Take Back Control of Hot Water With Electronic Technology

Transcription

1 Take Back Control of Hot Water With Electronic Technology Learn how electronic mixing valves can help better manage risks associated with Legionnaires and other waterborne diseases. by Paul Knight Last year news media reports in the United States and Canada renewed interest in and concern about a disease discovered 30 years ago: Legionnaires disease. Reports of hospital-associated outbreaks in major metropolitan areas prompted federal agencies and state health departments like the New York Department of Health to update their guidelines for prevention and control of the disease. While reported outbreaks are few, an estimated percent of Legionnaires cases go undetected, giving the false perception that the disease is rare, according to Matt Freije, a nationally recognized authority on Legionella and author of Legionellae Control in Health Care Facilities: A Guide for Minimizing Risk. Institutional plumbing systems are under increased scrutiny as the demand for a user-safe environment intensifies. The challenge to exert proactive control over the recirculating hot water system in environments where people wash and bathe while maintaining water temperatures high enough to retard the growth of waterborne pathogens but low enough at a point of use to protect from accidental scalding has proven to be daunting. As a result, a significant paradigm shift in domestic hot water system design, maintenance, and ongoing management is underway. This evolution includes a proactive approach toward system operation, a much tighter operating temperature control requirement, and improved system integration and communication capability. The fact that the primary water temperature control device in the mechanical room of a hospital, nursing home, high school, or other institutional-type installation is a simple mechanical thermostatic mixing valve (TMV), many of which are based upon a 1930s-era product development platform, needs to be re-evaluated in light of next-generation electronic mixing valves. Reviewing TMVs TMVs are designed to mix hot and cold water to deliver a blended temperature. For optimum performance they require a measurable differential between the inlet supply(s) and the mixed water outlet. According to the original design principle, the TMV would deliver that water to a point or points of use where the water would flow through a fixture or group of fixtures and down the drain. Installed in this application TMVs seem to work reasonably well. Some shut down upon inlet supply failure; some loosely restrict flow; and others simply run at full bore hot or cold. However, not all TMVs claim to have a thermal shutdown capability. With group control TMVs, attention must be paid to minimum flow rates so the internal thermostat can get a good read on what is flowing across it. In a group control application the good old TMV is just that: a good, 16 Plumbing Systems & Design MAY/JUNE 2006 PSDMAGAZINE.ORG

2 Figure 1 Manufacturer-recommended piping schematic including balancing valve on return-to-water-heater line and aquastat on pump Figure 2 Typical manufacturer pre-piped thermostatic mixing valve assembly includes aquastat on pump and balancing valve on return line to hot water source old, mostly reliable water temperature controller. Take the same out-of-the-box TMV design or manufacturer pre-piped assembly and place it in the mechanical room next to a hot water source, and then pump that water around a building to serve diversely operated points of use, and you have changed the equation completely. While you have changed the equation, you have not changed the product. Why not? Recirculating hot water systems experience significant periods of zero demand, which may be referred to as idling. Some systems idle up to 80 percent of the time in a 24-hour period. Recirculating hot water systems in idling mode require the TMV to mix hot water with returning hot water to give you hot water. Is a TMV really equipped to do that? The answer is no. The system idling control requirement is a fundamental misapplication of the product. Unless the system designer integrates a series of support components, the TMV cannot work. Unfortunately the prevailing performance bias will be to overdose the idling system with hot water from the hot water source at which point the system enters into a phenomenon known as temperature creep. To prevent temperature creep, some manufacturers and system designers recommend an aquastat on the pump set a few degrees above TMV set point. However, pumps were designed to move water, not control temperature. Circulating pumps may start to wear prematurely with frequent on/off cycling, and the use of an aquastat to intentionally stall the plumbing system directly conflicts with a specific Occupational Safety and Health Administration (OSHA) guideline for Legionella risk reduction. OSHA Technical Manual, Section III, Chapter 7: Legionnaires Disease states: Domestic hot-water recirculation pumps should run continuously. They should be excluded from energy conservation measures. In addition to the use of an aquastat to prevent temperature creep, almost all manufacturers suggest some level of flow restriction and control on the return to heater line. This line is required to return a portion of hot water from the system back to the hot water source, so the system can add hot water from the hot water source to make up the radiant heat loss within the circuit when the system is idling. Some systems utilize a manual throttling valve or balance valve (see Figures 1 and 2) while others use a thermostatic return-limiting device (see Figure 3). The idea behind the manual valve is that manually restricting the volume of Figure 3 Manufacturer-recommended piping schematic including thermostatic return limiter on return line to hot water source MAY/JUNE 2006 Plumbing Systems & Design 17

