PREFACE
The multi-branch DHW recirculation system shown in Figure 5-1 combines Caleffi ThermoSetter™ balancing valves, with an electronically-controlled point-of-distribution mixing valve and a variable-speed recirculation circulator. The electronically-controlled point-of-distribution mixing valve allows precise control of the water temperature entering the DHW distribution system. It can be configured to provide lower DHW temperatures, such as 120-130ºF into the delivery piping most of the time, while automatically increasing that temperature to execute a specific thermal disinfection protocol.
Operating the delivery system at temperatures lower than those required for thermal disinfection lowers piping heat loss, which reduces the building’s cooling load. The controller operating the mixing valve can also receive information from multiple temperature sensors to ensure that the entire system has undergone adequate thermal disinfection. If an abnormal condition is detected, the controller can automatically notify building maintenance staff.
The system shown in Figure 5-1 also uses a “sensorless” variable-speed circulator with an electronically-commutated motor. Figure 5-2 shows an example of such a circulator with a stainless steel volute for use in domestic hot water systems. This type of circulator is widely available from multiple suppliers.
The piping configuration in Figure 5-1 is similar to a “2-pipe direct return” hydronic heating distribution system. As such, the variable-speed circulator could be set for proportional differential pressure control mode, with the “design” head equal to the head loss of the branch circuit path of highest resistance.
This action is shown in Figure 5-3. As the ThermoSetter™ valves in each branch reduce recirculation flow rates, the variable-speed circulator will automatically reduce speed to maintain the required pressure differential on the recirculation system. Reducing circulator speed when appropriate can lower electrical energy use significantly.
It is also possible to control the speed of the circulator, as well as the on/off status of the actuators on the ThermoSetter™ valves, from a building automation system based on specific “normal operation” and “disinfection operation” conditions that need to be maintained.
SUMMARY
Recirculating domestic hot water delivery systems conserve water and reduce energy use. They also enhance occupant comfort by providing nearly instant hot water delivery at all fixtures, regardless of usage patterns. When used with the proper controls, they also greatly decrease the risk of dangerous pathogens such as Legionella bacteria in the system.
The specific needs of multi-branch domestic hot water delivery systems are optimally met using balancing valves that sense and react to water temperature at the farthest fixture in each branch. Such valves can reduce the flow rate through each branch of the recirculation system to the minimum needed to ensure adequate hot water delivery temperature to the farthest fixtures. Flow rate reductions at other times allow variable-speed recirculation circulators to significantly reduce input power, and thus save electrical energy.
Caleffi ThermoSetter™ valves are ideally matched to several types of DHW recirculation systems, including those using thermal, chemical or UV light disinfection details.