once the system is turned over to the owner, it must be maintained by professionals who in turn must understand the potential for harm being introduced into the system as repairs, revisions, and/or modifications occur to the various systems and associated equipment.
About the Author
David D. Dexter, FNSPE, FASPE, CPD, CPI, LEED BD+C, PE, is a registered Professional Engineer, Certified Plumbing Inspector, and Certified Plans Examiner with more than 40 years of experience in the installation and design of plumbing systems. He specializes in plumbing, fire protection, and HVAC design as well as forensics related to mechanical system failures. Dave serves as Chair of ASPE’s Main Design Standards Committee, Chair of the Bylaws Committee, Co-Chair of the College of Fellows Selection Committee, and Co-Chair of the Professional Engineer Working Group. He also was the 2008–2009 President of the Engineering Foundation of Ohio, 2010–2011 President of the Ohio Society of Professional Engineers, and 2012–2014 Central Region Director for the National Society of Professional Engineers.
The opinions expressed in this article are those of the author and not the American Society of Plumbing Engineers.