3 water returning to the hot water source equally restricts the amount of hot water, which can reenter the loop through the hot water port of the TMV. In fact, this action overrides the TMV (theoretically at this point, during system idling we do not need the TMV at all) and levels the playing field accordingly. A thermostatic return limiter essentially does the same thing but automates the process to a degree because the manual throttling is replaced by a return to hot water source flow restriction, which is based upon the temperature of the return water. Once again in idling mode the central TMV is now redundant and all of the temperature control is directed by the return limiter. An advantage of the thermostatic return limiter is that it takes the manual adjustment and, to a degree, the guesswork out of the equation. However, these modifications are necessary because of the TMV, which should be doing all the controlling but fundamentally, and by its very design principle, simply cannot. The installation manuals and maintenance tags of TMVs often carry statements such as these: Caution: All mixing valves have limitations. They will not provide the desired accuracy outside of their flow capacity range. Consult the manufacturer s flow capacity chart and do not oversize. Minimum flow must be no less than as indicated. Note: A limit stop set for 120 F is simply a mechanical setting to prevent excessive handle rotation. If incoming water is hotter than 150 F, the temperature of the factory test, the valve when turned to full hot may deliver water in excess of 120 F and the limit stop must be reset by the installer. These cautions and notes are a responsible inclusion by the manufacturer, but on the flip side they serve to amplify an important point. TMV technology has been around for 75 years with little if any significant product development in the interim. They are incredibly simple devices, which, at least in a recirculating hot water system application, have manufacturer- and designer-acknowledged limitations. With the system setup/commissioning requirements, which can consume endless contractor hours, plus an ongoing high level of maintenance, the era of the TMV might just have passed. Electronic Mixing Valves in the Mechanical Room The advent of electronic mixing valves (EMVs, see Figure 4), which are designed specifically and only for central recirculation system control, changes the equation completely. The low-flow control is very accurate, and there is no installeradjustable maximum temperature stop. An aquastat on the pump is not required, nor is there a specific requirement for any mechanical-room-based manual or thermostatic controls outside of the mixing unit. Figure 4 Electronic mixing valve EMVs are digital. You set the operating temperature the way you set a wall thermostat. They make independent decisions based upon the prevailing inlet supply conditions as these conditions relate to set point. EMVs exert control over the system by constantly monitoring the mixed water temperature whether that mix is a proportional hot and cold water mix (system under a demand greater than the circulating pump flow), a proportional blend of hot, cold, and system return water (system under a demand less than circulating pump flow), or hot water and system return water (zero demand, system idling). EMVs can exert this level of control and hold recirculating mixed water temperatures to within two degrees of set point. It can do this across the full diversity of flow rates with an unprecedented zero minimum system draw off requirement for one simple reason: Control of the recirculating hot water system is the basis of the product s design. EMVs resist water with high mineral content, which is often the bane of their thermostatic ancestry due to a positiveaction electronic motor that actuates the wet side proportioning mechanism (shuttle). The motor is powerful (40 foot pounds), so it resists mineral deposition and positively controls the shuttle from port to port. This feature allows the EMV to mix to within two degrees of either inlet supply. This results in the EMV s capability to control a recirculation system purely thermally. The EMV does not need to rely upon support components such as aquastats, throttling/balancing valves, and/or return limiters. EMVs offer remote set point adjustment and internal actual temperature communication capability. These are two critically important features. (Note: This claim is based upon water temperature readings 15 feet downstream of the EMV, not at points within the circuit.) Remote set point adjustment enables the building operator to respond to a series of thermal disinfection or pasteurization recommendations that are present within the OSHA guidelines previously noted, along with similar content from the Centers for Disease Control (CDC) and the most recent updated guidelines concerning Legionella risk reduction from the New York State Department of Health. The ability to both monitor the EMV mixing unit s internal temperature and install packaged solutions, which allow users to interrogate and report all tem- 18 Plumbing Systems & Design MAY/JUNE 2006 PSDMAGAZINE.ORG

