Tekmar 157 Difference Setpoint Control Variable Speed The Difference Setpoint Control 157 is designed to use a variable speed pump to transfer heat from an intermittent heat source to a storage tank. It can be used in many different solar applications, including closed systems, drainback, or heat dump installations. Two sensors are used to measure the temperature at the source and at the storage location, and begin heat transfer when there is a positive difference between them. This product uses variable speed technology to operate a permanent capacitor wet rotor circulator to vary the rate at which heat transfer occurs.