Taco 0011-VSF4 Variable Speed Setpoint Cast Iron Circulator, Accepts 3/4", 1", 1-1/4", 1-1/2" Flanged Connections, Flanges Not Included Variable speed circulators are one of the best ways to improve circulation, and system, efficiency. Taco's variable speed setpoint circulators will maintain a user-set fixed temperature or differential through shifting system conditions and demands. By scaling circulation and temperature mixing to your specific needs" to "By constantly monitoring system conditions and adjusting speed to match current conditions, the variable speed setpoint circulator improves system efficiency. The universal flange-to-flange dimension of Taco's "00" series circulators makes installation quick, even if you are replacing a different pump. With Taco, your hydronic circulation is in good hands.