tekmar’s Setpoint Difference Control 157 uses variable-speed pump operations to get more heat from a solar thermal system, even on less-than-ideal days. During mild solar radiation days, variable-speed pumping provides a higher amount of heat transfer when compared to on/off pump operation. An added benefit is the reduced electrical consumption that results from running at lower pump speeds. A second pump output provides for a wide range of applications including backup heating, external heat exchanger circulation or booster pump operation for drainback systems.