The Difference Setpoint Control 156 is Designed to Transfer Heat From an Intermittent Heat Source to a Storage Tank. This Control Can be Used in Applications Ranging From Solar Thermal to Wood Boiler Systems. 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 a Single 240 V (Ac) Rated Relay to Accomplish This. For Further Information And up to Date Literature Visit: Tekmar Documentation | Tekmar Controls | 156