Also known as aquastats, these devices are installed to maintain the piping at a lower temperature than water stored in the heater. The reason that might be desirable is because there is considerable heat loss from the piping of a recirculation line. It’s important to keep the tank thermostat at about 130 degrees to prevent the growth of legionella bacteria inside it. However, a recirc thermostat can keep the water in the recirc line at a minimum temperature that is considerably below that.
The one at right was set even higher than the thermostat on the tank, so it would never turn the pump off, thus utterly defeating the purpose for which it was installed.. We’ve found this to be the case in most instances.
The tank here should have been set at 130 and the recirc thermostat at about 110 instead of 190!