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5 Figure 5 Typical building automation system perature values within the hot water system from a resident building automation system (BAS), gives building management an important ongoing audit capability (see Figures 5 and 6). EMVs have an integral out of temperature range mode that enhances user safety. If the mixing unit is unable to exert satisfactory control over the circuit, it recognizes the condition and enters into an error mode. The EMV will attempt to correct the condition independently, but it displays an error message sequence and simultaneously notifies the BAS of the condition via a low-voltage signal. From within the BAS we can access mobile text messaging, , and other communication features to raise awareness. The ability to fully understand, track, log, and trend temperatures, as well as relay data to supervisory bodies as required in some states, is attractive. Controlling Water Temperature We typically seek to regulate water temperatures to prevent scalding, but ongoing guidance with respect to Legionella risk reduction presents a new safety consideration. Legionella colonization can be present at water temperatures below 122 F. While most experts previously agreed that stagnant water below 122 F represents the Source: SUMMA Healthcare, Akron City Hospital, Akron, Ohio most significant risk, hence the continuously running recirculating pump theory, regulatory agencies have released relevant advisories regarding water temperatures at all points within the circuit. For example, the State of New York Department of Health July 2005 Prevention and Control of Legionnaires Disease Guidance for Clinicians states: If your facility s building has the necessary mixing valves and/or anti-scald valves, hot water shall be stored above 140 F (60 C) and circulated with a minimum return temperature of 124 F (51 C). Mixing valves and/or anti-scald valves are necessary on such systems to reduce the final water temperature to no more than 110 F (43 C) in patient areas to prevent scalding. OSHA Technical Manual, Section III Chapter 7 states: To minimize the growth of Legionella in the system, domestic hot water should be stored at a minimum of 60 C (140 F) and delivered at a minimum of 50 C (122 F) to all outlets. While much has been documented regarding Legionella and the domestic hot water system, members of the plumbing engineering and system design community still have a lot of questions. More than a dozen guideline references can be found within the public domain, including those by OSHA, CDC, and NYDOH. Experts such as Freije would offer that each situation is different, and in many cases a single solution might prove to be inadequate. It is all about testing below acceptable levels, and often that achievement requires a combination of prevailing disciplines. One thing is cast in stone, however. Every building requires the ability to heat water and control the temperature of that Figure 6 Building automation system data logging and trending Source: Richmond State Hospital, Richmond, Indiana 20 Plumbing Systems & Design MAY/JUNE 2006 PSDMAGAZINE.ORG

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7 Figure 7 Two-temperature electronic mixing valve installation with BAS interface water, as well as a delivery mechanism to the point of use. By selecting EMVs and by choosing to elevate the complete plumbing system onto the BAS platform within the facility (see Figure 7), the plumbing designer accomplishes several things: User safety may be enhanced. The building will be positioned to meet the water temperature minimums and thermal disinfection guidelines should the owner opt to do so at some future juncture. EMVs at Points of Use While advanced electronic water temperature controls in the mechanical room with enhanced BAS interface options are becoming an accepted design standard, clearly the most intriguing application is at the point of use. EMVs at each individual point of use such as lavatories, showers, bathtubs, for combination bath/shower installations are already available. These products offer some helpful features, the most important being a credible solution to meeting Legionella risk reduction guidelines. Current features of today s EMVs for the point of use are hands-free user flow control and temperature selection, which are programmable by building management through the use of a personal digital assistant. When the EMV is tied into the BAS, flow times can be set and temperature adjustment bands (how hot and how cold) can be selected during commissioning. This feature can be helpful in several applications: In an assisted living, nursing, longterm care environment, the EMVs can streamline assisted bathing because temperature selection and flow control do not require a mechanical handle or even any contact. In a surgical scrub, neonatal, or other critical care/control environment, EMVs offer a more advanced solution to the foot pedal, knee panel design and enhance today s more modern sensor-operated systems by allowing temperature selection and a preset wash-up timer. EMVs can be programmed to automatically turn on and evacuate a dead leg of water after a preselected period of inactivity termed a service flush. A service flush eliminates the concern associated with infrequently used fixtures and the dead legs feeding them as possible incubation points for Legionella. Guaranteeing that the treatment will reach all points within the system enhances the efficacy of any selected method of Legionella abatement. On the surface, thermal disinfection of an entire plumbing system from water heater to fixture in an occupied building looks to have inherent user safety and/ or manpower and associated expense issues. It is very difficult to overcome this objection unless you are able to integrate some level of electronic technology. Consider this solution: A permanent water heater set point of 160 F (NYDOH suggests the use of a non-storage-type instantaneous water heater). Integrating an EMV in the mechanical room set to a F recirculation temperature with BAS interface allows one person to easily elevate the system to 160 F via the remote set point adjustment feature. With EMVs, which are thermally actuated and controlled to within one degree at each point of use, suitable user protection measures are in place. The elevated hot water temperatures will not reach the user at the fixture. Loop pasteurization is now ensured. To meet the fixture flushing guideline, EMVs have a PDA-directed fixture flushing protocol, which allows building management, on a per-fixture basis, to draw the 160 F water through a particular point of use in a controlled activity while other fixtures are still user accessible. Next generation EMVs for both the mechanical room and the point of use will offer wireless connectivity along with networking capability. The ability for each point of use within the building to report in on a periodic basis is a natural product enhancement, and the ability to have fixtures and the central recirculating hot water system fully integrated and communicating back and forth seems to be a reasonable expectation. Electronics are here. Current water temperature control technology is behind the times to an inordinate degree compared to the other utilities within the building. The accumulating pressure of enhanced user safety requirements for water that is both too hot and, due to the Legionella risk, often not hot enough has system owners, designers, and operators seeking a superior solution. Paul Knight has more than 20 years of field, laboratory, and general work experience involving the design, application, and marketing of mixed water temperature controls. He is a 20-year member of ASPE and ASSE, including consecutive terms on the board of the ASPE Northern New Jersey Chapter. 22 Plumbing Systems & Design MAY/JUNE 2006 PSDMAGAZINE.ORG