Smarter, Safer Hot Water: Digital Thermostatic Mixing Stations

Specifying digital mixing and recirculation stations provide a sustainable measurement and verification system that conserves water, saves energy, protects users from hot water burns, and helps minimize the occurrence of Legionella in tempered water delivery systems
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Sponsored by Powers
Celeste Allen Novak FAIA, LEED AP BD+C

Smart System Design Components

A digital water mixing and recirculation station can be integrated into a Building Automation System to allow facility managers control and visibility of their domestic hot water delivery. These units can control the entire tempered water recirculation loop safely and efficiently. Some of the more advanced technology can be configured in the field and do not require the use of a laptop or special software.

A manufactured system with integrated components is connected to any commercial boiler and mounted on small 2’ x 3’ plumbing grids. New systems are “plug and play,” and critical components can be easily replaced. An engineer can specify the recirculation pump make and model based on the specific requirements of a given project. Engineers can also provide system requirements. Other components include:

  • Control module connected to BAS systems;
  • Mixed outlet temperature and pressure sensors;
  • Electronically activated valve;
  • Hot water supply and return temperature and pressure sensors;
  • Hot and cold water temperature and pressure sensors; and
  • Flow sensors can also be added to measure the mixed and return flow of water as well as energy consumption.

The systems should be selected in conjunction with compatible plumbing fixtures to maximize water savings such as thermostats and valves at the point of use. Systems can be commissioned at the job site with direct field data input rather than factory settings that are more difficult to change.

Control modules are the intelligence of these systems. They should be able to be viewed on a home screen, and some of these systems can display as many as 196 combinations of critical system data, including mixed outlet temperature. Variables that can be monitored include pressures, temperatures, flows, and energy. Specific data can include:

  • Hot and cold supply inlet temperature;
  • Hot and cold supply inlet pressure;
  • Mixed outlet temperature and pressure;
  • Mixed and return outlet flow rate;
  • Energy consumption; and
  • Return temperature and return pressure.

The facility operator is able to access this data from a central data source, at the controller or through the Building Automation System. Error codes help to troubleshoot problems. Common adjustments and data flow allow for predictive temperature stability. When there is instability in the system due to low flows at the hot water inlet and/or cold water inlet and temperatures are fluctuating, the controller anticipates this problem and turns on the pumps to provide stability.

Energy is saved through the ability to detect when the system pump can no longer generate flow. The controller simply turns off the pump when the pressure exceeds the pump head. The pump is turned off until a minimum “off time” has elapsed and the system pressure drop across the pump is below the maximum pump head. It also turns the pump off when the desired recirculation temperature is attained; it is turned back on if the temperature falls below a pre-set limit.

Temperature “creep” occurs when there is little or no demand for hot water in a facility. This typically occurs during overnight hours when user demand is low. Mixed outlet temperature may rise when the internal hot water bleed-over exceeds the total demand on the system, potentially creating an elevated temperature condition. These units provide a commercial grade, electronically controlled valve that provides near 100 percent internal shut-off of hot water (no bleed-over) during zero-demand periods, eliminating the potential for temperature creep. This eliminates the need for a circuit setter (balancing valve) or automatic balancing valve, simplifying the overall plumbing system.

The actuators in these units have internal capacitors to position the valve to full cold water upon any power failure. There are also overrides that allow manual adjustment of the valve to a tempered water position at the station during a prolonged outage. Once power is restored, the controller returns the system to the previous setting. If the outage occurred during a sanitation mode, the facility manager can reprogram the valve as a safety precaution. Actuators should be located outside the actual mixing valve to protect the controls from water leakage.

Digitally controlled, industrial-grade, lead-free, electronic valve modules respond in near real time to pressure, temperature, and flow. Plumbing fixtures and fittings specified after 2014 should meet National Sanitation Foundation, NSF 372 standards as in compliance with Section 1417.

A digital water mixing and recirculation station can be encased in enclosures constructed for either indoor or outdoor use. An enclosure can protect components from physical damage due to falling objects or from exposure to indoor flooding. If the system is on the exterior, an enclosure will protect the units from ice build-up, rain, snow, or sleet. Enclosures are used to secure controller modules to limit access to authorized personnel.
High-Performance and Safety Standards Compliance
As with any specified system, there are numerous standards that high-performance equipment must meet. Digital water recirculation and mixing stations exceed performance values for many of these standards, increasing the ability to conserve water and save energy. ASSE 1017 – 2009 requirements for Temperature Activated Mixing Valves or Hot Water Distribution Systems requires that certified products must perform as follows:

  • Tested at 50 percent of valves flow at a 10 psi differential;
  • Increase hot water temperature 25 degrees F within 5 minutes, 5 degrees F/ 1 minute; and
  • Must hold +/- 7 degrees F (valves >40 gpm)

By incorporating electronic valves, fast temperature sensors, and digital controls, new systems can sample water temperature ten times per second and make necessary adjustments to stabilize mixed outlet set point temperature to within +/– 2 degrees F.

Other applicable ASSE standards include the following:

ASSE 1016 covers the delivery of water at the individual shower fixtures that are adjusted and controlled by the user. Therefore, it addresses very precise and immediate temperature regulation requirements. This standard covers three types of valves: Pressure Balancing, Thermostatic, and Combined Pressure Balancing/Thermostatic.

ASSE 1070 covers control of maximum temperature to a fixture or group of fixtures such as sinks, lavatories, or bathtubs. The device may either be the final temperature regulation or have water further tempered downstream with the addition of cold water.

New construction advanced building commission required by USGBC LEED v4 and other green rating systems require verification of building performance and the inclusion of recommissioning management manuals. Throughout the life of a building, the domestic water supply is the building system that needs to be carefully monitored to prevent the accumulation of harmful bacteria such as Legionella. OSHA publishes guidelines and recommendations for the control of Legionella. Hot water at the point of source should be maintained at 140 degrees F and frequently re-circulated throughout a system which “steps” down the temperature to prevent scalding at the point of source. ASHRAE Standard 188P lists requirements for sanitation modes that can disinfect contaminated hot water systems. One effective method for emergency disinfection of contaminated hot water is the implementation of a thermal shock treatment using the following procedure as recommended by the Legionella Prevention Association:

“ASHRAE Standard 188P An effective method for emergency disinfection of contaminated hot water systems is thermal shock treatment to be implemented using the following procedure:

  • Water temperatures shall be maintained at 71 to 77 degrees C (160 to 170 degrees F) while progressively flushing each outlet in the system BSR/ASHRAE Standard 188P
  • A flush time of thirty minutes shall be attempted. The intent is to provide thermal eradication for as long as possible up to thirty minutes.
  • Sanitization modes that conform to this standard can be quickly initiated in case of suspected water contamination. The digital controls on these water mixing and recirculation stations can also be set to provide regular maintenance by facility managers to prevent Legionella contamination.
Important features of digital mixing valves as rated by plumbing engineers.

Image courtesy of Powers

Important features of digital mixing valves as rated by plumbing engineers.


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Originally published in Architectural Record