Grundfos 1/2" clip-on aquastat control 115 volts #595443 automatically turns the circulator "OFF" when the temperature in the hot water line exceeds 105 degrees Fahrenheit and back "ON" when the temperature falls below 85 degrees. May be used alone or with timer control. Surface sending design with 1/2" clip-on mount for 5/8" O.D. system piping size (1/2" I.D. copper tubing)